1. I/O PORTS AND TIMERS
IN 8051 MICRO CONTROLLER
Dr. C. Saritha
Lecturer in Electronics
SSBN Degree & PG College
Anantapur - 515001

2. Input/Output Ports (I/O Ports)
 All8051 microcontrollers have 4 I/O ports
each comprising 8 bits which can be
configured as inputs or outputs.
 Allthe ports upon reset are configured as
output, ready to use it as output port.
 To use any of these ports as an input port,
it must be programmed.

3. Port 0
Port0 occupies a total of 8 pins (pins
32-39).
It can be used for input or output.
To use the pins of port o as both input
and output ports, each pin must be
connected externally to 10k-ohm pull up
resistor.
This is due to fact that p0 is an open
drain with external pull up resistors
connected upon the reset.

4. Port0 with pull up resistor

5. Continue…
 Port0 is configured as o/p port:-
BACK MOV A,#55H
MOV PO,A
ACALL DELAY
CPL A
SJMP BACK

6. Continue…
 To make the port o as i/p port, the port
must be programmed by writing 1 to all the
bits.
MOV A,#FFH
MOV PO,A
BACK MOV A,PO
SJMP BACK

7. Dual role of port 0
 Port0 is also designated as ADO-
AD7,allowing it to be used for both address
and data .
 When connecting 8051 to an external
memory, port0 provides both address and
data
 The 8051 multiplexes address and data
through port0 to save pins.

8. Continue…
 When ALE=0, it provides data D0-D7 but
when ALE=1 it has address A0-A7.
.
 ALE is used for multiplexing address and
data with help of 74LS73.

9. Port1
 Port
1 occupies a total of 8 pins, it can be
used as input or output.
 This port does not need any pull-up
resistors since it already has pull-up
resistor internally.
 Upon reset ,port1 is configured as an o/p
port.

10. Continue…
 Port1 is configured as o/p port:-
BACK MOV A,#55H
MOV P1,A
ACALL DELAY
CPL A
SJMP BACK

11. Continue…
 To make port1 an i/p port, it must be
programmed as such writing 1 to all its bits
MOV A,#FFH
MOV P1,A
BACK MOV A,P1
SJMP BACK

12. PORT 2
 Port2occupies a total of 8 pins, it can be
used as i/p or o/p.
 Justlike p1,port2 does not need any pull-
up resistor since it has pull-up resistor
internally.
 Upon reset, port2 configured as an o/p.

13. Continue…
 Port2 is configured as o/p port:-
BACK MOV A,#55H
MOV P2,A
ACALL DELAY
CPL A
SJMP BACK

14. Continue…
 To make port2 an i/p port, it must be
programmed as such writing 1 to all its bits
MOV A,#FFH
MOV P2,A
BACK MOV A,P2
SJMP BACK

15. Dual role of port2
 Port2 must be used along with p0 to
provide the 16 bit address for the external
memory.
 Port2 is designated as A8-A15,indicating
its dual function

16. PORT 3
 port3 occupies a total of 8 pins. It can be
used as i/p or o/p .
 P3 does not need any pull up resistor same
as p1 and p2 did not.
 Port3 is configured as o/p port upon reset .
 Port3 has the additional function of providing
some extremely important signals such as
interrupts

17. Alternate Pin-functions

18. Timers/counters
Timers/Counters
Timer0/Counter0 Timer1/Counter1

19. Continue…
They can be used as:
1 Timer:
 Timers are used as time delay generator.
 The clock source is the internal crystal
frequency of the 8051.
2 counter:
 Which counts number of events.
 External input from input pin to count the
number of events on register

20. Registers used in Timer/counter
 Register pairs (TH0,TH1),(TL0,TL1) form
16-bit Timer/Counter registers.
 These are instructions for reading and
writing and they are bit wise
 One 8 bit mode register-TMOD.
 One 8 bit control register-TCON.

21. TMOD Register
 Both timer 0 and timer 1 use the same
register called TMOD.
 It is an 8-bit register in which the lower 4
bits are set aside for timer 0 and the upper 4
bits are set aside for timer 1
 The lower 2 bits are used to set the timer
and the upper 2 bits to specify the

22. Timer Mode (TMOD)
GATE C/T M1 M0 GATE C/T M1 M0
Timer 1 Timer 0

23.  GATE :-
Gating control when set, the
timer/counter is enabled only while the INTX
Pin is high and the TRX control pin is set.
 C/T :-
This bit in the TMOD register is used
to decide whether the timer is used as a
delay generation or an event counter.
If C/T=0,it is used as a delay
generator.

24. Continue…
 M1:Mode bit 1
M0: Mode bit 0
M0 and M1 select the timer mode, there
are three modes i.e., mode 0,mode 1,
mode 2.

25. Modes of timers/counters
 Mode 0 : 5 bit prescaler(13 bit )
 Mode 1 : 16 bit timer
 Mode 2 : 8 bit Auto reload
 Mode 3 : TL 0 and TH0 are independent
but TH0 controlled by TR1
and TF1.

26. TCON Register
 This is an 8 bit register.
 The upper 4 bits of the register is used to
store the TF and TR bits of both timer 0
and timer 1.
 The lower 4 bits are set aside for
controlling the interrupt bit.

27. Timer Control Register (TCON)
TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0

28.  TF1 [Timer1 over flow flag] :- Set by
hardware when timer 1 counters over
flows, cleared by hardware as the
processor vectors to the service routine.
 TR1 [Timer1 run control bit] :- Set/cleared
by software to turn timer1 counter1 on/off.
 TF0 [Timer0 over flow flag] :- Set by
hardware when timer /counters over flows,
cleared by hardware as the processor
vectors to the service routine.

29.  TR0 [Timer0 run control bit] :- Set/cleared by
software to turn timer/counter on/off.
 IE1 [External interrupt1 edge flag] :- Set by
CPU when the external interrupt edge is
detected, cleared by the CPU when the
interrupt is processed.
 IT1 [Interrupt 1 type control bit] :-
Set/cleared by software to specify falling
edge/low level triggered external interrupt.

30.  IE0 [External interrupt0 edge flag] :- Set by
CPU when the external interrupt edge is
detected, cleared by the CPU when the
interrupt is processed.
 IT0 [Interrupt 0 type control bit] :-
Set/cleared by software to specify falling
edge/low level triggered external interrupt.

31. THANK YOU