Programmable Peripheral Interface Devices Support chips: 8155, 8255, 8279, 8254, DMA Controller, Interrupt Controller, USART
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Programmable Peripheral
Interface DevicesSupport chips: 8155, 8255, 8279, 8254, DMA
Controller, Interrupt Controller, USART
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
8085Programmable
I/O
Interface
I/O
Devices
System Bus
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
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
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/O D
evic
es
8085
Application design with 8155
Objectives
◦ Interfacing 8155 with 8085
◦ Programming 8155
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
Configurable Device Example
Latch Direction
A
B
Direction Chip
Select
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
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
DIRG: Enable
7B
0BControl
Reg
D0
D1
D7A7
A6
A5
A4
A3
A2
A1
A0
3 to 8
Decoder
CWR
Port
A
Port
B
Port
C
TimerMSB
LSB
Latch
Clock for timer
PA0-PA7
PB0-
PB7
PC0-
PC5
Timer
Out
A0
A1
A2ALE
AD0-AD7A0-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
3 to 8
Decoder
04A13
A12
A11
A15
A14
A2
A1
A0
5
V
3 to 8
Decoder
RAM
Cont
rolCWR
Port
A
Port
B
Port
C
TimerMSB
LSB
Latc
h
Clock for timer
PA0-
PA7
PB0-
PB7
PC0-
PC5
Timer
Out
Reset in
RD
WR
A0
A1
A2
ALE
C
EIO/M
IO/
M
AD0-AD7
A0-
A7
D7-
D0
20H
21H
22H
23H
24H
25H
CS
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
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
Control Word Definition for
8155D0
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
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
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
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.
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.
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
InputP
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
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
Application Programming
Logic1. 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
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
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
Program Instructions for configuration
Data
◦ Control word : 0AH
Port Address
◦ Address of Control register : 10H
Instructions
◦ MVI A, 0AH
◦ OUT 10H
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
InstructionsStart: MVI A, 20H
OUT 13H
MVI A, 00H
OUT 13H
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
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 RESETand◦ starts conversion process again
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
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
LED’s
8085 reads status register
◦ Address of status register : 10H
◦ Starts conversion again if BFA = 0
Again: IN 10H
ANI 02H
JNZ Again
JMP Start
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
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
04A13
A12
A11
A15
A14
A2
A1
A0
5V
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
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