ECE 353Introduction to Microprocessor Systems

Michael G. Morrow, P.E.

Week 10

TopicsADuC7026 External Memory Interface Implementation Demultipexing

Bus Timing Bus cycle timing modification

Wait states and more

Assessing timing compatibility

Basic System Bus Operation

Address Unidirectional from CPU

Data Bidirectional

Control /RS or /RD – output from CPU

Indicates a read operation in progress /WS or /WR – output from CPU

Indicates a write operation in progress /WAIT or /READY – input to CPU

Used by external device to signal that it is not able to complete transfer yet (not present on ADuC7026)

ADuC7026 Bus OperationThe ADuC7026 external memory interface consists of 16-bit multiplexed address/data bus (AD15:0) High address bit for 8-bit operation (A16)

128kB regions require 17 bits of address Read and write strobes (/RS, /WS) Memory select signals (/MS3:0)

Internal decodes of upper 15 bits of address Byte enables (/BHE, /BLE)

Used when memory interface is operating at 16-bit width

Demultiplexing control signal (AE) Used to control logic that holds address valid to

memory system There is no WAIT/READY signal

Basic Read Cycle at Bus Level

Basic Write Cycle at Bus Level

ADuC7026 DemultiplexingMultiplexed Signal Timing Read Cycle

Dealing with a multiplexed bus Demultiplexing by the device Demultiplexing logic to create an

address bus

Implementation Devices Connections AE timing

16-Bit Memory System

SRAM Timing CompatibilityIn order to properly read and write the device, we need to ensure that the processor-to-memory interface is compatible with the memory device.This is accomplished by analyzing the timing for all relevant parameters, and ensuring that the operation can be completed successfully.We will work through the read cycle analysis for the ADuC7026...

Assessing Timing Compatibility

Need to know whether CPU could operate with the tAA for given device. We designate a CPU characteristic tAVDV, which is the delay from When the address becomes valid at the

CPU Until the data must be driven back to

CPUThis establishes an upper bound on tAA tAA < tAVDV

Read cycle parametersRead cycle timing control

Basic Read Cycle at Bus Level

Read Cycle Controls

System Timing Compatibility

Need to account for all delays in a system to assess timing compatibility.Consider this system.Analyze the read timing with regard to: tAA – address access time tACS – chip enable to valid data tOE – output enable to valid data tDF – output hold/float time

tAA – address access time

tACS – chip enable to valid data

tOE – output enable to valid data

tDF – output hold/float time

ADuC7026 External Memory Interface Configuration

The external memory interface supports four independently configured memory regions, each of which is 128kB in size.In order to use the external memory interface, we need to Configure the required pins (GPxCON) Enable the external interface (XMCFG[0]

= 1) Region enable and bus width (XMxCON) Configure region for the desired bus

timing (XMxPAR)

ADuC7026 XMxCONThe XMxCON registers configure the bus width and enable the interface for the respective memory region. Only D[1:0] are used. XM0CON 0x10000000-0x1001FFFF XM1CON 0x20000000-0x2001FFFF XM2CON 0x30000000-0x3001FFFF XM3CON 0x40000000-0x4001FFFF

ADuC7026 XMxPARThe XMxPAR MMR configures the bus timing for a region 0x70FF at reset

[14:12] – AE extend[9] – implements bus turn-around[8] – provides additional hold time [7:4],[3:0] – extend write/read strobes

Write cycle timing controlRead cycle timing control

System Timing Compatibility

Consider again the system.Analyzing write cycle timing.SRAM write characteristics tWC

tAS, tAW, tCW

tWR tWDS, tWDH

Write cycle controls

Timing Wrap-UpDevice characteristics are just part of the total timing analysis picture Line/device capacitive loading and driver

slew rates Transmission line effects and parasitic

reactances Impedance mismatch and reflections Skew and physical/electrical trace length

mismatch Signal integrity

Ensuring that signals are correct in spite of all of the above issues and mutual coupling effects

Wrapping UpHave a great Spring Break!Complete Pre-Quiz #5 by the start of class on Monday, April 9th

Homework #5 will be due on Wednesday, April 11th

Reading for next week (interrupts and exceptions) Textbook chapter 10 ADuC 74-75 ARM7 2.8-2.10

Basic Read Cycle

Basic Write Cycle

Read Cycle Parameters

Write Cycle Parameters

Read Cycle Controls

Write Cycle Controls

16-Bit Memory System

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