ECE 353Introduction to Microprocessor Systems

Michael G. Morrow, P.E.

Week 10

Decoding ExerciseUsing only a single 74HC138 decoder, place a 32Kx8 RAM at 00000h and a 64Kx8 ROM at F0000h.Decode as exhaustively as possible.Draw memory map, show fold-back and block sizes (decoder granularity).

TopicsI/O port basicsI/O ports with MSI devicesP compatible devicesAddress decoding for isolated and memory-mapped I/OConditional I/O80C188EB integrated I/O unit82C55A PPI

I/O Port BasicsI/O subsystems allow CPU to interact with the outside worldInput, output, and combined I/O blocksInput ports Byte Word

Output ports Byte

Unconditional I/O

MSI I/O PortsMedium Scale Integration (MSI) circuits are available to construct portsSimple byte input ports can be constructed from… Octal buffers Octal latches

Simple byte output ports can be constructed from octal latches

P Compatible I/O DevicesComplex I/O devices typically require complex interface and control logicP compatible I/O devices have the necessary logic built in to the device itself Interface designed to be reasonably

compatible with many microprocessor buses Need to add decoding/selection logic Examples

Device controllers Used to control complex I/O devices (LCD, disk

drives, etc.) Generic model Example – Hitachi HD44780U LCD Controller

I/O Address DecodingI/O address decoding determines the logical location of the I/O device Isolated I/O Memory-mapped I/O

Input vs. output ports Same address does not guarantee

same function!

Device select pulsesWait statesUsing the CSU with I/O devices

I/O Address Decoding (cont.)

PAL/PLA DecodersNonspecific I/O strobes /IOW /IOR

Linear selectionConventional decoders Device select strobes Cascading

Conditional I/OConditional vs. unconditional transfersHardware examplePolling Overhead Flags / semaphores Wait loops Timeouts

Software examplePossible race condition

80C188EB Integrated I/O Unit

Port 1 FunctionsPort 2 FunctionsBidirectional pin structure Synchronizer

Programming Port Control Register Port Direction Register Port Data Latch Register Port Pin State Register

82C55A Programmable Peripheral Interface (PPI)

LSI device providing 24 bits of I/O Logical organization Block diagram

Software configurable ports Three modes of operation

Mode 0 Basic Input/Output ports

Mode 1 Strobed Input/Output

Mode 2 Bidirectional data bus

Bit set/reset capability

Real-World ExampleInterface the MAX158 8-bit, 8-channel ADC to the 80C188EB Hardware interface

Use /GCS0 at I/O address 1000h (CSU) Poll conversion status using Port 2.

P2CON / P2DIR / P2LTCH / P2PIN Software interfacing

Write a procedure that does an ADC conversion and then reads the ADC value using mode 1

Input: AL = ADC input channel to use (0-7) Output: ADC value returned in AL

What about mode 0? Timing?

Wrapping UpHomework #5 due Friday, 11/9/2001

Byte Input Port Example

Byte Output Port Example

74HC540/541

74HC573

74HC574

MAX1200

AD7865

Generic Device Controller(Fig 12.3-2)

control registers

TIMING ANDCONTROL

I/ODEVICE

A(n-1):0

D7:0

/CS

/WE

/OE

data registers

status registersCPU

CLOCK

address

data

/RD

/WR

chip select

HitachiHD44780ULCDController

Port 1 Functions

Port 2 Functions

BidirectionalPort Pin

Port Control Register

Port Direction Register

Port Data Latch Register

Port Pin State Register

Conditional I/O ExerciseWrite a procedure to read data from an input device like the hardware example. Assume that the flag is a READY signal (active high). If the device does not become ready after 1 million polling attempts, return with the carry flag set, otherwise, return with the data in AL and the carry flag cleared.

82C55A Block Diagram

82C55A Modes of Operation

82C55A Mode 1 Input

82C55A Mode 1 Output

Chip-Select Start Reg

Chip-Select Stop Register-Part 1

Chip-Select Stop Register -Part 2

Conditional I/O ExampleD7:0

INPUTDEVICE

Q1

Q2

Q3

Q4

D1

D2

D3

D4

74HC574

CLK <

OC

Q5

Q6

Q7

Q8

D5

D6

D7

D8

A13A14A15

/S2

V CC

A0

A1

A2

Y0

Y1

Y2

Y3

74HC138

E1

E2

E3

Y4

Y5

Y6

Y7

/RD

D7Q D

CLK <

PR

CL

74HC7474HC125

vcc

Synchronization