The document discusses I/O ports and timers in the 8051 microcontroller. It describes the four 8-bit I/O ports (Port 0, Port 1, Port 2, Port 3) that can be configured as inputs or outputs. It also discusses the two 16-bit timer/counters (Timer 0 and Timer 1), their associated registers (TMOD and TCON), and operating modes. The ports and timers provide capabilities for interfacing with external devices and generating time delays or counting events.

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