Interfacing of 8051 with 8255 Programmable Peripheral Interface

1. Interfacing of 8051 with 8255 PPI T.Srikrishna, M.Sc, M.Tech, GVP.
Interfacing of 8051 with 8255
Programmable Peripheral Interface
Why do we need to interface 8255 PPI with 8051?
Why do we need additional pins when we have enough on the 8051
already?
 For some applications, it is always recommended to use 8255 PPI to
avoid data loss and damage to the microcontroller.
 The 8051 has four ports: Out of four ports Port 0, Port 2, Port 3 – all
these ports also have alternate functions.
 But Port 1, this is the only port with no alternate functions. This is the
reason why we need to interface 8255 to provide an additional three
ports, namely Port A, Port B, and Port C. and the best part about the 8255
is that it is programmable.
Components Required
1. 8051 Microcontroller.
2. 8255 Programmable Peripheral Interface
3. Decoder circuit – 74LS373 (for address demultiplexing)
4. NAND gate IC – 74LS00
5. NOT gate IC – 74LS04
Chip Select Logic
For a microcontroller to interface with 8255, a LOW pulse is given to the Chip
Select pin of 8255 as it is an active low pin
Let’s allot the address to the ports of 8255 PPI
Port A: 0010 0000 (20H)
Port B: 0010 0001 (21H)
Port C: 0010 0010 (22H)
Control port: 0010 0011 (23H)

2. Interfacing of 8051 with 8255 PPI T.Srikrishna, M.Sc, M.Tech, GVP.
Note that here only the last two bits – A0 and A1, are changed to select
different ports.
Hence, we can design a combinational logic circuit using a NAND gate for
which the input will be A2 – A7 address lines and CS’ as the output.
The output of the NAND gate is LOW only when ALL the inputs are HIGH.
Therefore, from the address allocations that we have done, we have:
A7 = 0, A6 = 0, A5 = 1, A4 = 0, A3 = 0, A2 = 0
Algorithm
Step 1: Construct the control word register
D7 = 1, this will make sure that all the ports of 8255 PPI are in the I/O mode.
D6 = D5 = D2 = 0: selects mode 0 for Port A, Port B, and Port C.
D4 = D1 = 1: To select Port A and Port B as input ports.
D3 = D0 = 0: To select both lower and upper Port C as output port.
Step 2: Input the data from Port A and Port B
Step 3: Add the contents of port A and port B
Step 4: Display the result in port C

3. Interfacing of 8051 with 8255 PPI T.Srikrishna, M.Sc, M.Tech, GVP.
Circuit Diagram to interface 8255 PPI with 8051
Assembly Language Program to interface 8255 with 8051
PORTA EQU 0020H
PORTB EQU 0021H
PORTC EQU 0022H
CNTPORT EQU 0023H;
ORG 0000H
MOV A, #92H
MOV DPTR, #CNTPORT
MOVX @DPTR, A

4. Interfacing of 8051 with 8255 PPI T.Srikrishna, M.Sc, M.Tech, GVP.
MOV DPTR, #PORTA
MOVX A, @DPTR
MOV R0, A
MOV DPTR, #PORTB
MOVX A, @DPTR
MOV R1, A
MOV A, R0
ADD A, R1;
MOV DPTR, #PORTC
MOVX @DPTR, A
END
**Note: For Referece

5. Interfacing of 8051 with 8255 PPI T.Srikrishna, M.Sc, M.Tech, GVP.