IEEE 1149.1 Boundary-Scan Standard Part 1: Chip Levelpeople.ucalgary.ca/~smithmr/2007webs/encm415_07/07...IEEE Standard 1149.1-2001 “Test Access Port and Boundary-Scan Architecture,”

1149.1 Boundary Scan Standard: Chip Level Architecture June, 2006

© Bennetts Associates, 2006 Page 1

Slide 1© 2006, Bennetts Associates1149-1-chip.ppt, Last revised: June 2006

AB Boundary-Scan Technology

IEEE 1149.1Boundary-Scan Standard

Part 1: Chip Level

IEEE 1149.1Boundary-Scan Standard

Part 1: Chip Level

Ben Bennetts, DFT ConsultantBennetts Associates, UK

Tel: +44 1489 581276 E-mail: [email protected]://www.dft.co.uk/

In this tutorial, you will learn the basic elements of boundary-scan architecture — where it came from, what problem it solves, and the implications on the design of an integrated-circuit device.




AB The Standard

IEEE Standard 1149.1–90, 1a-93, 1b-94, 1c-01

“Test Access Port And Boundary-Scan Architecture”

Available from:

http://standards.ieee.org/ or http://www.techstreet.com/info/ieee.html or

http://global.ihs.com/

IEEE Standard 1149.1–90, 1a-93, 1b-94, 1c-01

“Test Access Port And Boundary-Scan Architecture”

Available from:

http://standards.ieee.org/ or http://www.techstreet.com/info/ieee.html or

http://global.ihs.com/

The core reference is the IEEE 1149.1 Standard:IEEE Standard 1149.1-2001 “Test Access Port and Boundary-Scan Architecture,”available from the IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, New Jersey 08855-1331, USA. The standard was first published in 1990, revised in 1993 and 1994, and most recently in 2001. You can obtain a copy of the Standard via the world wide web on the IEEE home page at: http://standards.ieee.org/catalog.The 1993 revision to the standard, referred to as “1149.1a-1993,” contained many clarifications, corrections, and minor enhancements. Two new instructions were introduced in 1149.1a and these are described in this tutorial.The 1149.1b-1994 supplement contained a description of the Boundary-Scan Description Language (BSDL).The 1149.1-2001 version contains enhancements to the wording, plus removal of the use of the all-0s code for the Extest instruction. In addition, the mandatory Sample/Preloadinstruction has been spit into two separate instructions: Preload and Sample, both still mandatory.




AB In-Circuit & Functional Board Test

Bed-Of-Nails (ICT, FPT, MDA)Functional

Since the mid-1970s, the structural testing of loaded printed circuit boards has relied very heavily on the use of the so-called in-circuit bed-of-nails technique (see above). This method of testing makes use of a fixture containing a bed-of-nails to access individual devices on the board through test lands laid into the copper interconnect, or into other convenient physical contact points. Testing then proceeds in two phases: power-off tests followed by power-on tests. Power-off tests check the integrity of the physical contact between nail and the on-board access point, followed by open and shorts tests based on impedance measurements.

Power-on tests apply stimulus to a chosen device, or collection of devices (known as a cluster), with an accompanying measurement of the response from that device or cluster. Other devices that are electrically connected to the device-under-test are usually placed into a safe state (a process called “guarding”). In this way, the tester is able to check the presence, orientation, and bonding of the device-under-test in place on the board.




AB In-Circuit Test & Test Objectives

PresencePresence

OrientationOrientation

BondingBonding

Bed-Of-Nails (ICT, FPT, MDA)




AB Defect Coverage: Bed-Of-Nails

Driver(Sensor)

Sensor(Driver)

Driver(Sensor)

Defects covered: nail - plated-through-hole - interconnect - solder - leg - bond wire - device - bond wire - leg - solder - interconnect - plated through hole - nail

The use of boundary-scan cells to test the presence, orientation, and bonding of devices was the original motivation for inclusion in a device. The use of scan cells as a means of applying tests to individual devices is not the major application of boundary-scan architecture. Consider the reason for boundary-scan architecture in the first place. The prime function of the bed-of-nails in-circuit tester was to test for manufacturing defects, such as missing devices, damaged devices, open and short circuits, misaligned devices, and wrong devices. See diagram above.

In-circuit testers were not intended to prove the overall functionality of the on-board devices. It was assumed that devices had already been tested for functionality when they existed only as devices (i.e., prior to assembly on the board). Unfortunately, in-circuit test techniques had to make use of device functionality in order to test the interconnect structure — hence the rather large libraries of merchant device functions and the problems caused by increasing use of ASICs.




AB Device Packaging Styles

DIP PGA SOIC TSOP

SOJ PLCC QFP BGA

Thru-Hole Surface-Mount

Fundamentally, the in-circuit bed-of-nails technique relied on physical access to all devices on a board. For plated-through-hole technology, the access is usually gained by adding test lands into the interconnects on the “B” side of the board — that is, the solder side of the board. The advent of onserted devices packaged in surface mount styles –see Figure above - meant that system manufacturers began to place components on both sides of the board — the “A” side and the “B” side. The smaller pitch between the leads of surface-mount components caused a corresponding decrease in the physical distance between the interconnects.




AB Multi-Layer Board Construction

Vias

InnerLayers

A Side

B Side

Components

Components




AB Probing Multi-Layer Boards

The move to surface-mount packaging had a serious impact on the ability to place a nail accurately onto a target test land, as shown in the Figure. The whole question of access was further compounded by the development of multi-layer boards created to accommodate the increased number of interconnects between all the devices. Basically, the ability to physically probe onto the board with a bed-of-nails system was going away: physical access was becoming limited.




AB So, What Went Wrong?

DIP PGA SOIC TSOP

SOJ PLCC QFP BGA

Thru-Hole Surface-Mount




AB What Problem Did This Cause?

Nowhere to probe onto the board.

Nowhere to probe onto the board.




AB What Was The Solution?

Joint (European) Test Action Group, 1985 - 90




AB Principle of Boundary Scan

Any Digital Chip

Each boundary-scan cell can:Capture (parallel load) data on its parallel input PIUpdate (parallel unload) data onto its parallel output POSerially scan data from SO to its neighbour’s SIBehave transparently: PI passes to PONote: all digital logic is contained inside the boundary-scan register

PI

PO

SOSI

Test Data In (TDI)

Test Clock (TCK)

Test Mode Select (TMS)

Test Data Out (TDO)

Input Scan Cell

Output Scan Cell

In a boundary-scan device, each digital primary input signal and primary output signal is supplemented with a multi-purpose memory element called a boundary-scan cell. Cells on device primary inputs are referred to as “input cells”; cells on primary outputs are referred to as “output cells.” “Input” and “output” is relative to the internal logic of the device. (Later, we will see that it is more convenient to reference the terms “input” and “output” to the interconnect between two or more devices.) See diagram above.

The collection of boundary-scan cells is configured into a parallel-in, parallel-out shift register. A parallel load operation — called a Capture operation — causes signal values on device input pins to be loaded into input cells, and signal values passing from the internal logic to device output pins to be loaded into output cells. A parallel unload operation — called an Update operation — causes signal values already present in the output scan cells to be passed out through the device output pins. Signal values already present in the input scan cells will be passed into the internal logic.

Data can also be Shifted around the shift register, in serial mode, starting from a dedicated device input pin called Test Data In (TDI) and terminating at a dedicated device output pin called Test Data Out (TDO). The Test ClocK, TCK, is fed in via yet another dedicated device input pin and the various modes of operation are controlled by a dedicated Test Mode Select (TMS) serial control signal.




AB Motivation For Boundary Scan: Summary

Major change of device packaging: from thru-hole (DIL) to surface mount (SOIC, QFP, BGA, etc), leading to …… increasing density of devices on the boards, leading to……development of double-sided and, eventually, multi-layer Printed-Circuit Boards (PCBs), leading to ...… reduced physical access for MDA/ICT nails

Question: how to test for manufacturing defects?Answer: use boundary scan structures

To summarize, the basic motivation for boundary scan was the miniaturization of device packaging, the development of surface-mounted packaging, and the associated development of the multi-layer board to accommodate the extra interconnects between the increased density of devices on the board. These factors led to a reduction of the one thing an in-circuit tester requires: physical access for the bed-of-nails probes.

The long-term solution to this reduction in physical probe access was to consider building the access inside the device i.e. a boundary scan register. In the next section, we will take a look at the device-level architecture of a boundary-scan device, and begin to understand how the boundary-scan register solves the limited-access board-test problem




AB JTAG Meeting, 17 September, 1988

Such was the situation in the mid-1980s when a group of concerned test engineers in a number of European electronics systems companies got together to examine the board-test problem of limited access and its possible solutions. The group of people initially called themselves the Joint European Test Action Group (JETAG). Their preferred method of solution was to bring back the access to device pins by means of an internal serial shift register around the boundary of the device - a boundary scan register.

Later, the group was joined by representatives from North American companies and the ‘E’ for “European” was dropped from the title of the organization leaving it Joint Test Action Group, JTAG – see photograph above. (The author is in the front row, third from the right-hand end.) JTAG did not invent the concept of boundary scan. Several companies, such as IBM, Texas Instruments and Philips, were already working on the idea. What JTAG did was to convert the ideas into an international Standard, the IEEE 1149.1-1990 Standard, first published in April 1990.




AB

Any Digital Chip

Any Digital ChipAny Digital Chip

Any Digital Chip

Using The Boundary-Scan Path

TDI

TCK

TMS

TDO

Input Scan Cell:Sensor (RX)

Output Scan Cell:Driver (TX)

At the device level, the boundary-scan elements contribute nothing to the functionality of the internal logic. In fact, the boundary-scan path is independent of the function of the device. The value of the scan path is at the board level as shown in the diagram aboveThe figure shows a board containing four boundary-scan devices. Notice that there is an edge-connector input called TDI connected to the TDI of the first device. TDO from the first device is permanently connected to TDI of the second device, and so on, creating a global scan path terminating at the edge connector output called TDO. TCK is connected in parallel to each device TCK input. TMS is connected in parallel to each device TMS input.




AB What The Tester Sees

TDI

TCK

TMS

TDO

What the tester sees from the edge connector is simply the concatenation of the various boundary-scan registers – that is, a single register that provides access to all device outputs now considered to be drivers (sometimes called a transmitter) onto an interconnect) and all device inputs (now considered to be sensors (sometimes called a receiver) from the interconnect – see diagram above.

In this way, particular tests can be applied across the device interconnects via the global scan path by loading the stimulus values into the appropriate device-output scan cells via the edge connector TDI (shift-in operation), applying the stimulus across the interconnects (update operation), capturing the responses at device-input scan cells (capture operation), and shifting the response values out to the edge connector TDO (shift-out operation).

Essentially, boundary-scan cells can be thought of as virtual nails, having an ability to set up and apply tests across the interconnect structures on the board.




AB Even Simpler

TDI

Driver (TX) boundary-scan cells

TDO

Sensor (RX) boundary-scan cells

Question: how many tests to catch all opens (modelled by the s-a-1/s-a-0 model), and all 2-net shorts (modelled by the wired-AND/wired-OR model)?

Question: how many tests to catch all opens (modelled by the s-a-1/s-a-0 model), and all 2-net shorts (modelled by the wired-AND/wired-OR model)?

20 interconnects

We will answer the question later in the course.




AB Basic Boundary-Scan Cell (BC_1)

D

Clk

Q D

Clk

Q01

01

01

01

Data In(PI)

Scan In(SI)

ShiftDR ClockDR UpdateDR

Mode

Data Out(PO)

Scan Out(SO)

CaptureScan Cell

UpdateHold Cell

CS U

= 0, Functional mode= 1, Test mode(for BC_1)

The figure shows a basic universal boundary-scan cell, known as a BC_1. The cell has four modes of operation: normal, update, capture, and serial shift. The memory elements are two D-type flip-flops with front-end and back-end multiplexing of data. (As with all circuits in this tutorial, it is important to note that the circuit shown in the Figure is only an example of how the requirement defined in the Standard could be realized. The IEEE 1149.1 Standard does not mandate the design of the circuit, only its functional specification.)

During normal mode, Data_In is passed straight through to Data_Out.

During update mode, the content of the Update Hold cell is passed through to Data_Out.

During capture mode, the Data_In signal is routed to the input Capture Scan cell and the value is captured by the next ClockDR. ClockDR is a derivative of TCK.

During shift mode, the Scan_Out of one Capture Scan cell is passed to the Scan_In of the next Capture Scan cell via a hard-wired path.

Note that both capture and shift operations do not interfere with the normal passing of data from the parallel-in terminal to the parallel-out terminal. This allows on the flycapture of operational values and the shifting out of these values for inspection without interference. This application of the boundary-scan register has tremendous potential for real-time monitoring of the operational status of a system — a sort of electronic camera taking snapshots — and is one reason why TCK is kept separate from any system clocks.




AB Summary

Boundary scan targets manufacturing defects around the boundary of a device and between the interconnects between devices caused by

Electrical stress: electro-static dischargeMechanical stress: bent pins (causing opens and shorts), mis-oriented positioning, missing devicesThermal stress: solder-to-solder shorts, open-circuit bond wires

The region targeted (driver cell - driver amplifier - pad - bond wire - leg - solder - interconnect - solder - leg - bond wire -pad - sensor amplifier – sensor cell) is the region most likely to be damaged during board assembly

Given that boundary-scan registers were seen as an alternative way of testing for the presence of manufacturing defects, we should question what these defects are, what causes them, and where they occur.

An examination of the root cause for board manufacturing defects shows them to be caused by any one of three reasons: electrical stress (e.g., electrostatic discharge causing damage to input or output amplifiers), mechanical stress (e.g., bent legs caused by clumsy handling when mounting devices on the board), or thermal stress (e.g., hot spots caused by the solder operation). A defect, if it occurs, is likely present either in the periphery of the device (leg, bond wire, driver amplifier), in the solder, or in the interconnect between devices. It is very unusual to find damage to the internal logic without some associated damage to the periphery of the device.

In this respect, the boundary-scan cells are precisely where we want them — at the beginning and ends of the region most likely to be damaged during board assembly i.e. the region between the output driver scan cell and the input sensor scan cell. This region is more-commonly referred to as the interconnect region.




AB

In this mode (EXternal TEST), defects covered: driver (TX) scan cell - driver amp - bond wire - leg - solder -

interconnect- solder - leg - bond wire - sensor amp - sensor (RX) scan cell

In this mode (EXternal TEST), defects covered: driver (TX) scan cell - driver amp - bond wire - leg - solder -

interconnect- solder - leg - bond wire - sensor amp - sensor (RX) scan cell

Defect Coverage: ExtestDriver Sensor

Virtualnails

Using the boundary-scan cells to test the interconnect structure between two devices is called External Test, shortened to Extest – see diagram above. The use of the cells for Extest is the major application of boundary-scan structures, searching for opens and shorts plus damage to the periphery of the device. In this mode, the boundary-scan cells are often referred to as virtual nails




AB

In this mode (INternal TEST), defects covered: driver scan cell - device - sensor scan cell

In this mode (INternal TEST), defects covered: driver scan cell - device - sensor scan cell

Defect Coverage: IntestDriver Sensor Driver Sensor

It is also possible to use boundary-scan cells to test the internal functionality of a device. This use of the boundary-scan register is called Internal Test, shortened to Intest. Intest is only really used for very limited testing of the internal functionality to identify defects such as the wrong variant of a device, or to detect some gross internal defect.

In the next section, we will take a closer look at the overall architecture of an 1149.1-compliant device and, particularly, the function of the Instruction register.




AB 1149.1 Chip Architecture

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip

Identification Register

Boundary-Scan Register

Instruction Register

TAPController

1

1

1

Internal Register

After nearly five year’s discussion, the JTAG organization finally proposed the device architecture shown above

The Figure shows the following elements:

• A set of four dedicated test pins — Test Data In (TDI), Test Mode Select (TMS), Test Clock (TCK), Test Data Out (TDO) — and one optional test pin Test Reset (TRST*). These pins are collectively referred to as the Test Access Port (TAP).

• A boundary-scan cell on each device primary input and primary output pin, connected internally to form a serial boundary-scan register (Boundary Scan).

• A finite-state machine TAP controller with inputs TCK, TMS, and TRST*.• An n-bit (n >= 2) Instruction register, holding the current instruction.• A 1-bit Bypass register (Bypass).• An optional 32-bit Identification register capable of being loaded with a permanent

device identification code.

At any time, only one Data register can be connected between TDI and TDO e.g., the Instruction register, Bypass, Boundary-Scan, Identification, or even some appropriate register internal to the device. The selected Data register is identified by the decoded parallel outputs of the Instruction register. Certain instructions are mandatory, such as Extest (boundary-scan register selected), whereas others are optional, such as the Idcode instruction (Identification register selected).




AB Mandatory Instructions and Reset Modes

IR ≥ 2Reset: TMS = 1, 5 x TCK

IR ≥ 2Reset: TMS = 1, 5 x TCK

UndefinedBoundary ScanSample

UndefinedBoundary ScanPreload

All-1sBypassBypass

Formerly All-0sBoundary ScanExtest

CodeTarget (Active) Register

Instruction

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

Internal Register

1

1

1

Before we look closer at each part of this architecture there are three general points to note:

Point 1. Since 1149.1-2001, the latest version of the Standard, there are only four mandatory instructions: Extest, Bypass, Sample and Preload. The behaviour of these instructions is explained in a little while but the fact that there are only four mandatory instructions dictates the minimum size of the Instruction register to be 2-bits long. The maximum size is left undefined in the Standard. One of the instructions – Bypass –takes a mandatory codes of all-1s. Extest used to take a mandatory code of all-0s but this requirement has now been relaxed in the 2001 version of the Standard. The codes for all other instruction are not defined by the Standard and are left to the device designer to define.

Point 2. Until the publication of 1149.1-2001, the Sample and Preload instructions were merged into a single Sample/Preload instruction, often abbreviated to just Sample. It is now possible to allocate a different code to Sample compared to Preload and consider them to be two completely separate instructions. It is also possible to allocate the same code to each instruction and consider them still to be a single merged Sample/Preloadinstruction.

Point 3. The asynchronous Reset signal, TRST*, is optional. If present, the signal is active low. If not present, there is always a synchronous reset available within the TAP controller. If TMS is held at logic 1, a maximum of five consecutive TCKs is guaranteed to return the TAP controller to the reset state of Test_Logic Reset. This will be referred to as the TMS = 1, 5 x TCK synchronous reset.




AB Target Register Modes

Capture

Shift

Update

Select

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

Internal Register

1

1

1

Whenever a register is selected to become active between TDI and TDO, it is always possible to perform three operations on the register: parallel Capture followed by serial Shift followed by parallel Update. The order of these operations is fixed by the state-sequencing design of the TAP controller. For some target Data registers, some of these operations will be effectively null operations, no ops.




AB Loading The Instruction Registers

Problem: Set device 1 in Bypass, devices 2 and 3 in Extest ready for interconnect test between 2 and 3.Step 1: Using TMS and TCK, select IRs as active registers in all devices. Step 2: Load Bypass code into 1 (all-1s); Extest code into 2 and 3 (assumed to be all-0s)

1 2 3

TDI

TDO11111111 00000000 00000000

TMS

TCKTRST*

Before proceeding with a description of other parts of the architecture, we will examine how to load the Instruction register and decode its contents. Consider the board circuit shown above.

Assume that what we want to do is to place Chip 1 into Bypass mode (to shorten the time it takes to get test stimulus to follow-on devices) and place chips 2 and 3 into Extestmode preparatory to setting up tests to check the four interconnects between Chips 2 and 3. This set-up requires loading the Bypass instruction (all-1s) into the Instruction register of chip 1, and the Extest instruction (assumed to be all-0s) into the Instruction registers of Chips 2 and 3.

Step 1 is to connect the Instruction registers of all three devices between their respective TDI and TDO pins. This is achieved by a special sequence of values on the board control line TMS going to each TAP controller in each device. Note that the TMS (and TCK) lines are connected to all devices in parallel. Any sequence of values on TMS will be interpreted in the same way by each device TAP controller. Later, we will see the precise TMS sequence to select the Instruction register between TDI and TDO. For now, we will assume that such a sequence exists.

Step 2 is to load the appropriate instructions into the various Instruction registers via the global connection of Instruction registers. If we assume simple two-bit Instruction registers per device, this operation amounts to a 6 x TCK serial load of the sequence 110000 into the edge-connector TDO entry to place 00 in the Instruction registers of Chips 2 and 3, and 11 in the Instruction register of Chip 1. The Instruction registers are now set up with the correct instructions loaded in their serial scan sections




AB Interconnect Test Setup

Step 3: decode and execute new instructions. New target registers are selectedDevices now set up to apply interconnect tests between devices 2 and 3

1 2 3

TDO

TDI

TMS

TCKTRST*

Step 3 is to continue with values on TMS to cause each TAP controller to issue the control-signal values to transfer the instruction codes in the scan sections of the Instruction registers to the hold sections where they become the current instruction. This is the Update operation. At this point, the various instructions are executed — that is, Chip 1 deselects the Instruction register and selects the Bypass register between its TDI and TDO (Bypass instruction), and Chips 2 and 3 deselect their Instruction registers and select their Boundary-Scan registers between their TDI and TDO (Extest instruction). Devices 2 and 3 are now set up ready for Extest operation.




AB Open-Circuit TDI, TMS and TRST*?

An open-circuit TDI, TMS or TRST* must go to logic-1. Why?

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

Internal Register

1

1

1

The 1149.1 Standard mandates that an open circuit TDI, TMS or TRST* input must go to logic 1. This can be achieved with internal weak resistive pull ups, or with active transistor pull ups. The reasons are explained in subsequent slides.




AB TDI Open-Circuit Behaviour

Open-circuit TDI. A safe instruction (Bypass, all-1s code) is loaded and executed into the device with the open-circuit TDI and to all devices downstream of this device.

11111111 00000000 11111111

1 2 3

TDO

TDI

TMS

TCKTRST*

For TDI. If the Instruction register is selected as the target register between TDI and TDO ready to be loaded with a new instruction, then a safe instruction (Bypass, all-1s code) is loaded and executed into the device with the open-circuit TDI and to all devices downstream of this device.




AB TMS Open-Circuit Behaviour

Open-circuit TMS. TMS floats high. In a maximum of 5 x TCK cycles, the TAP controller of this device will be placed into its Test_Logic Reset state.

1 2 3

TDO

TDI

TMS

TCKTRST*

For TMS. In a maximum of 5 x TCK cycles, the TAP controller of this device will be placed into its Test_Logic Reset state. In this state, the boundary-scan logic is inactive but the device can continue to operate functionally.




AB TRST* Open-Circuit Behaviour

Open circuit TRST*. TRST* floats at inactive level (logic-1). Reset reverts back to the synchronous reset cycle (TMS = 1, 5 x TCK) rather than through its asynchronous TRST* signal.

1 2 3

TDO

TDI

TMS

TCKTRST*

For TRST*, logic 1 is the inactive state and so the device is not prevented from being used either in functional mode or in 1149.1 modes. The device must be reset with the synchronous reset cycle (TMS = 1, 5 x TCK) rather than through its asynchronous TRST* signal.




AB Warning

In the following slides, any circuit diagrams are for illustrative purposes only. The Standard mandates what should happen but not how to implement.




AB Instruction Register

Scan Register (Scan-in new instruction/scan-out capture bits)


Decode Logic, If any.Decode Logic, If any.

Hold register(Holds current instruction)


10Higher order bits:current instruction, status bits, informal identification, results of a power-up self test, … (recommended to define)

TAPController

TAPController IR Control

FromTDI

ToTDO

DR select and control signals routed to selected target register

An Instruction register has a shift scan section that can be connected between TDI and TDO, and a hold section that holds the current instruction. There may be some decoding logic beyond the hold section depending on the width of the register and the number of different instructions. The control signals to the Instruction register originate from the TAP controller and either cause a shift-in/shift-out through the Instruction register shift section, or cause the contents of the shift section to be passed across to the hold section (parallel Update operation). It is also possible to load (Capture) internal hard-wired values into the shift section of the Instruction register. The Instruction register must be at least two-bits long to allow coding of the four mandatory instructions — Extest, Bypass, Sample, Preload — but the maximum length of the Instruction register is not defined. In capture mode, the two least significant bits must capture a 01 pattern. (Note: by convention, the least-significant bit of any register connected between the device TDI and TDO pins, is always the bit closest to TDO.) The values captured into higher-order bits of the Instruction register are not defined in the Standard. One possible use of these higher-order bits is to capture an informal identification code if the optional 32-bit Identification register is not implemented. In practice, the only mandated bits for the Instruction register capture is the 01 pattern in the two least-significant bits. We will return to the value of capturing this pattern later in this section.




AB Use of The “Capture 01” Feature

Problem: how to “test the tester” i.e. chip-to-chip TDO-to-TDI, plus TMS and TCK connections

… 01 … 01 … 01

1 2 3

TDO

TDI

TMS

TCKTRST*

In an earlier section, we discussed the capture of the fixed 01 pattern into the least two significant positions of the Instruction scan register. Normally, we would think only of shift and update operations for the Instruction register. The question arises — what is the use of the capture 01 pattern?To answer this question, we need to think about the use of the boundary-scan registers at the board level. Consider again the circuit shown above.Previously, we saw how to set up a test environment preparatory to carrying out interconnect tests. To do this, we made use of the test infrastructure i.e., the on-chip boundary-scan features plus the board-level TMS and TCK connections and the chip-to-chip TDO-to-TDI interconnects. It is important to know that this infrastructure is fault-free before making use of it. In other words, we must first test the tester before using the tester to test other parts of the board. This is the purpose of the Instruction register capture 01 operation.Essentially, what happens is as follows: (see next slide also)

Step 1: Apply the sequence to TMS which causes each device to place its Instruction register between TDI and TDO. At this stage, there is a serial shift register that starts at the board TDI and ends at the board TDO and which is made up of the various Instruction registers in the devices — an Instruction register chain.Step 2: Apply an additional sequence to TMS to cause each Instruction register to capture the hardwired 01 into the least two significant positions of each Instruction scan register. Higher-order bits capture what they are set up to capture. These values are not mandated by the Standard. The captured 01 values constitute a checkerboard flush test for the serial Instruction register chain.Step 3: As the new Instructions codes are clocked into the Instruction scan registers via TDI onto the board, then so the captured 01 values will be clocked out via TDO off the board. If the sequence …01…01…01 emerges at the board TDO, then we can be reasonably sure of the following facts:




AB Use of The “Capture 01” Feature

Solution: select IRs, go to “Capture_IR” to load the hard-wired … 01 patterns, shift out … 01 patterns. Terminate with …10 sentinel bits

… 01 … 01 … 01

1 2 3

10

Sentinelbits

TDO

TDI

TMS

TCKTRST*

1. The TMS control signal is properly connected from the board’s TMS input to the TMS inputs of every device.

2. The TCK control signal is properly connected from the board’s TCK input to the TCK inputs of every device.

3. The TDO from one device is properly connected to the TDI of its logical neighbor.4. Each internal TAP controller is at least capable of responding correctly to the sequence on

TMS that causes the Instruction register both to capture and to shift.5. The TDO of the last device in the chain is properly connected to the TDO off the board6. The TDI of the first device in the chain is properly connected to the TDI onto the board. Note

that it is usual to feed the inverse values 10 into the board TDI so as to know when to terminate the shift-out phase (Step 3) and also to test this part of the chain. These bits are called the sentinel bits. They have an added benefit as they help to remove a possible cause of incorrect diagnosis if there is a TDI-to-TDO short circuit on one of the devices.

Steps 1 to 3 represent a minimum integrity test for the boundary-scan infrastructure. Additional tests can be included. For example: load and execute the Bypassinstruction into all devices to show that the Bypass registers are functioning correctly; load a non-test instruction (e.g., Preload) to select the boundary-scan register and pass a flush test through the register to check the integrity of the Boundary-Scan cells. The question that is raised is why do all these additional integrity tests? If our purpose is just to test for manufacturing defects on the test infrastructure, the Instruction register checkerboard test is probably sufficient. All additional integrity tests deal with testing the functionality of the IEEE 1149.1 features on the devices. We could argue that this is more a chip test requirement, not a board test requirement - in fact, the same argument used earlier to explain why the Intest instruction is not mandatory.Most test programmers run the extra integrity tests as time permits. These tests provide additional confidence that the test infrastructure is healthy before using it to test other parts of the board.




AB Standard InstructionsInstruction Selected Data Register

Mandatory:Bypass Bypass (initialised state, all-1s code)Sample Boundary scan (device in functional mode)Preload Boundary scan (device in functional mode)Extest Boundary scan (formerly all-0s code)

Optional:Intest Boundary scanIdcode Identification (initialised state if present)Usercode Identification (for dual personality devices)Runbist Result registerClamp Bypass (output pins in safe logic state)HighZ Bypass (output pins in high-Z state)NB. All unused instruction codes must default to Bypass

The IEEE 1149.1 Standard describes four mandatory instructions: Extest, Bypass, Sample, and Preload, and six optional instructions: Intest, Idcode, Usercode, Runbist, Clamp and HighZ. These nine instructions are known as the public instructions. We will look first at the mandatory instructions




AB Extest Instruction

Boundary-scan register selectedUsed to apply patterns to the interconnect structures on the boardBoundary-scan cells have permission to write to their outputs (device in test mode)

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

The Extest instruction selects the boundary-scan register when executed, preparatory to interconnect testing. The code for Extest used to be defined to be the all-0s code. This requirement has been relaxed in the 2001 version of the Standard.

Extest puts the device in test mode – that is, the boundary-scan cells on the input side of the device have permission to write logic values into the device, and the boundary-scan cells on the output side of the device have permission to write logic values onto the output pins of the device. This means that there should be no risk of the device being placed into its boundary-scan test mode of operation by means of the accidental loading and execution of the Extest instruction. If this happened, then either this device or other devices connected to this device might be placed into an unsafe state. This is the reason why the requirement for the Extest instruction code be all-0s has been relaxed in 1149.1-2001. A stuck-at-0 short to Ground on a TDI input pin could load an all-0s code into the Instruction register causing the device to be placed in an unsafe mode when executed.




AB Bypass Instruction

Bypass register selectedUsed to allow quick passage through this device to another device connected in the chain

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

The Bypass instruction must be assigned an all-1s code and when executed, causes the Bypass register to be placed between the TDI and TDO pins. By definition, the initialized state of the hold section of the Instruction register should contain the code for the Bypassinstruction unless the optional Identification register has been implemented, in which case, the code for the Idcode instruction should be present in the hold section.

Note: all unused instruction codes should be interpreted as the Bypass instruction




AB Sample and Preload Instructions

Boundary scan register selectedPreload: used to preload known values in the boundary scan cells.Sample: used to capture mission-mode signals into the boundary-scan cellsDevice in functional mode, not test mode

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

The Sample and Preload instructions, and their predecessor the Sample/Preloadinstruction, selects the Boundary-Scan register when executed. The instruction sets up the boundary-scan cells either to sample (capture) values or to preload known values into the boundary-scan cells prior to some follow-on operation. The codes for the Sample and Preload instructions are not defined. The codes may either be the same or they may be different.

Both instructions leave the device in functional mode, not test mode. This means that any values preloaded or sampled into the boundary scan cells are not passed through into the device or to the device output pins, unlike Extest. Thus the device is still under the control of the mission mode signals i.e. is in functional mode. This is an important distinction between Preload and Sample, compared to Extest.

Note that the 1149.1-2001 Sample instruction now captures values coming into the chip on signal Input pins and values being generated by the chip logic and passing through tothe signal Output pins. Previous versions of the Standard did not allow capture of values generated by the chip logic and passing through to the signal Output pins.




AB Intest Instruction

Boundary scan register selectedUsed to apply patterns to the device itselfBoundary scan cells have permission to write to their outputs (device in test mode)

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

The IEEE 1149.1 Standard defines a number of optional instructions – that is, instructions that do not need to be implemented but which have a prescribed operation if they are implemented.

The instruction illustrated here is Intest, the instruction that selects the boundary-scan register preparatory to applying tests to the internal logic of the device.




AB Idcode Instruction

Optional Identification register selected, if available, else Bypass register selectedUsed to capture internal 32-bit identification code (manufacturer, part number, version number) and then shift out through TDO

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

Idcode is the instruction to select the Identification register between TDI and TDO, preparatory to loading the internally-held 32-bit identification code and reading it out through TDO. Note that if the Idcode instruction is loaded and there is no Identification register present in the device, then the Idcode instruction code must be interpreted as if it were the Bypass instruction.




AB Usercode Instruction

Optional Identification register selected, if available, else Bypass register selectedUse to capture an alternative 32-bit identification code for dual personality devices e.g. PLDs

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

Usercode selects the same 32-bit register as Idcode, but allows an alternative 32 bits of identity data to be loaded and serially shifted out. This instruction is used for dual-personality devices, such as Complex Programmable Logic Devices and Field Programmable Gate Arrays. Such devices have data pertaining to the original PLD manufacturer e.g. Altera and, once programmed, to the final user of the device,. The original manufacturer’s data is accessed though Idcode, whereas the new user identity data is accessed through Usercode..




AB RunBist Instruction

Control registers for initiating internal BIST (Memory or Logic) Pass/fail register targeted as final selected registerChip in test mode with safe values set on the output pins using Preloadinstruction

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

BIST Registers

An important optional instruction is RunBist. Because of the growing importance of internal self-test structures, the behavior of RunBist is defined in the Standard. The self-test routine must be self-initializing (i.e., no external seed values are allowed), and the execution of RunBist essentially targets a self-test result register between TDI and TDO. Once the self-test routine is initiated, the TAP controller is held in its Run_Test/Idle state for the duration of the test. The self-test clock could be TCK but is more-usually a high-speed system clock.

At the end of the self-test cycle, the targeted data register holds the Pass/Fail result. It is important that any subsequent pulses on TCK do not change this result. In this way, parallel self-tests of different lengths on different devices on the same board can be carried out. When the final (i.e., the longest in run time) self-test is complete, all results can be clocked out along the register path made up of the linked individual Pass/Failresult registers.




AB Clamp InstructionSafe logic values are pre-loaded into boundary scan cells using Preload instruction.Clamp drives these values to the output pins but leaves the Bypass register as the selected register.Note: if some outputs can be placed into a high-Z mode (3-state) or Input mode (bidirectionals), Preload is used to load the appropriate logic values in the control cells prior to execution of Clamp.

TDI TDO

TMS

TCK

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

Bypass register


Two new instructions introduced in the 1993 revision, 1149.1a-1993, were Clamp and HighZ. Clamp is an instruction that uses boundary-scan cells to drive preset values established initially with the Preload instruction onto the outputs of devices, and then selects the Bypass register between TDI and TDO (unlike the Preload instruction which leaves the device with the boundary-scan register still selected until a new instruction is executed or the device is returned to the Test_Logic Reset state: see later).

Clamp would be used to set up safe guarding values on the outputs of certain devices in order to avoid bus contention problems, for example.




AB HighZ InstructionThe device is designed such that all outputs can be placed in either a high-Z mode (3-state outputs) or input receive mode (bidirectionals).HighZ establishes these values on the output pins but leaves Bypass register as the selected registerNote: the Enable control signal to do this is supplied directly from the Instruction Register upon execution of the HighZ instruction. Preload is not used.

TDI TDO

TMS

TCK

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Register

Bypass register


HighZ is similar to Clamp, but it leaves the device output pins in a high-impedance state rather than drive fixed logic-1 or logic-0 values. HighZ also selects the Bypass register between TDI and TDO.

Note that HighZ does not require an a priori Preload activity. The ability to drive all outputs to a high-Z (3-state outputs) or input-only mode (bidirectionals) is an inherent feature of the design and must be designed in by the designer.




AB Accessing Other Internal Registers

Any internal register can be accessed by means of “private” instructionsAn example could be access to internal scan registers through an instruction called InScanTDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Scan Registers

With the exception of Bypass (and previously Extest), the codes for all instructions are undefined in the Standard. Given the need for four mandatory instructions, the minimum length of the Instruction register is two bits. The maximum length is undefined. Any instruction can have more than one code and all unused codes are interpreted as Bypass.

Note that the designer may use certain codes to implement private instructions to access any suitable internal data registers — that is, instructions that are not defined in the Standard and whose detailed functions are not made public. An example could be an instruction InScan to allow access to an internal scan-path register via the TDI-TDO route. In these circumstances, the designer must state that these codes are private so that the board test-programmer can avoid loading the codes.




AB Test Access Port (TAP)

Test Reset Optional async reset(TRST*) Active low

Default = 1

Test Data In Serial data in(TDI) Sampled on rising edge

Default = 1

Test Data Out Serial data out(TDO) Changes on falling edge

Default = Z (only activeduring a shift operation)

Test Mode Select Input Control(TMS) Sampled on rising edge

Default = 1

Test Clock Dedicated clock(TCK) Any frequency

TDI TDO

TMS

TCK

Bypass register

TRST* (optional)

Any Digital Chip




TAPController

1

1

1

Internal Scan Registers

We turn now to the Test Access Port (TAP) and its Controller. The TAP consists of four mandatory terminals plus one optional terminal.

The mandatory terminals are:

• Test Data In (TDI): serial test data in with a default value of 1.• Test Data Out (TDO): serial test data out with a default value of Z and only active

during a shift operation.• Test Mode Select (TMS): serial input control signal with a default value of 1.• Test Clock (TCK): dedicated test clock, any convenient frequency (usually determined

by the maximum TCK frequency of the external tester).

The optional terminal is:

• Test Reset (TRST*): asynchronous TAP controller reset with default value of 1 and active low.




AB TAP Controller

16-state FSMTAP Controller

(Moore machine)

16-state FSMTAP Controller

(Moore machine)

ClockDRShiftDR

UpdateDR

Reset*Select

ClockIR

ShiftIR

UpdateIR

Enable

TMS

TCK

TRST*

TMS and TCK (and the optional TRST*) go to a 16-state finite-state machine controller, which produces the various control signals. These signals include dedicated signals to the Instruction register (ClockIR, ShiftIR, UpdateIR) and generic signals to all data registers (ClockDR, ShiftDR, UpdateDR). The data register that actually responds is the one enabled by the conditional control signals generated at the parallel outputs of the Instruction register, according to the particular instruction.

The other signals, Reset, Select and Enable are distributed as follows:

• Reset is distributed to the Instruction register and to the target Data Register• Select is distributed to the output multiplexer• Enable is distributed to the output driver amplifier

Note: the Standard uses the term Data Register to mean any target register except the Instruction register




AB Distribution of Signals

TAPController

IR (Shift Section)

IR (Hold Section)

TDI

TCK

TMS

TRST*

TDO

Reset ShiftIRClockIRUpdateIR

Select

ShiftDRClockDRUpdateDRReset

Enable

To Data Registers (DRs)

IR (Decoder) FromDRs

The slide shows how the various signals generated by the TAP controller are distributed to the Instruction register, the various data registers and also how they control the selection of the Instruction register, plus ensure that TDO is inactive except when required to shift data out of the selected register.




AB TAP Controller State Diagram

SelectDR_Scan

Capture_DR

Shift_DR

Exit1_DR

Pause_DR

Exit2_DR

Update_DR

01

1

1

0

0

0

0

0

SelectIR_Scan

Capture_IR

Shift_IR

Exit1_IR

Pause_IR

Exit2_IR

Update_IR

0

1

1

1

1

1

0

0

0

0

0

Test_LogicReset

Run_Test/Idle

0

0

1

1

010

11

1 1

1

Transitions between states are controlled by the logic value on TMS (as shown in the state diagram) plus the rising edge of TCK.

The diagram shows the 16-state state table for the TAP controller. The value on the statetransition arcs is the value of TMS. A state transition occurs on the positive edge of TCK and the controller output values change on the negative edge of TCK.

The TAP controller initializes in the Test_Logic Reset state (“Asleep” state). While TMS remains a 1 (the default value), the state remains unchanged. In the Test_Logic Resetstate and the active (selected) register is determined by the contents of the Hold section of the Instruction register. The selected register is either the Identification register, if present, else the Bypass register. Pulling TMS low causes a transition to the Run_Test/Idle state (“Awake, and do nothing” state). Normally, we want to move to the Select IR_Scan state ready to load and execute a new instruction.

An additional 11 sequence on TMS will achieve this. From here, we can move through the various Capture_IR, Shift_IR, and Update_IR states as required. The last operation is the Update_IR operation and, at this point, the instruction loaded into the shift section of the Instruction register is transferred to the Hold section of the Instruction register to become the new current instruction. This causes the Instruction register to be de-selected as the register connected between TDI and TDO and the Data register identified by the new current instruction to be selected as the new target Data register between TDI and TDO. For example, if the instruction is Bypass, the Bypass register becomes the selected data register. From now on, we can manipulate the target data register with the generic Capture_DR, Shift_DR, and Update_DR control signals.




AB Initialising The TAP Controller

SelectDR_Scan

Capture_DR

Shift_DR

Exit1_DR

Pause_DR

Exit2_DR

Update_DR

01

1

1

0

0

0

0

0

SelectIR_Scan

Capture_IR

Shift_IR

Exit1_IR

Pause_IR

Exit2_IR

Update_IR

0

1

1

1

1

1

0

0

0

0

0

Test_LogicReset

Run_Test/Idle

0

0

1

1

010

11

1 1

1

Note that there is no master reset to the TAP controller if the optional TRST* is not implemented. The TAP controller is mandated to power up in the Test_Logic Resetstate. If there is a need to re-initialize the controller, it can be done by observing the following properties of the controller state diagram.

In general, TMS = 1 causes a state-to-state transition to be made when the positive edge of TCK occurs. The exception is the Test_Logic Reset state. This state is one of six stable states i.e. a state that can be maintained for consecutive TCK clock pulses as long as TMS is constant. The other five stable states are Run_Test/Idle, Shift_DR, Pause_DR, Shift_IR and Pause_IR. For these five stable states, TMS = 0 will maintain the state for consecutive TCK clock pulses.

Consequently, no matter what state the TAP controller is in, if TMS is held at logic 1, the next five consecutive TCK clocks are guaranteed to drive the controller to its Test_LogicReset state. Note that five is the maximum number of TCKs required to do this. Depending on the start state, the Test_Logic Reset state may be reached in less than five TCKs. The student is invited to verify that from any start state, five TCKs is sufficient to return the controller to the Test_Logic Reset state, given that TMS remains at logic 1.

In this way, the TAP controller can always be initialized using a TMS = 1, 5 x TCKprotocol – the synchronous reset referred to earlier in the tutorial.




AB TAP Controller: Output Signals

The diagram above, taken straight from the Standard, illustrates the details of the control signals generated by each of the sixteen states. In this diagram, the first two rows show TCK and TMS respectively. The third row shows the transmission status of the device TDO output: active or high impedance. The fourth row defines the sixteen states in a hexadecimal format. The allocation of the hexadecimal identifier is shown in the next slide. The remaining rows show the values on the various output control signals that are distributed to the Instruction register and to the selected Data register.




AB TAP Controller: States

SelectDR_Scan

Capture_DR

Shift_DR

Exit1_DR

Pause_DR

Exit2_DR

Update_DR

01

1

1

0

0

0

0

0

SelectIR_Scan

Capture_IR

Shift_IR

Exit1_IR

Pause_IR

Exit2_IR

Update_IR

0

1

1

1

1

1

0

0

0

0

0

Test_LogicReset

Run_Test/Idle

0

0

1

1

010

11

1 1

1

0

5

4

3

2

1

C

B

A

9

8

7

6

D

F

E

The allocation of the sixteen state hexadecimal identifiers are shown in this figure.




AB

Assume 30 devices at 100-bits/device

= 3000-bit BScan Register

RAM

RAM

RAM

RAM

1024 bits

1024 bits

Driver

Sensor

A Reason for the Pause State

TDI

TDO

Each of the main branches of the state table contains additional Exit and Pause states. The Exit1 state allows a transition from the shift operation to Update. It also allows the controller to be placed in a Pause state. This might be necessary if, for example, all devices have their boundary-scan registers selected as the data registers and an external tester driver/sensor pin channel is either loading or unloading test data e.g., as in the use of Extest to test interconnect structures. If the length of the chained boundary-scan registers is longer than the memory associated with the tester driver/sensor pin channel then it will become necessary periodically to update or unload the content of the channel memory before resuming the shift operation through the boundary-scan path. The Pausestate enables this action and Exit2 state allows a return to the shift operation. The slide illustrates this situation.

Note: it is also possible to stop the TCK clock whilst in the Shift_DR state but some global board-level boundary-scan controllers have a free-running TCK to maintain the boundary-scan devices in their Test_Logic Reset state. If this is the case, then the Pause_DR state is the only other way of temporarily suspending the shift operation.




AB Instruction Register: Default Values



Decode LogicDecode Logic

10



In the Test-Logic/Resetstate, the value forced into the Hold section of the Instruction Register is Idcode if the Identification Register is present, else Bypass

In the Test-Logic/Resetstate, the value forced into the Hold section of the Instruction Register is Idcode if the Identification Register is present, else Bypass




AB Bypass registerOne-bit shift register, selected by the Bypass instructionCaptures a hard-wired 0Note: in the Test-Logic/Reset state, the Bypass register is the default register selected between TDI and TDO if no Identification Register is present

D

Clk

Q0

From TDITo TDO

ShiftDR

ClockDR

The slide shows a typical design for a Bypass register. It is a 1-bit register, selected by the Bypass instruction and provides a basic serial-shift function. There is no parallel output (which means that the Update_DR control has no effect on the register), but there is a defined effect with the Capture_DR control — the register captures a hard-wired value of logic 0. We will shortly explain the value of this.




AB Identification Register32-bit shift register, selected by Idcodeand Usercode instructionCaptures an internal hard-wired 32-bit wordMain function: identify device owner and part numberCan be the last 32 bits of the boundary scan register (next slide)Note: in the Test-Logic/Reset state, the Identification register is the default register selected between TDI and TDO.

The optional Identification register is a 32-bit register with capture and shift modes of operation. The register is selected by the Idcode and Usercode instructions and the 32-bits of internal data are loaded into the shift part of the register and scanned out through the device TDO pin. Recall also that this register, if present, is the selected active register when the TAP controller is in the Test_Logic Reset state, else the Bypass register is selected in this state.




AB 32-Bit Identification Format

VersionVersion Part NumberPart Number ManufacturerIdentityManufacturer

Identity

4-bitsAny format

16-bitsAny format

11-bitsCoded formof JEDEC

D

Clk

QID Code bit

Shift inShift out

ShiftDR

ClockDR

TDI TDOLSBLSB

1

When the register is selected, the Capture_DR state captures 32 bits of internal data. The captured 32 bits identify the device through the following four fields:

• Bit 0 (least significant bit) is always a logic 1.• Bits 1 - 11 identify the manufacturer or “owner” of the device using a compact form of

the JEDEC identification code – see next slide for more detail on how JEDEC allocates their numbers.

• Bits 12 - 27 provide a 16-bit free-format part number field.• Bits 28 - 31 provide a 4-bit free-format field to specify up to 16 different versions of the

same basic device.

Once captured, the 32-bit identification code can be shifted out through TDO for inspection off the board. The diagram also shows a possible implementation of one cell in the 32-bit register.




AB JEDEC Coding

Bank 07 bits + Parity






Each manufacturer receives a unique JEDEC numberAll-1s codes are used as continuation codes from one bank to the next i.e. not allocated to any company# Codes = 16 (27 - 1) = 2032 (Bank 6, May 2006) www.jedec.orgSearch for JEP106T

www.jedec.orgSearch for JEP106T

The Joint Electron Device Engineering Council, JEDEC, is an organization in Arlington, Virginia, that allocates unique identification codes to electronic semiconductor manufacturers and to system companies. The codes are based on a 7-bit number, plus odd-1 parity bit, and placed in one of sixteen “banks”. The 11-bit code in the Identification register is built from the 7-bit number plus 4-bits to identify in which bank the number is located. The parity bit is discarded. Note that JEDEC does not use the last (all-1s) binary numbers in any bank.

We will now investigate why the least significant bit (lsb) of the Identification register is a 1 and why the Bypass register captures a hard-wired value of 0




AB TAP Controller State Diagram

SelectDR_Scan

Capture_DR

Shift_DR

Exit1_DR

Pause_DR

Exit2_DR

Update_DR

01

1

1

0

0

0

0

0

SelectIR_Scan

Capture_IR

Shift_IR

Exit1_IR

Pause_IR

Exit2_IR

Update_IR

0

1

1

1

1

1

0

0

0

0

0

Test_LogicReset

Run_Test/Idle

0

0

1

1

010

11

1 1

1

This slide is required to follow the reasoning in the next slide.




AB Blind Interrogation

TDI

TMS

TCK

TDO

1 2 3

Problem: determine how many devices on the board have boundary scan; where they are in the chain; and identify those with Identification RegistersAll this from the edge connector!! (Blind interrogation)

Consider the following field servicing scenario. A customer’s computer system has broken down. The cause is suspected to be a hardware fault on a particular board. There are many variations of the board and the service engineer needs to identify the board type and the component versions. All the engineer knows is that there are boundary-scan components on the board and the location of the primary (edge-connector) TDI entry, TMS, TCK and TDO exit ports plus Power and Ground. The following procedure, known as a “blind interrogation”, identifies the boundary-scan components on the board and whether or not they have Identification registers.




AB

1

23

4

5

Blind Interrogation – Seven QuestionsQ1: Do on-board devices meet the 1149.1 Standard?

Q2: Simple board TMS and TCK infrastructure?

Q3: TDO off the board?

Q4: TMS onto the board?

Q5: TCK onto the board?

Q6: Power (pin and value)?

Q7: Ground (pin)?TDOTMSTCKGND 3.5v




AB How It Works

1

23

4

5

Contain Identification

registers

0

0

31+1

1+31 0

TDOTMSTCKGND 3.5v

SelectDR_Scan

Capture_DR

Shift_DR

Exit1_DR

Pause_DR

Exit2_DR

Update_DR

01

1

1

0

0

0

0

0

SelectIR_Scan

Capture_IR

Shift_IR

Exit1_IR

Pause_IR

Exit2_IR

Update_IR

0

1

1

1

1

1

0

0

0

0

0

Test_LogicReset

Run_Test/Idle

0

0

1

1

010

11

1 1

1

1 + 1111111 = end of chain

Logic 1

0 = 1149.1 device, no identification

1 = 1149.1 device, with 31-bits identification

Apply power to the boardApply TMS = 1, 5 x TCK to enter TLR state

Step 1: Power up the board and apply the 010sequence to TMS to enter the Select DR_Scan state. By default, the instruction loaded into the hold stage of every boundary-scan Instruction register on power-up must be Idcode if the device contains an Identification register, or Bypass if the device does not contain an Identification register. This is mandated by the Standard and is shown above.. Devices 2 and 4 have Identification registers; devices 1, 3 and 5 do not.

Step 2: Capture the hard-wired values (Capture_DR state) in the default selected Bypass or Identification register. Bypass captures 0; Identification captures 1 + 31 bits of interest.

Step 3: Shift (Shift_DR state) the captured values out through the primary TDO exit output. A leading 0 identifies a device without an Identification register. A leading 1identifies a device with an Identification register, in which case the next 31 bits identifies the device JEDEC code, part number and version number.

In the situation of a true “blind” interrogation (i.e., one in which it is not known how many boundary-scan devices there are on the board), the process can be terminated by feeding in an illegal sequence through the primary TDI entry input and waiting for this sequence to appear at the primary output TDO. Such a sequence is seven consecutive 1s. The JEDEC coding system avoids this sequence and it should never appear naturally from the content of Identification registers. It is usual to add a further 0 to this sequence just in case the primary TDI entry input is stuck-at-1.




AB Boundary-Scan Register

Shift register with boundary-scan cells on:

device input pinsdevice output pinscontrol of three-state outputscontrol of bidirectional cells

Selected by the Extest, Intest, Preload and Sample instructions

We are now ready to take a more detailed look at the boundary-scan cells and their concatenation into a general-purpose boundary-scan register. For a given device, boundary-scan cells are placed on the device digital input ports, digital output ports, and on the control lines of bidirectional (IO) ports and tristate (0Z) ports. The scan cells are linked together to form the boundary-scan register. The order of linking within the device is determined by the physical adjacency of the pins and/or by other layout constraints. The boundary-scan register is selected by the Extest, Sample, Preload, and Intestinstructions.




AB Full-Featured BS Cell (BC_1)

D

Clk

Q D

Clk

Q01

01

01

01

Data In(PI)

Scan In(SI)


Mode

Data Out(PO)

Scan Out(SO)

CaptureScan Cell

UpdateHold Cell

The figure shows a more universal design for a boundary-scan cell – a BC_1. The BC_1is capable of all three operations of capture, shift, and update, and is suitable as a cell on the device inputs or device outputs. This design has separate flip-flops for capture-and-shift and for hold functions. Data can be shifted through the boundary-scan shift path without interfering with the value in the hold section (which could be routed to the data-out port through the output multiplexer).




AB Full-Featured BS Cell (BC_2)

D

Clk

Q D

Clk

Q01

01

0

1

0

1

Data In(PI)

Scan In(SI)


Mode

Data Out(PO)

Scan Out(SO)

CaptureScan Cell

UpdateHold Cell

A simple variation of the BC_1 is shown above. In this design, known as a BC_2, the captured signal is taken downstream of the output multiplexer. This design has the advantage of being able to sense the signal passing to the output pin (if the cell is connected directly to a digital output pin of the device) or the signal going into the internal logic (if the cell is connected directly to a digital input to the device). In this way, if the device output or input is itself faulty, the cell can capture the faulty value, thereby increasing the overall defect coverage.




AB Reduced-Feature Input Cell (BC_4)

D

Clk

Q01

01

Data In(PI)

Scan In(SI)

ShiftDR ClockDR

Data Out(PO)

Scan Out(SO)

CaptureScan Cell

Only Capture and Shift: no UpdateNote: mandatory instructions do not require Update on input pinsUsed on timing-sensitive inputs e.g. Clock

There are many different designs for boundary-scan cells. The slide shows a simple design, known as a BC_4, capable only of capture and shift operations. Such a cell could be used on device inputs that are especially sensitive to extra loading on the Data_Insignal e.g., a high-speed system clock. (Note: the four mandatory instructions do not require an update operation on the input scan cells.)




AB Providing Boundary-Scan Cells: OZ

On all device signal IO, control of three-state:dual-mode input signal or additional scan cell

Primarily, boundary-scan cells must be provided on all device digital input and digital output signal pins, with the exception of Power and Ground. Note that there must be no circuitry between the pin and the boundary-scan cell other than driver amplifiers or other forms of analog circuitry such as electro-static discharge protection circuitry.

In the case of pin fan-in, boundary-scan cells should be provided on each primary input to the internal logic. In this way, each input can be set up with an independent value. This provides the maximum flexibility for Intest.

Similarly, for the case of pin fan-out: if each output pin has a boundary-scan cell, then so Extest is able to set different and independent values on the multiple outputs.

Where there are OZ tristate output pins, there must be a boundary-scan cell on the status control signal into the output driver amplifier. The diagram shows this situation. The input pin has two modes: as an input and as an output status-control signal. For this situation, an extra boundary-scan cell is not required.




AB Providing Boundary-Scan Cells: OZ

On all device signal IO, control of three-state:dual-mode input signal or additional scan cell

For some devices, the control signal for tristate outputs may be generated internally. If this is so, an extra control boundary-scan cell is inserted in the boundary-scan register with the sole purpose of allowing boundary-scan control of the status of the outputs. The slide shows this extra scan cell. Usually, just one control cell controls all tristate outputs simultaneously, not one control cell per tristate output.




AB Providing Boundary-Scan Cells: IO

On control of bidirectional IO:dual-mode input signal

Special cells:BC_7, BC_8, BC_9

This slide shows the set up for a bidirectional IO pin. Here, we see that, conceptually at least, three boundary-scan cells are required: one on the input side, one on the output side, and one to allow control of the IO status. In practice, the two IO scan cells are usually combined into a single multi-function cell called a BC_7.

This is the end of the part of the tutorial that has mostly concentrated on the device-level features of an 1149.1-compliant device. From now on, we will develop the board-level application of 1149.1, starting with the interconnect test-pattern generation process.




AB

BC_7 Bidirect Cell

BC_7 Boundary Scan Cell

This slide shows the set up for a bidirectional IO pin. Here,

Documents

IEEE 1149.1 Boundary-Scan Standard Part 1: Chip Levelpeople.ucalgary.ca/~smithmr/2007webs/encm415_07/07...IEEE Standard 1149.1-2001 “Test Access Port and Boundary-Scan Architecture,”