The document describes the timers/counters functionality of the 8051 microcontroller. It contains the following key details: - The 8051 has two 16-bit timer/counters that can be independently programmed as timers or event counters. - There are four special function registers (SFRs) associated with timer/counter operation: TMOD for timer mode control, TCON for timer control, and TH0/TL0 and TH1/TL1 for Timer 0 and Timer 1 values. - The timers can be configured into four modes using the M1 and M0 bits in TMOD: 13-bit counter, 16-bit counter, 8-bit counter with auto-reload, and split operation

1. TIMERS / COUNTERS
- Two 16 bit timer/counters
- Can be programmed independently as – timer or event
counter.
- Four-SFR’s connected with TIMER/COUNTER operation
- TMOD – Timer Mode Register
- TCON – Timer Control Register
- TH0, TL0 – Timer/Counter - 0
- TH1, TL1 – Timer/Counter - 1
- Two pins of 8051 connected with Timer/counter.
T0 – Timer 0 external input – P3.4
T1 – Timer 1 external input – P3.5
- INT0 and INT1 are also used for controlling the
timer/counters.

2. Timer Operation
Timer Register (TH0, TL0 or TH1, TL1) incremented every m/c cycle. Thus
working at increment frequency of 1/12 of oscillator frequency ( for 12
oscillator machine cycle ). Any preset value i.e. initial count can be loaded to
TH0, TL0 or TH1, TL1.
For Example – Clock frequency = 12 MHZ
Clock period = 1/12 µ sec
Machine cycle time = 1 µ sec
Thus timer register will be incremented every microsecond.
- If timer is initialized to 0000H
Max. count = FFFFH
max. time measured = 216 µ sec
= 26 x 210 µ sec
≈ 26 millisecond
≈ 64 millisecond
= 65.5 millisecond

3. Counter Operation
- Counts pulses occurring at T0 pin (Timer/Counter 0) and/or
T1 pin (Timer/counter 1).
- May correspond to event like
• Passing of railway coach from a point – axle counter
• Rotation of speedometer cable
– speedometer of vehicle
• No. of persons visiting exhibition.
- T0, T1 scanned every m/c cycle
• nth m/c cycle – T1 or T0 = High
• (n+1)th m/c – T1 or T0 = Low
• Timer 0 or timer 1 incremented in (n+1)th m/c cycle
- Count frequency = min 2 m/c cycle per count
- T0- P3.4, T1- P3.5

4. 2 m/c
cycle
In 12 MHz 8051 – m/c cycle = 1 µ sec
- 8051 can count at the rate of 2 µ sec per count or higher
- Any event when takes less than 2 µ sec may go unnoticed
- C/T bit of TMOD selects Timer or counter operation for
Timer 0 or Timer 1.
- Timer/Counter operations are controlled by
- Gate bit of TMOD
- TR0 bit of TCON
When Gate = 0 then
TR0, TR1 act as
Timer run control bits.

5. - Thus make Gate = 0 in TMOD
By making TR0 (TCON. 4) or TR1 (TCON. 6) = 1 .
through instruction, Timer/Counter 0 or
Timer/Counter 1 may be started.
- For starting and stopping the Timer/Counter
from outside through hardware.
 Make Gate = 1, TR0 = 1 through software.
- By making INT0 or INT1 pin High will start
Timer/Counter 0 or Timer/Counter 1
 Make INT0 or INT1 Low to stop the Timer/Counter.
 INT0 - P3.2
 INT1 - P3.3

6. As value in Timer register rolls from all ones
(i.e. FFFFH) to all zero’s (i.e. 0000H) interrupt
flag (TF0 or TF1) will be set.
- TF0 (for Timer 0) and TF1 (for Timer 1) are
bits of TCON SFR.
 IF Timer 0 or Timer 1 interrupt is enabled then
program control will branch to interrupt servicing
routine.

7. Timer
Register
Interrupt
TF0
Program
Timer ISR
RETI
Timer modes
Mode 0
Mode 1
Mode 2
Mode 3
-
4 modes
13 bit counter
16 bit counter
8 bit counter + auto reload
Split operation – Timer 0

8. - Modes are set by M1 M0 bits of TMOD register.
Mode 0 - 13 bit counter operation
- TH0, TL0 (for Timer 0) or TH1, TL1 (for Timer 1)
used as 13 bit counter.
- All 8 bits of TH0 or TH1
- 5 lower bits of TL0 or TL1
are used, for counting.
- When count rolls over from all 1’s to all 0’s, interrupt flag TF0 or TF1 is set.

9. OSC
÷ 12
C/T = 0
TH0
TL0
(8 bits) (5 bits)
C/T = 1
TF0
Control
T0 pin
TR0
Gate
INT0 pin
Figure Timer 0, mode 0 - 13-bit counter
Interrupt

10. In above figure when C/T = 0 - timer operation
 count incremented every m/c cycle.
provided
TR0 (TCON. 4) or TR1 (TCON. 6) = 1
and Gate (TMOD. 3) or (TMOD. 7) = 0
Other way is- TR0 or TR1 =1
- Gate = 1 and INT0 or INT1 = 1
- Thus by sending Logic High signal on INT0 (or INT1) pins
Timer 0 or Timer 1 can be started.

11. - This can be used for finding pulse width in the
following way.
C/T = 0 – Timer operation
TR0 or TR1 = 1
Gate = 1
Source of pulse connected to INT0 or INT1 pin
- When pulse goes high timer starts counting at the
rate 1/12 clock frequency
- Which pulse goes low – Timer stops.
INT0 or INT1 = Low - causes interrupt.

12. Timer
Starts
Timer
Stops
+
Interrupt
Generated
- ISR can read the timer value.
- ISR can store the timer value and process it as
required by the application.

13. Mode 1 – 16 bit counter
OSC
÷ 12
C/T = 0
TH0
TL0
(8 bits) (8 bits)
C/T = 1
TF0
Control
T0 pin
TR0
Gate
INT0 pin
Figure
Timer 0, mode 1 - 16-bit counter.
Interrupt

14. - Operation same as mode 0 except that all
bits of TH0, TL0 or TH1, TL1 are used.
When count rolls over from all 1’s to all 0’s –
TF0 or TF1 interrupt flag is set.
- Causes interrupt if enabled.

15. Mode 2 – 8 bit operation with auto reload
OSC
÷ 12
C/T = 0
TL0
(8 bits)
C/T = 1
TF0
Control
T0 pin
TR0
TH0
(8 bits)
Gate
INT0 pin
Figure
Timer 0, mode 2 - autoreload.
Interrupt

16. - Only TL0 or TL1 are used i.e. 8 bit counting.
- Initial preset value is loaded to TH0 or TH1 by
software.
- The value is loaded to TL0 or TL1 by hardware
automatically before starts of counting.
- When count rolls from all 1’s (i.e. FFH) to all 0’s
(i.e. 00H)
- TF0 or TF1 flag is set
- Preset value in TH0 or TH1 is reloaded to TL0 or TL1
- Operation i.e. Counting starts automatically.

17. Mode 3 – Split operation – Timer 0
OSC
÷ 12
(1/12) fosc
(1/12) fosc
C/T = 0
TL0
(8 bits)
TH0
(8 bits)
C/T = 1
TF0
TF1
Interrupt
Control
T0 pin
TR0
Gate
INT0 pin
(1/12) fosc
Control
TR1
Figure
Timer 0, mode 3-split to two 8-bit counters.
Interrupt

18. - When Timer 0 is put in mode 3
- Acts as two 8 bit counters i.e. TL0 and TH0
become two separate counter.
TL0 – 8 bit operation in mode 0 or mode 1
(Timer or Counter) controlled by C/T, TR0,
Gate, INT0
– Sets TF0 when count rolls to all 0’s from all 1’s.
TH0 – Timer function only.
– Controlled by TR1 i.e. starts when TR1 = 1.
When count rolls to all 0’s from all 1’s – TF1 flag is
set.

19. Note – TR1 and TF1 are used in
Timer 0 (TH0) even though they are
bits for Timer 1.
When Timer 1 is put in mode 3 –
It just holds the preset count
– same as TR0 = 0 i.e. opening the switch.
[Modes 0, 1 and 2 are mostly used]

20. Timer Mode Control Register - TMOD
Timer 1
Timer 0
Bit no.
7
6
5
4
3
2
1
0
Symbol
Gate
C/T
M1
M0
Gate
C/T
M1
M0
M1 and M0 specify the mode as follows:
M1
M0
Mode
Description in brief
0
0
0
13-bit counter
0
1
1
16-bit counter
1
0
2
8-bit counter with autoreload
1
1
3
Split Timer 0 into two 8-bit counters or to stop Timer 1

21. If C/T = 1, the timers function as counters to
count the negative transitions at T0 or T1 pins.
If C/T = 0, the timers function as timers, that is,
they basically count the number of machine
cycles.
Gate = 0 means that the timer is controlled by
TR1 or TR0 only, irrespective of INT0 or INT1.
Gate = 1 means that the timer control will
depend on INT0 or INT1 and also on TR0 or TR1
bits

22. When data is written it gets latched.
TMOD is used for setting mode bits M1, M0,
Gate bit and C/T for Timer 0 and Timer 1.
Bit 0 to 3 for Timer 0.
Bit 4 to 7 for Timer 1.

23. Timer Control Register - TCON
Bit 0 to 3 – used for interrupt functions
Bit 4 to 7 – used for setting TR0, TR1 by software
- Setting TF0, TF1 by counter i.e. hardware
When count rolls from all 1’s to all 0’s.

24. Bit no.
7
6
5
4
3
2
1
0
Symbol
TF1
TR1
TF0
TR0
IE1
IT1
IE0
IT0
TF1:
Timer 1 overflow flag. Set by hardware when the timer/counter overflows.
Cleared by hardware when the processor vectors to the interrupt routine.
TR1: Timer 1 run control bit. Set/cleared by software to turn the timer/counter
on/off.
TF0:
Timer 0 overflow flag. Set by hardware when the timer/counter overflows.
Cleared by hardware when the processor vectors to the interrupt routine.
TR0: Timer 0 run control bit. Set/cleared by software to turn the timer/counter
on/off.

25. Example –
a. Configuring Timer/Counter using TMOD
Timer 1
7
Gate
Timer 0
6
5
4
3
2
1
0
C/T
M1
M0
Gate
C/T
M1
M0

26. TIMER 1 - TIMER
Mode
= 00
(13 bit operation)
TR1
C/T
= 0
M1 M0 = 00
Gate
= 0
TIMER 0 - Counter
Mode
= 01
(16 bit operation)
TR0
C/T
= 1
M1 M0 = 01
Gate
= 0
TMOD = 0 0 0 0 0 1 0 1 = 0 5 H
MOV TMOD, #05H

27. b. To load initial count as preset value
- Work out the preset value = ABCDH – Timer 0
- Load the preset value
= 0000H - Timer 1
MOV
MOV
MOV
MOV
TL0,
TH0,
TL1,
TH1,
#CDH
#ABH
#00H
#00H
Timer 0
Timer 1

28. c. Start Timer/Counter through TR0, TR1
7
TCON =
TF1
6
5
4
3
2
1
0
TR1
TF0
TR0
IE1
IT1
IE0
IT0
Timer
Interrupt
Make TR0 = 1, TR1 = 1
TCON = 0 1 0 1 0 0 0 0 = 5 0 H
MOV TCON, #50H
or SETB TCON.4
SETB TCON.6

29. d. When count value in Timer Register
transits from all 1’s to all 0’s
- Following tasks need to be done.
Preset value to be loaded to Timer Register
Timer interrupt flag (TF0 or TF1) to be cleared
For continuous operation of Timer/Counter
Time clock
Pulse train generation etc.

30. - Can be achieved in 2 ways:
1. - Check Timer interrupt flag in loop.
JNB TCON.5, $ or JNB TCON.7, $
- When interrupt flag is set then clear the flag.
CLR TCON.5 or CLR TCON.7
- Load the preset count and restart
SJMP to b

31. 2. Write ISR for Timer 0 or Timer 1 and store at
location 000BH (for Timer 0) or 001BH (for Timer
1)
- Enable Timer 0 or Timer 1 interrupt
by making bits ET0 (IE.1) or ET1(IE.3) = 1.
SETB IE.1 or SETB IE.3
- When TF0 or TF1 is set
- Interrupt will occur and program will branch to
ISR location (000BH for Timer 0) or (001BH for
Timer 1).

32. - ISR
- clear flag TF0 or TF1
- load preset value
- Restart timer/counter
RETI
Step d will be different for different applications.
Example -1
- Generate a square wave of 50% duty cycle at pin p1.7.
Use Timer 1 to generate time delay.
Clock frequency = 12 MHz, 12 oscillator clock. Pulse
width = 50 millisecond.
- Let us work out the initial preset value.

33. 50 ms
50 ms
1 m/c cycle = 1 microsecond
50 millisecond = 50 x 103 microsecond
= 50, 000 m/c cycle
FFFF = 65535
Difference = 65535 - 50000 = 15535 m/c cycle
- Since count will roll from FFFF to 0000 additional m/c cycle
will be required to set TF0 or TF1 .

34. Thus initial count must be 15536
i.e. = 3CB0H
By putting initial preset count of
3CB0H (or 15536 decimal), the register will
reach FFFF in 49999 m/c cycle and roll over to
0000 in 50,000th m/c cycle accounting for 50
millisecond

35. a. Configure Timer 1
7
0
6
0
5
0
4
1
3
0
2
0
1
0
Gate = 0, C/T = 0, Mode = 01
(16 bit operator)
MOV TMOD , # 1 0 H
Make P1.7 = Low initially
CLR P1.7
b.
Load Preset Value
KK :
MOV
MOV
TL1, #B0H
TH1, #3CH
0
0
= 10H

36. c. Complement P1.7
CPL
P1.7
d. Start Timer 1 (TR1 = 1)
SETB TCON.6
e. Check for TF1=1 in loop
JNB TCON.7, $
f. TF1=1, Make TF1=0
CLR TCON.7
g. Stop Timer 1 Make TR1=0
CLR TCON.6
h. SJMP KK
To reload preset value
Complement
P1.7
Start Timer 1.

37. Steps d to g can be written as subroutine.
Modified Program
MOV
CLR
KK. MOV
MOV
CPL
ACALL
SJMP
TMOD, #10H
P1.7
TL1, #B0H
TH1, #3CH
P1.7
TDELY
KK

38. TDELY:
SETB
JNB
CLR
CLR
RET
TCON.6
TCON.7, $
TCON.7
TCON.6

39. Example -2
- Generate a square wave of ON time of 3 ms and
OFF time of 2 ms on P1.0.
Clock frequency = 16 MHz, 12 clock m/c cycle
- 16 MHz Frequency using Timer 1.
1 clock period = 1/16 µ sec

40. - 1 m/c cycle = 12/16 µ sec = ¾ µ sec
1 ms = 1000 µ sec = 4000/3 m/c cycle
≈ 1333 m/c cycle
- 2 ms ≈ 2666 m/c cycle.
Accounting for additional m/c cycle – 2665
- 3 ms ≈ 4000 m/c cycle. Accounting for
additional m/c cycle – 3999

41. Count can be very well represented in 16 bits. Thus
for 2 ms delay –
65535 – 2665 = 62870 =F596H
for 3 ms delay –
65535 – 3999 = 61536 = F060H
; Configure Timer 1
MOV
TMOD =
TMOD, #10H
0
0
0
1
0
0
0
0
= 10H

42. ; Load preset value for 3 ms
KK :
MOV
TL1, #60H
MOV
TH1, #F0H
; Make P1.0 = High
SETB
P1.0
; Start Timer 1
SETB
TCON.6
; Check for TF1 in loop
JNB
TCON.7, $
; Make P1.0 = Low
CLR
P1.0

43. ; TF1 = 1,
Make TF1 = 0
CLR
TCON.7
; Stop Timer 1, Take TR1 = 0
CLR
TCON.6
; Repeat for 2 ms.
.
MOV TL1, #96H
.
MOV TH1, #F5H
.
SETB TCON.6
.
JNB TCON.7, $
.
SJMP KK.

44. Example – 8051 with clock frequency = 18 MHz
a. Generate a square wave of frequency 2 KHz
on pin P1.0 using mode 2.
b. Calculate the smallest frequency possible
without using software counter.
 Clock frequency = 18 MHz
Clock period = 1/18 µ sec.
1 m/c cycle = 12/18 µ sec = 2/3 µ sec.

45.  2 KHz square wave
clock period = ½ 10-3 sec. = 0.5 millisecond
<- - - - - - 0.5 ms - - - - ->
Up time = 0.25 ms
Dn time = 0.25 ms
Up time = 0.25 ms = 0.25 x 103 µ sec
No. of m/c cycles in up time = (¼ x 103)/(2/3)
= ¼ x 103 x 3/2 = 3/8 x 103
= 3000/8 = (30 x 25)/2
= 15 x 25 = 375

46. - Delay of 375 m/c cycle can be achieved in many
ways.
375/3 = 125 – Generate delay of 125 m/c cycle 3 times
375/5 = 75 – Generate delay of 75 m/c cycle 5 times
375/15 = 25 – Generate delay of 25 m/c cycle 15 times
:
:
:
We can take any of the options. Let us take 1st one.

47. To generate delay of 125 m/c cycle
Preset =
(FFH) 255 – 125 = 130
Accounting for additional m/c cycle
Preset = 131 = 83H
; Configure TMOD.
Let us use Timer 0
TMOD =
MOV
0
0
0
0
TMOD, #02H
0
0
1
0
= 02H

48. ;
Set P1.0 = High
SETB
P1.0
; Load preset count
MOV
TH0, #83H
; Declare software counter
;Loop: MOV R3, 03H
; Start Timer

49. KK:
; Check
;
;
;
;
SETB
TCON.4
TF0 in loop
JNB
TCON.5, $
Stop
timer by making TR0=0
CLR
TCON.4
Clear
TF0
Flag
CLR
TCON.5
Decrement and Branch to start timer.
DJNZ
R3, KK.
Delay of 375 m/c cycle completed.
CPL
P1.0
SJMP
Loop.

50. b. – Frequency is smallest when
clock period = maximum
i.e. Up time and Down time = maximum
i.e. Delay is maximum.
Delay is maximum when preset value =0
i.e. No. of m/c cycles in Up time = FF+1
No. of m/c cycles in Dn time = FF+1
Up time = 256 x 2/3 µ sec = 512/3 ≈ 170 µ sec
Clock period = 341 µ sec.
Frequency = 1/341 MHz = 1000/341 KHz
= 2.92 KHz

51. Example – Counter Operation
- Design a counter to count pulses input at P3.4.
- Determine the no. of pulses received in 1 minute.
- 8051 is 12 MHz, 12 Clock m/c cycle.
-> P3.4 pin is T0 i.e. -> external input to Timer 0
TMOD =
0
0
0
0
0
1
0
1
C/T = 1 – Counter operation
Gate = 0, M1 M0 = 01 - 16 bit operation
= 05H

52. ;
; Initialize
REPT:
MOV
TMOD, #05H
Timer value to 0000H
MOV
TH0, #00H
MOV
TL0, #00H
; Start Counterby making TR0 = 1
SETB
TCON.4
ACALL DELIM
; Read timer value and output on P2, P1
MOV
A, TL0
MOV
P1, A
MOV
A, TH0
MOV
P2, A
SJMP
REPT

53. DELIM :
MOV
MOV
MOV
DJNZ
DJNZ
DJNZ
R2,
R3,
R4,
R4,
R3,
R2,
-

54. Let us calculate the maximum no. of pulses
that can be counted.
- In 16 bit operation i.e. Mode 01 –
(FFFF+1) = 65535 + 1 = 65536
- In 13 bit operation i.e. Mode 00 –
1FFF+1 = 8191 = 8192
- In 8 bit operation i.e. Mode 02 & Mode 03 –
FF+1 = 255+1 = 256

55. Let us assume that 1 pulse corresponds to
– one rotation of is wheel
– circumference of wheel = 1 meter
Max. distance travelled in 1 minute can be
measured as
8 bit operation – 256 meter.
13 bit operation – 8192 meter
16 bit operation – 65536 meter.
Considering that overflow takes place in 1 minute
duration

56. Max. distance travelled in 1 hour that can be
measured with be
8 bit operation – 256 x 60 = 14760 meter
= 14.76 KMPH
13 bit operation – 8192 x 60
= 491520 = 491.52 KMPH
16 bit operation – 65536 x 60 = 3932160 meter.
= 3932.16 KMPH

57. For measuring automobile speed –
13 bit or 16 bit operation will be o.k.
- Delay operation can also be managed using hardware timer
so that micro controller is free for carrying out other tasks.
Let us assume that Timer 1 is used for incorporating 1
minute delay
12 MHz clock
1 m/c cycle = 1 µ sec
1 second = 106 µ sec = 106 m/c cycle
= 220 m/c cycle
= 24 x 216 m/c cycle.

58.  Thus 0000 to FFFF+1, counter has to repeat 16 times for delay
of 1 sec.
 For 1 Minute delay
1 minute = 60 x 16 x216
– 1 second delay must be repeated in loop by 60 times
Now, TMOD will become
0
0
0
1
0
1
0
1
= 15 H
C/T = 0 for Timer 1 – Timer operation
MOV
TMOD, #05H will get modified
to MOV TMOD, #15H

59. DELIM Subroutine
DELIM:
MOV
LOOP1:
MOV
LOOP2:
; Start
MOV
MOV
Timer 1
SETB
JNB
CLR
CLR
DJNZ
DJNZ
RET
R3, #3CH ; for 60 seconds
R4, #10H ; for 16 times
repeat for 1 second
TH1, #00H
TL1, #00H
TCON.6
TCON.7, $
TCON.6
TCON.7
R4, LOOP2
R3, LOOP1
 We could also use interrupt servicing routine
of timer interrupt for this purpose.