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8051 timer counter

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8051 timer counter
Introduction
TMOD Register

TCON Register
Modes of Operation
Counters
The microcontroller 8051 has two 16 bit Timer/ Counter registers namely Timer 0 (T0) and Timer 1 (T1) .
When used as a “Timer” the microcontroller is programmed to count the internal clock pulse.
When used as a “Counter” the microcontroller is programmed to count external pulses.

Maximum count rate is 1/24 of the oscillator frequency.

Published in: Engineering
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8051 timer counter

  1. 1. 8051 Timer / Counter
  2. 2.  The microcontroller 8051 has two 16 bit Timer/ Counter registers namely Timer 0 (T0) and Timer 1 (T1) .  When used as a “Timer” the microcontroller is programmed to count the internal clock pulse.  When used as a “Counter” the microcontroller is programmed to count external pulses. o Maximum count rate is 1/24 of the oscillator frequency.
  3. 3.  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)
  4. 4.  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
  5. 5. Timer Registers D15 D8D9D10D11D12D13D14 D7 D0D1D2D3D4D5D6 TH0 TL0 D15 D8D9D10D11D12D13D14 D7 D0D1D2D3D4D5D6 TH1 TL1 Timer 0 Timer 1
  6. 6.  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
  7. 7. 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
  8. 8. 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)
  9. 9.  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
  10. 10. 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
  11. 11.  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..
  12. 12.  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.
  13. 13. ÷ 12 TR TH TL TF Timer overflow flag C/T = 0 TF goes high when FFFF 0 XTAL oscillator
  14. 14.  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.
  15. 15. XTAL oscillator ÷ 12 TR1 TL1 TH1 TF1 overflow flag reload C/T = 0
  16. 16.  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.
  17. 17.  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
  18. 18. 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
  19. 19. TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 Timer 1 Timer0 for Interrupt (MSB) (LSB)
  20. 20.  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.
  21. 21. 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)

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