Lecture 13Basic I/O Interface

Presented ByDr. Shazzad Hosain

Asst. Prof. EECS, NSU

I/O Port Address Decoding

• I/O port address decoding is very similar to address decoding

• Any I/O device is addressed by a port number• Reading or writing to an I/O device is to read

or write to that port address

INS Instructions

• INSB – Input String Byte, from I/O device to memory location

Table 4-14: From Brey’s Book

Example

• Read 50 bytes of data from an I/O device whose address in 03ACH and store the data in LISTS array

Example 4-7: From Brey’s Book

OUTS Instructions

• OUTSB – Output String Byte, from string memory location to I/O device

Table 4-15: From Brey’s Book

Example

• Transfer data form memory array (ARRAY) to an I/O device at I/O address 3ACH

Example 4-8: From Brey’s Book

Decoding 8-Bit I/O Addresses• Decode 8-bit I/O ports F0H – F7H using 74ALS138 decoder

1111 0000 = F0H1111 0111 = F7H

Decoding 8-Bit I/O Addresses• Develop an I/O port decoder, using a 74ALS138, for the 8-bit I/O port

addresses 10H, 12H, 14H, 16H, 18H, 1AH, 1CH and 1EH

The Programmable Peripheral Interface (PPI)• 82C55 is a very popular low-cost

interfacing component found in many applications

• It sits between microprocessor and the I/O devices

• It has three ports– Port A– Port B– Port C

• Also has an internal command register

Why Peripheral?

I/O Devices

Interfacing 82C55• Connect 82C55 so that it functions at 8-bit I/O port addresses C0H (port A), C2H

(port B), C4H (port C), and C6H (command register).

1100 0000 = C0H1100 0010 = C2H1100 0100 = C4H1100 0110 = C6H

Interfacing 82C55• Draw the diagram for interfacing 82C55 for the following addresses 8C (Port A), 9C• (Port B), AC (Port C) and BC (command register).

CBA011

References

• Chapter 9 The Intel Microprocessors – by Barry B. Brey