I/O Interfacing

The two methods of data transfer are:

· Serial I/O : One bit is transferred using one data line

· Parallel I/O : Data can enter (or exit) in groups of eight bits using the entire data bus.

Thus the I/O devices (Keyboards and displays) can be interfaced using two methods namely:

· Peripheral-mapped I/O: The device is identified with an 8-bit address and enabled by I/O – related control signals.

· Memory-mapped I/O: The device is identified with a 16-bit address and enabled by memory related control signals.

Concepts Review for peripheral mapped I/O

a) When an I/O instruction is executed, 8085 places the device address (Port number) on the de multiplexed low – order and high – order address bus.

b) The address is decoded to generate the pulse corresponding to the device.

c) The device address pulse is AND ed with the appropriate control signals like IO/M, RD and WR to assert the I/O device.

d) A latch is used for an output port and a tri-state buffer is used for an input port..

e) The address bus can be decoded by using either the absolute or the linear select decoding reduces the component cost but the I/O device ends up with multiple addresses.

Memory Mapped I/O :

Here, the input and output devices are assigned and identified by 16-bit addresses. To transfer data between 8085 and I/C devices, memory – related instructions like LDA, STA etc are used. The control signals MEMR and MEMW should be connected to I/O devices.

STA 8000H; Address 2050H; stores the contents of A to 8000.

2050 32;

2051 00;

2052 80;

8085 requires 4 Machine cycles to execute STA; Instruction fetch and decode in M1; read 2051 and 2052 in M2 an M3; In M4 8085 places the entire address (8000H) on the address lines, the contents of the accumulator on data bus and generates MEMW