Answers
Handshaking
The making of inter relation between slower peripheral device and microprocessor is called handshaking.
Handshaking Signal
Before making the inter-relation between peripheral device and microprocessor the PPI send some signals to microprocessor and peripheral device to perform the process, these signals are called handshaking signal. 8255-based devices that perform handshaking support following handshaking signals:
Read OperationSTB
goes low indicates that data are loads into port latch.
IBF Becomes high (at high to low transition of STB) indicates that input latch contains data.
INTR Becomes high (at low to high transition of STB) uPgoes interrupt subroutine to read data.
RD becomes low.IBFbecomes low when read complete, RD becomes high and IBF goes low.
IBF low means input latch has no data (Read complete)
It consists of three 8-bit bidirectional I/O ports i.e. PORT A, PORT B and PORT C. We can assign different ports as input or output functions.
Block diagram –
It consists of 40 pins and operates in +5V regulated power supply. Port C is further divided into two 4-bit ports i.e.
Port C lower and port C upper and port C can work in either BSR (bit set rest) mode or in mode 0 of input-output mode of 8255. Port B can work in either mode or in mode 1 of input-output mode. Port A can work either in mode 0, mode 1 or mode 2 of input-output mode.
It has two control groups, control group A and control group B. Control group A consist of port A and port C upper. Control group B consists of port C lower and port B.
Depending upon the value if CS’, A1 and A0 we can select different ports in different modes as input-output function or BSR.
This is done by writing a suitable word in control register (control word D0-D7).
| S’ | A1 | A0 | Selection | Address |
|---|---|---|---|---|
| 0 | 0 | 0 | PORT A | 80 H |
| 0 | 0 | 1 | PORT B | 81 H |
| 0 | 1 | 0 | PORT C | 82 H |
| 0 | 1 | 1 | Control Register | 83 H |
| 1 | X | X | No Seletion | X |
Pin diagram –
- PA0 – PA7 – Pins of port A
- PB0 – PB7 – Pins of port B
- PC0 – PC7 – Pins of port C
- D0 – D7 – Data pins for the transfer of data
- RESET – Reset input
- RD’ – Read input
- WR’ – Write input
- CS’ – Chip select
- A1 and A0 – Address pin
Write a control word to configure port A as input port in mode 0 and port B in mode 1 as output port.
Solution:The control word is 94H.N.B. D0 and D3 are low if port C is used as output or if unused
solution of d) Program
PPICR EQU 00FBH
PPIC EQU 00FAH
PPIB EQU 00F9H
PPIA EQU 00F8H
MOV AL, 83HOUT
PPICR , ALIN
AL, PPIBOUT
PPIA, ALIN
AL, PPICAND
AL, 0FH
MOV CL,
04HROL
AL, CLOUT
PPIC, AL
HLT