8051 timer counter


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8051 timer counter

  1. 1. Presented by :  AKASH GUPTA 111257  ANKIT SAHA 111340
  2. 2.  Introduction  TMOD Register  Modes of Operation  TCON Register  Counters
  3. 3.  The 8051 comes equipped with two timers, both of which may be controlled, set, read, and configured individually.  The 8051 timer has three general functions:  Keeping time and calculating the amount of time between events.  Counting the events .  Generating baud rates for the serial port.
  4. 4. When a timer is in interval timer mode and correctly configured, it will increment by „ONE‟ every machine cycle. We have: 1,10,59,000 / 12 = 9,21,583 machine cycles / sec  The 8051 uses a 11.059 MHz crystal.  A machine cycle equals 12 clock (crystal) pulses. Thus the counter increments 9,21,583 times per second. Thus if a timer has counted from 0 to 50,000 you may calculate: 50,000 / 9,21,583 = .0542 seconds
  5. 5.  The 8051 has 2 timers/counters: ◦ Timer/Counter 0 ◦ Timer/Counter 1  Registers Used in the Timer : ◦ Timer 0 registers: TH0, TL0 Exclusive ◦ Timer 1 registers: TH1, TL1 ◦ TMOD (Timer mode register) Shared by both ◦ TCON (Timer control register)
  6. 6.  Registers THx & TLx They are 16 bit wide.  These registers store: The time delay as a timer. The number of events as a counter.  Timer 0: TH0 & TL0 Timer 0 high byte , timer 0 low byte  Timer 1: TH1 & TL1 Timer 1 high byte, timer 1 low byte
  7. 7. D15 D8D9D10D11D12D13D14 D7 D0D1D2D3D4D5D6 TH0 TL0 D15 D8D9D10D11D12D13D14 D7 D0D1D2D3D4D5D6 TH1 TL1 Timer 0 Timer 1 Timer Registers
  8. 8.  Timer mode register = TMOD ◦ An 8-bit register  lower 4 bits : Timer 0 Mode setting (0000 : not used)  upper 4 bits : Timer 1 Mode setting (0000 : not used) ◦ Not bit-addressable
  9. 9. GATE C/T M1 M0 GATE C/T M1 M0 Timer 1 Timer 0 (MSB) (LSB) BIT NAME EXPLANATION OF THE FUNCTION TIMER 7 GATE1 When this bit is set the timer will only run when INT1 (P3.3) is high. When this bit is clear the timer will run regardless of the state of INT1. 1 6 C/T1 When this bit is set the timer will count events on T1 (P3.5). When this bit is clear the timer will be incremented every machine cycle. 1 5 T1M1 Timer mode bit 1 4 T1M0 Timer mode bit 1 3 GATE0 When this bit is set the timer will only run when INT0 (P3.2) is high. When this bit is clear the timer will run regardless of the state of INT0. 0 2 C/T0 When this bit is set the timer will count events on T0 (P3.4). When this bit is clear the timer will be incremented every machine cycle. 0 1 T0M1 Timer mode bit 0 0 T0M0 Timer mode bit 0
  10. 10. 0 : Timer operation (clock : Machine cycle) 1 : Counter operation (clock : Tx input pin) GATE C/T M1 M0 GATE C/T M1 M0 Timer 1 Timer 0 (MSB) (LSB)
  11. 11.  Every timer has a mean of starting and stopping. Timer is enabled only while the INT pin is high and TR control pin(in TCON) is set. ◦ GATE = 0  Internal control  The start and stop of the timer are controlled by the software (Set/clear the TR) ◦ GATE = 1  External control  The hardware way of starting and stopping the timer by software and an external source.
  12. 12.  M0 and M1 select the timer mode for timers 0 & 1. M1 M0 Mode Operating Mode 0 0 0 13-bit timer mode 8-bit THx + 5-bit TLx (x= 0 or 1) 0 1 1 16-bit timer mode 8-bit THx + 8-bit TLx 1 0 2 8-bit auto reload 8-bit auto reload timer/counter; THx holds a value which is to be reloaded into TLx each time it overflows. 1 1 3 Split timer mode
  13. 13. TxM1 TxM0 Timer Mode Description of Mode 0 0 0 13-bit Timer 0 1 1 16-bit Timer 1 0 2 8-bit Auto Reload 1 1 3 Split Timer Mode 4 Operating Modes
  14. 14.  This is a relic mode. ◦ Included in 8051 to maintain compatibility with its predecessor 8048.  The counters are counting up: ◦ TLx will count from 0 to 31. ◦ When TLx is incremented from 31, it will “reset” (overflow) to 0. ◦ Now THx will be incremented.  Hence effectively only 13 bits are used. ◦ Bits 0-4 of TLx. ◦ Bits 0-7 of THx..
  15. 15.  This is the most commonly used mode.  This mode operates in a fashion almost like the Mode 0, only this time all 16 bits are used.  The counting: ◦ TLx is incremented from 0(00h) to 255(FFh). ◦ When TLx is incremented from 255, it resets to 0 and causes THx to be incremented by 1. ◦ Hence we have a maximum count of „65,025‟ (255*255) machine cycles.
  16. 16. 1. Choose mode 1 timer 0 MOV TMOD,#01H 2. Load registers TL and TH with initial count values MOV TH0,#FFH MOV TL0,#FCH 3. Clear the flag TF0 CLR TF0 4. Start the timer. SETB TR0
  17. 17. 5. The 8051 starts to count up by incrementing the TH0-TL0. ◦ TH0-TL0= FFFCH,FFFDH,FFFEH,FFFFH,0000H FFFC FFFD FFFE FFFF 0000 TF = 0 TF = 0 TF = 0 TF = 0 TF = 1 TH0 TL0Start timer Stop timer Monitor TF until TF=1 TR0=1 TR0=0 TF
  18. 18. ÷ 12 TR TH TL TF Timer overflow flag C/T = 0 TF goes high when FFFF 0 XTAL oscillator
  19. 19.  When a timer is in mode 2, THx holds the "reload value" and TLx is the timer itself.  Thus the counting proceeds as: ◦ TLx starts counting up. ◦ TLx reaches 255 and is subsequently incremented. ◦ Now instead of resetting to 0 (as in the case of modes 0 and 1), it will be reset to the value stored in THx.
  20. 20. 1. Choose mode 2 timer 0 MOV TMOD,#02H 2. Set the original value to TH0. MOV TH0,#38H 3. Clear the flag to TF0=0. CLR TF0 4. After TH0 is loaded with the 8-bit value, the 8051 gives a copy of it to TL0. TL0=TH0=38H
  21. 21. 5. Start the timer. SETB TR0 6. The 8051 starts to count up by incrementing the TL0. TL0= 38H, 39H, 3AH,.... 7. When TL0 rolls over from FFH to 00, the 8051 set TF0=1. Also, TL0 is reloaded automatically with the value kept by the TH0. TL0= FEH, FFH, 00H (Now TF0=1) The 8051 auto reload TL0=TH0=38H. Clr TF0 Go to Step 6 (i.e., TL0 is incrementing continuously).  Note that we must clear TF0 when TL0 rolls over. Thus, we can monitor TF0 in next process. 8.Clear TR0 to stop the process. Clr TR0
  22. 22. XTAL oscillator ÷ 12 TR1 TL1 TH1 TF1 overflow flag reload C/T = 0
  23. 23.  When Timer 0 is placed in mode 3, it essentially becomes two separate 8- bit timers.  That is to say, Timer 0 is TL0 and Timer 1 is TH0. Both timers count from 0 to 255 and overflow back to 0 independently.  What happens to timer1? ◦ All the bits that are related to Timer 1 will now be tied to TH0. ◦ While Timer 0 is in split mode, the real Timer 1 (i.e. TH1 and TL1) can be put into modes 0, 1 or 2 normally. ◦ However, you may not start or stop the real timer 1 since the bits that do that are now linked to TH0. ◦ The real timer 1, in this case, will be incremented every machine cycle no matter what.
  24. 24. A good possible use of using split timer mode is if you need to have two separate timers and, additionally, a baud rate generator. In such case you can use the real Timer 1 as a baud rate generator and use TH0/TL0 as two separate timers.
  25. 25.  Finally, there is one more SFR that controls the two timers and provides valuable information about them.  Timer control register: TCON ◦ Upper nibble : TIMER ◦ Lower nibble : INTERRUPTS
  26. 26. The Register bits represent the following values : BIT NAME BIT ADDRESS EXPLANATION OF THE FUNCTION TIMER 7 TF1 8Fh Timer 1 Overflow. This bit is set by the microcontroller when Timer 1 overflows. 1 6 TR1 8Eh Timer 1 Run. When this bit is set Timer 1 is turned on. When this bit is clear Timer 1 is off. 1 5 TF0 8Dh Timer 0 Overflow. This bit is set by the microcontroller when Timer 0 overflows. 0 4 TR0 8Ch Timer 0 Run. When this bit is set Timer 0 is turned on. When this bit is clear Timer 0 is off. 0
  27. 27.  TF (timer flag, control flag) ◦ TF0 : timer 0 ◦ TF1 : timer 1. ◦ TF is like a carry. Originally, TF=0. When TH-TL roll over to 0000 from FFFFH Then , TF is set to 1  TF=0:not reached  TF=1:reached TR (run control bit) TR0 : Timer 0 TR1: Timer 1 Turn timer - ON or OFF TR = 0 : OFF (stop) TR = 1 : ON (start) TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 Timer 1 Timer0 for Interrupt (MSB) (LSB)
  28. 28. This SFR is “Bit-Addressable” What does this mean ? It means that if you want to set the bit TF1 (which is the highest bit of TCON) you could execute the command : SETB TF1 Rather than executing this command : MOV TCON , #80h Because it is bit-addressable. It decreases program execution time.
  29. 29.  As far as the use of a timer/counter as an event counter is concerned ,everything that we have talked about in the last section also applies to programming it as a counter ,except the source of the frequency.  When used as a timer ,the 8051‟s crystal is used as the source of the frequency.  However ,when used as a counter ,it is a pulse outside of the 8051 that increments the TH,TL registers.  These timers can also be used as counters counting events happening outside the 8051.
  30. 30. Pin Port Pin Function Description 14 P3.4 T0 Timer/Counter 0 external input 15 P3.5 T1 Timer/Counter 1 external input GATE C/T=1 M1 M0 GATE C/T=1 M1 M0 Timer 1 Timer 0 (MSB) (LSB)
  31. 31.  16-bit counter (TH0 and TL0)  TH0-TL0 is incremented when TR0 is set to 1 and an external pulse (in T0) occurs.  When the counter (TH0-TL0) reaches its maximum of FFFFH, it rolls over to 0000, and TF0 is raised.  Programmers should monitor TF0 continuously and stop the counter 0.  Programmers can set the initial value of TH0-TL0 and let TF0=1 as an indicator to show a special condition. (ex: 100 people have come).
  32. 32. Timer 0 external input Pin 3.4 TR0 TH0 TL0 TF0 TF0 goes high when FFFF 0 overflow flag C/T = 1
  33. 33.  8-bit counter. It allows only values of 00 to FFH to be loaded into TH0  Auto-reloading • TL0 is incremented if TR0=1 and external pulse occurs.
  34. 34. Thank You!