This document discusses the I/O ports on the 8051 microcontroller and how to interface it with an external 8255 parallel I/O chip. It describes the internal structure and functionality of the 8051's four 8-bit I/O ports P0-P3. It also covers how to configure the ports for input or output, read from and write to the ports, and how the port pins are multiplexed with other signals. The document then discusses using the 8255 chip to expand the number of available I/O ports and provides an example of simple interfacing between the 8051 and 8255 with code.

1. Pari vallal Kannan
Center for Integrated Circuits and Systems
University of Texas at Dallas
8051 I/O and 8255
Class 7
EE4380 Fall 2002

2. 17-Sep-02 2
Why I/O Ports
l Controllers need to get external inputs and
produce external outputs
– I/O ports serve the purpose
l 8051 has 4 built-in I/O ports
l Too many ports increase pin-count and device
cost. Too few makes it inadequate for complex
control needs
l Generally, Ports are scarce and Port
usage/allotment is an engineering decision

3. 17-Sep-02 3
8051 I/O Ports – Internal Structure
l 32 pins are allotted for 4
eight bit I/O ports
– P0, P1, P2, P3
l At power-on all are
output ports by default
l To configure any port for
input, write all 1’s (0xFF)
to the port
– Q=1, QB=0, M1=OFF,
Read_Pin asserted by
read instruction
Q
QB
D
Write
CLK
READ LATCH
READ PIN
INTERNAL
BUS
PORT
LATCH
P1.X
Vcc
Internal
Load
Pin
P1.X
M1

4. 17-Sep-02 4
8051 I/O Ports (contd.)
l Ports can be read and written to like normal registers
mov A, #55H ; can use A
mov P0, A ; write A to P0
mov P1, A
mov P2, #0AAH ; can use immediate mode
xlr P1, #0FFH ; read-modify-write (ex-or)
mov P0, #0FFH ; configure P0 for input
mov A, P0 ; read from P0
l Ports can be bit manipulated (single bit addressable)
using cpl and setb instructions
cpl P1.2 ; complement bit 2 of Port1
setb P1.3 ; set bit 3 for Port1 to 1
clr P0.0 ; clear bit 0 of Port0

5. 17-Sep-02 5
8051 I/O Ports – Pin Muxing
l Port pins are muxed with other signals
– P0 : Also carry A0:A7 and D0:D7
– P1 : dedicated
– P2 : Also carry A8:A15
– P3 : Also carry serial I/O (TxD, RxD), Timer inputs (T0, T1),
external interrupts (INT0, INT1) and read write signals (RD,
WR)
l For 8051 or DS5000, with no external memory, P0, P1
and P2 are available. For 8031, only P2 is available
l To increase the number of ports, use a Parallel port
interface chip like 8255

6. 17-Sep-02 6
8051 I/O Ports : Hardware Specs
l P0 is open drain.
– Has to be pulled high by external 10K resistors.
– Not needed if P0 is used for address lines
l P1, P2, P3 have internal pull-ups
l Port fan-out (number of devices it can drive) is limited.
– Use buffers (74LS244, 74LS245, etc) to increase drive.
– P1, P2, P3 can drive up to 4 LS-TTL inputs
– P0 fan-out is dependant on the pull-up resistor value, limited
by the max current it can sink on the output stage.

7. 17-Sep-02 7
8051 I/O Ports : Input Quirks
l Port read instructions either
– Read from the 8051 pins (“voltage”
levels on the pins)
– Read from an internal latch on the
ports
l Writing 1 to the latch
– Q=1, QB=0
– M1 off
– P1.x is available at tristate buffer
l Writing 0 to the latch
– Q=0, QB=1
– M1 ON
– Input always gets 0
– Can damage the port (M1) if P1.x is Vcc
– Use 10K resistance between switch on P1.x
and Vcc
– Or use a SPST switch connected to GND
Q
QB
D
Write
CLK
READ LATCH
READ PIN
INTERNAL
BUS
PORT
LATCH
P1.X
Vcc
Internal
Load
Pin
P1.X
M1

8. 17-Sep-02 8
Input Quirks (contd.)
l Instructions that read the pins (READ_PIN is asserted)
– mov A, Px
– jnb Px.y …
– jb Px.y …
– mov C, Px.y
l Instructions that read the latch (READ_LATCH is
asserted)
– They read the last output value and not the value on the pins
– [anl, orl, xrl] Px
– [jbc, djnz] Px.y, target
– [cpl, clr, setb] Px.y
– [inc, dec] Px

9. 17-Sep-02 9
8051 - Switch On IO Ports
l Case-1:
– Gives a logic 0 on switch
close
– Current is 0.5ma on
switch close
l Case-2:
– Gives a logic 1 on switch
close
– High current on switch
close
l Case-3:
– Can damage port if 0 is
output
Port Pin
1.Good
10K
Port Pin
2.Poor
GND
470
Port Pin
InternalPullup
GND GND
Vcc+5V
Vcc+5V
Port Pin
3.Poor
Vcc+5V

10. 17-Sep-02 10
DIP Switches on IO port
l DIP switches usually
have 8 switches
l Use the case-1 from
previous page
l Can use a Resistor
Pack, instead of
discrete resistors
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
Vcc
+5V
10KResPack
SW-DIP8
GND
8051

11. 17-Sep-02 11
LED on IO Port
l Try to use current sinking
l Case-1
– LED is ON for an output of
zero
– Most LEDs drop 1.7 to 2.5
volts and need about 10ma
– Current is (5-2)/470
l Case-2
– Too much current
– Failure of Port or LED
l Case-3
– Not enough drive (1ma)
– LED too dim
l Seven Segment LEDs
– Common Anode/ Cathode
– CA preferred
– Case-1 may have LEDs of
different brightness
Vcc +5V Vcc +5V
GND
Port Pin Port Pin Port Pin
1. Good 2. Poor 3. Poor
470
Vcc
+5V
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
8051
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
P1.7
8051
7segLED-CommonAnode
a
b
c
d
e
f
g
dp
CA
Vcc
+5V
330
7segLED-CommonAnode
CA
330

12. 17-Sep-02 12
8051 Interfacing with the 8255
l 8255 - Widely used I/O chip
– 40 pins
– Provides 3 eight bit ports PA, PB and PC
– Port PC can be used as two 4 bit ports PCL and PCU
– Ports have handshaking ability
– Two address lines A0,A1 and a Chip select CS
l Address space of 4 bytes
l 00b selects Port A
l 01b selects Port B
l 10b selects Port C
l 11b selects a control register

13. 17-Sep-02 13
8255 Functional Diagram
l CS is used to interface with 8051
l If CS is generated from lets say
Address lines A15:A12 as
follows,
A15:A12 = 1000
l Base address of 8255 is
– 1000 xxxx xxxx xx00b
– 8000H
l Address of the registers
– PA = 8000H
– PB = 8001H
– PC = 8002H
– CR = 8003H
8
2
5
5
PA
PB
PC
CS RESET
D7-D0
RD
WR
A0
A1

14. 17-Sep-02 14
8255 Operating Modes
l Mode 0 : Simple I/O
– Any of A, B, CL and CU can be programmed as input or output
l Mode 1: I/O with Handshake
– A and B can be used for I/O
– C provides the handshake signals
l Mode 2: Bi-directional with handshake
– A is bi-directional with C providing handshake signals
– B is simple I/O (mode-0) or handshake I/O (mode-1)
l BSR (Bit Set Reset) Mode
– C alone is available for bit mode access

15. 17-Sep-02 15
8255 Configuration
l Configured by writing a control-word in CR register
l CR definition
– D7 : 1àI/O mode, 0àBSR
– D6,D5 : Mode selection for A and CU
l 00àMode0, 01àMode1, 1xàMode2
– D4 : Port A control
l 1àA input, 0àA output
– D3 : Port CU control
l 1àCU input, 0àCU output
– D2 : Port B Mode selection
l 0àB is in mode 0, 1àB is in mode 1
– D1 : Port B control
l 1àB input, 0àB output
– D0 : Port CL control
l 1àCL input, 0àCL output
l Refer to 8255 datasheet for additional options

16. 17-Sep-02 16
8255 Usage: Simple Example
l 8255 memory mapped to 8051 at address 8000H base
– PA = 8000H, PB = 8001H, PC = 8002H, CR = 8003H
l Control word for all ports as outputs in mode0
– CR : 1000 0000b = 80H
l Code snippet
test: mov A, #80H ; control word
mov DPTR, #8003H ; address of CR
movx @DPTR, A ; write control word
mov A, #55h ; will try to write 55 and AA alternatively
repeat: mov DPTR, #8000H ; address of PA
movx @DPTR, A ; write 55H to PA
inc DPTR ; now DPTR points to PB
movx @DPTR, A ; write 55H to PB
inc DPTR ; now DPTR points to PC
movx @DPTR, A ; write 55H to PC
cpl A ; toggle A (55àAA, AAà55)
acall MY_DELAY ; small delay subroutine
sjmp repeat ; for (1)

17. 17-Sep-02 17
Next Class
l Interfacing a keypad
l Interfacing a character LCD
l Scanned LED displays