8051 Timers and Counters

Learn about 8051 microcontroller's Timer and Counter Applications.

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 = 2 16 µ sec = 2 6 x 2 10 µ sec ≈ 2 6 millisecond ≈ 64 millisecond = 65.5 millisecond

3. 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 n th 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 Counter Operation

4. 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 . 2 m/c cycle

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

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

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

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

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 Figure Timer 0, mode 2 - autoreload . OSC ÷ 12 TL0 (8 bits) TF0 C/T = 0 C/T = 1 T0 pin Control Interrupt TR0 Gate INT0 pin TH0 (8 bits)

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

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

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. 7 6 5 4 3 2 1 TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 Bit no. Symbol 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 7 6 5 4 3 2 1 Gate C/T M1 M0 Gate C/T M1 M0 Timer 1 Timer 0

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

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

28. 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 c . Start Timer/Counter through TR0, TR1 TCON = 7 6 5 4 3 2 1 TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 Timer Interrupt

29. Following tasks need to be done. Preset value to be loaded to Timer Register Timer interrupt flag (TF0 or TF1) to be cleared d . When count value in Timer Register transits from all 1’s to all 0’s 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. 1 m/c cycle = 1 microsecond 50 millisecond = 50 x 10 3 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 . 50 ms 50 ms

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 50,000 th m/c cycle accounting for 50 millisecond

35. a. Configure Timer 1 7 6 5 4 3 2 1 1 = 1 0 H 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 TL1, # B0 H MOV TH1, #3CH

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 TMOD, #10H CLR P1.7 MOV TL1, #B0H MOV TH1, #3CH CPL P1.7 ACALL TDELY SJMP KK

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

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, #10H 1 = 10H TMOD =

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. . . . . . SJMP KK. MOV TL1, #96H MOV TH1, #F5H SETB TCON.6 JNB TCON.7, $

44. Example – 8051 with clock frequency = 18 MHz Generate a square wave of frequency 2 KHz on pin P1.0 using mode 2. 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 Up time = 0.25 ms Dn time = 0.25 ms Up time = 0.25 ms = 0.25 x 10 3 µ sec No. of m/c cycles in up time = (¼ x 10 3 )/(2/3) = ¼ x 10 3 x 3/2 = 3/8 x 10 3 = 3000/8 = (30 x 25)/2 = 15 x 25 = 375 <- - - - - - 0.5 ms - - - - ->

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 1 st 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 MOV TMOD, #02H 1 = 02H TMOD =

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: SETB TCON.4 ; Check 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 1 1 = 05H TMOD = C/T = 1 – Counter operation Gate = 0, M1 M0 = 01 - 16 bit operation

52. ; MOV TMOD, #05H ; Initialize Timer value to 0000H REPT: MOV TH0, #00H MOV TL0, #00H ; Start Counter by 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 R2, - MOV R3, - MOV R4, - DJNZ R4, - DJNZ R3, - DJNZ 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. L et us assume that Timer 1 is used for incorporating 1 minute delay 12 MHz clock 1 m/c cycle = 1 µ sec 1 second = 10 6 µ sec = 10 6 m/c cycle = 2 20 m/c cycle = 2 4 x 2 16 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 x2 16 – 1 second delay must be repeated in loop by 60 times Now, TMOD will become C/T = 0 for Timer 1 – Timer operation MOV TMOD, #05H will get modified to MOV TMOD, #15H 1 1 1 = 15 H

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

60. THE END