The document discusses the 8155 Programmable Peripheral Interface chip. It can be used as an interface between a microprocessor and I/O devices. The 8155 contains RAM, I/O ports, and a timer. It has ports A, B, and C that can be configured as input or output. The timer can operate in different modes. Programming the 8155 involves writing control words to its control register to configure the ports and timer. An example application shows how an 8155 can be used to interface an ADC and read temperature values using handshaking between the ADC and 8155 ports.

1. Programmable Peripheral
Interface Devices
Support chips: 8155, 8255, 8279, 8254, DMA
Controller, Interrupt Controller, USART

2. Programmable Interface
Devices
 Programmable Interface Device
 used to interface a I/O device to the microprocessor
 It is a multifunction device designed to use in minimum
mode system
 It contain RAM, I/O ports and Timer
8085
Programmable
I/O
Interface
I/O
Devices
System Bus

3. Requirement for PPI
 I/P & O/P Regs: A group of latches to
hold data
 Tri-State Buffer
 Capability of Bidirectional data flow
 Handshake & Interrupt signal
 Control Logic
 Chip Select Logic
 Interrupt control logic

4. 8155 – A multipurpose
programmable interface
 Designed to be compatible with 8085
 It includes
◦ 256 bytes of Read/Write memory
◦ Three I/O ports
 Port A (8-bit)
 Port B (8-bit)
 Port C (6-bit)
◦ A 14-bit timer
◦ Internal address latch to Demux AD0-
AD7, using ALE line

5. Block Diagram - 8155
256 X 8
Static
RAM
A
B
CTimer
8
8
6
Port A
Port B
Port C
PA0-7
PB0-7
PC0-5
8AD0-7
IO/M
CE
ALE
RD
WR
Timer CLK
TIMER OUT
Vcc (+5 V)
Vss (0 V)
RESET
I/ODevices
8085

6. Application design with 8155
 Objectives
◦ Interfacing 8155 with 8085
◦ Programming 8155

7. 8085
Interfacing 8085 with 8155
256 X 8
Static
RAM
A
B
CTimer
8
8
6
Port A
Port B
Port C
8AD0-7
IO/M
CE
ALE
RD
WR
RESET
A15
8085 8155

8.  Configurable Device Example
 Latch Direction
A
B
Direction Chip
Select

9. Programming 8155
 8155 is a Programmable Peripheral
Interface
 8085 can send data to 8155 using
data bus
 This data can be
◦ For I/O devices connected to 8155
◦ Timer registers of 8155
◦ Instruction/Command word for 8155
 Commands for 8155 are stored in a 8-
bit Control Register inside 8155

10.  Program
MVI A,01 H ; Set Do=1, D1-D7==0
OUTFFH ;Write in control register
MVI A,BYTE1 ;Load data bye
OUTFEH ; Send Data out
7A
0A
DIR
G: Enable
7B
0B
Control
Reg
D0
D1
D7A7
A6
A5
A4
A3
A2
A1
A0

11. 3 to 8
Decoder
CWR
Port
A
Port
B
Port
C
Timer
MSB
LSB
Latch
Clock for timer
PA0-PA7
PB0-
PB7
PC0-
PC5
Timer
Out
A0
A1
A2
ALE
AD0-AD7
A0-A7
D7-D0
0
1
2
3
4
5
CEb
A2 A
1
A0 Port (ALE
high,
AD0=A0)
0 0 0 Command
/Status
Register
0 0 1 PA
0 1 0 PB
0 1 1 PC
1 0 0 Timer LSB
1 0 1 Timer MSB

12. 3 to 8
Decoder
04
A13
A12
A11
A15
A14
A2
A1
A0
5
V
3 to 8
Decoder
RAM
Cont
rol
CWR
Port
A
Port
B
Port
C
Timer
MSB
LSB
Latc
h
Clock for timer
PA0-
PA7
PB0-
PB7
PC0-
PC5
Timer
Out
Reset in
RD
WR
A0
A1
A2
ALE
C
E
IO/M
IO/
M
AD0-AD7
A0-
A7
D7-
D0
20H
21H
22H
23H
24H
25H
CS

13. What type of Commands can be
given to 8155?
 To configure the I/O ports as Input or
Output
 To start/stop timer etc.
 To use handshake mode or not

14. Control word for 8155
 A command/instruction for 8155 is also
called control word
 This control word is written to control
register of 8155
 Control word of 8155 is of 8-bits

15. Control Word Definition for
8155
D0
D1
D2
D3
D4
D5
D6
D7
D3 D2 PORT C
0 0 ALT1
0 1 ALT2
1 0 ALT3
1 1 ALT4
D7 D6 Timer
0 0 NOP
0 1 Stop
1 0 Stop
after TC
1 0 Start
Port A
Port B
0, Input
1, Output
Port C
Interrupt Enable Port A
Interrupt Enable Port B
0, Disable
1, Enable
Timer

16. I/O functions of Port C
ALT D3 D2 PC5 PC4 PC3 PC2 PC1 PC0
ALT1 0 0 I I I I I I
ALT2 0 1 O O O O O O
ALT3 1 0 O O O STBA BFA INTRA
ALT4 1 1 STBB BFB INTRB STBA BFA INTRA
I = Input O = Output
STB = Strobe BF = Buffer Full INTR = Interrupt Request

17. The 8155 timer consists of two 8-bit registers.
1. 8-bit LSB and 8-bit MSB.
2. In these 16 bits, 14 bits are used for counter and two bit for
mode selection.
3. The counter is a 14 bit down counter. It can operate in 4
different modes of operation.
We can select mode using two bits M2 and M1
 00(Mode 0)- Single Square Wave
 01(Mode 1)- Square Wave
 10(Mode 2)- Single Pulse on TC(terminal count)
 11(Mode 3)- Pulse every TC

18.  Mode 0: In this mode, timer gives only one cycle of
square wave, the output remains high for 1/2 count
and remain s low for 1/2 count. If count is odd it
remains high for (n+1)/2 and low for (n-1)/2. Where n
is count value. Wave width depends on two factor: one
is Input clock pulse frequency, and the other is count
loaded in counter.
 Mode 1: This mode is similar to single square wave in
operation but the when counter becomes zero, the
count value is automatically reloaded. Thus it provides
continuous square wave.

19.  Mode 2: This mode gives a single clock pulse as a
output of the end of the count The output is high
normally, but it becomes low for 1 clock pulse and
again it will become high and remain high.
 Mode 3: This mode is similar to mode 2 but when the
counter becomes zero the count value is
automatically reloaded. Thus it provides continuous
pulses.

20. Design an interfacing circuit to read data from an A/D
converter using the 8155A in the peripheral mapped I/O.
(Example)
8085
3-to-8 Decoder
A/D
Converter
Analog
Input
Digital
Input
P
o
r
t
A
AD0-AD7
O0O7
8155
P
o
r
t
C
P
o
r
t
B
SOC
OE
EOCO2
CE
E2
E1
A2
A1
A0A11
A12
A13
A14
A15
IO/M
RD
WR
RESET
ALE
LED
Display
BFA
STBA
PC5

21. Port Addresses of 8155
A2 A1 A0 Port
0 0 0 Control/Status
Register
0 0 1 Port A
0 1 0 Port B
0 1 1 Port C
1 0 0 LSB Timer
1 0 1 MSB Timer
Chip Selection
A7 A6 A5 A4 A3
0 0 0 1 0
= 10H
= 11H
= 12H
= 13H
= 14H
= 15H

22. Application Programming
Logic
1. Configure 8155
2. 8085 sends SOC command to ADC
3. 8085 communicates with ADC
using PortC in handshake mode
4. 8085 reads 8-bit temperature value
from port A
5. 8085 displays the temperature
value on the LED display

23. 1. Configure 8155 I/O Ports
 Port A as INPUT port
 Port B as OUTPUT
port
 Port C in ALT3 mode
ALT D3 D2 PC5 PC4 PC3 PC2 PC1 PC0
ALT3 1 0 O O O STBA BFA INTRA
SOC EOC OE

24. Control Word for configuration
D0
D1
D2
D3
D4
D5
D6
D7
D3 D2 PORT C
0 0 ALT1
0 1 ALT2
1 0 ALT3
1 1 ALT4
D7 D6 Timer
0 0 NOP
0 1 Stop
1 0 Stop
after TC
1 0 Start
Port A
Port B
0, Input
1, Output
Port C
IE Port A
IE Port B
0, Disable
1, Enable
Timer
0
1
0
1
0
0
0
0

25. Program Instructions for configuration
 Data
◦ Control word : 0AH
 Port Address
◦ Address of Control register : 10H
 Instructions
◦ MVI A, 0AH
◦ OUT 10H

26. 2. 8085 sends SOC command to
ADC
 A small duration pulse on SOC input of
ADC will start conversion process
 SOC input of ADC is connected to port
pin PC5 of Port C (Port address 13H)
 Data
◦ Data value 20H followed by 00H
ALT D3 D2 PC5 PC4 PC3 PC2 PC1 PC0
ALT3 1 0 O O O STBA BFA INTRA
SOC
1 0 0 0 0 0

27.  Instructions
Start: MVI A, 20H
OUT 13H
MVI A, 00H
OUT 13H

28. 3. 8085 communicates with ADC
using Port C in handshake mode
 Port A used to input 8-bit data from
ADC
 Port C used for handshakingALT D3 D2 PC5 PC4 PC3 PC2 PC1 PC0
ALT3 1 0 O O O STBA BFA INTRA
EOC OE

29.  ADC resets EOC to LOW at end of
conversion
 8155 sets BF to 1 to enable ADC output
latch
 8085 is waiting for BF to be SET to 1 and
◦ Reads port A by making RD signal active (0)
 At the end of read cycle RD goes HIGH
(1)
 This resets BF signal to 0.
 8085 waits for BF signal to be RESET
and
◦ starts conversion process again

30. 8085 is waiting for BF
 To know the status of BF signal 8085
reads status register of 8155
 Status register shares same address as
control register
X Timer INTEB BFB INTRB INTEA BFA INTRA
D7 D6 D5 D4 D3 D2 D1 D0
Status Word Definition

31. Program Instructions
 8085 reads status register
◦ Address of status register : 10H
◦ Reads port A if BFA = 1
◦ Output temperature on port B
Status: IN 10H
ANI 02H
JZ Status
IN 11H ;Step -4 Reads temperature value
OUT 12H ; Step –5 display on

32.  8085 reads status register
◦ Address of status register : 10H
◦ Starts conversion again if BFA = 0
Again: IN 10H
ANI 02H
JNZ Again
JMP Start

33. Complete Program
MVI A, 0AH
OUT 10H
Start: MVI A, 20H
OUT 13H
MVI A, 00H
OUT 13H
Status: IN 10H
ANI 02H
JZ Status
IN 11H
OUT 12H
Again: IN 10H
ANI 02H
JNZ Again
JMP Start
 Configure 8155
 Start Conversion
 Read Status register
 Wait till BFA = 1
 Read temperature value
 Display on LED
 Read status register
 Wait till BFA = 0
 Start Conversion again

34. 8155
PA7
PA6
PA5
PA4
PA3
PA2
PA1
PA0
PB7
PB6
PB5
PB4
PB3
PB2
PB1
PB0
AD7
to
AD0
IO/Mb
ALE
RDb
WRb
RESET OUT
IO/Mb
ALE
RDb
WRb
RESET OUT
7 Seg
LED
Driver
7 Seg
LED
Driver
7 Seg
LED
Driver
7 Seg
LED
Driver
3 to 8
Decoder
04
A13
A12
A11
A15
A14
A2
A1
A0
5V

35.  Port Address
◦ Control Register=20H, Port A= 21H, Port B= 22H
 Control word:
 Program
◦ MVI A,03 ; initialize Port A &B for O/P
◦ OUT 20H
◦ MVI A, BYTE1 ; Display BYTE1 at port A
◦ OUT 21H
◦ MVI A, BYTE2 ; Display BYTE2 at port B
◦ OUT 22H
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 0 0 0 1 1
Timer Not
Applicable
Use for
Port C
Port B
Output
Port A
Output