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Timer/Counter Programming

Timer/Counter Programming. Timers/Counters Programming. The 8051 has 2 timers/counters: timer/counter 0 timer/counter 1 They can be used as The timer to generate time delay. The clock source is the internal crystal frequency of the 8051. An event counter .

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Timer/Counter Programming

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  1. Timer/Counter Programming

  2. Timers/Counters Programming • The 8051 has 2 timers/counters: • timer/counter 0 • timer/counter 1 They can be used as • The timer to generate time delay. • The clock source is the internal crystal frequency of the 8051. • An event counter. • External pulse input from input pin to count the number of events on registers. • These clock pulses could represent the number of people passing through an entrance, or the number of wheel rotations, or any other event that can be converted to pulses.

  3. Registers • TH0, TL0 : timer/counter register of timer 0 • TH1, TL1 : timer/counter register of timer 1 • TMOD : Mode Select register • TCON : Control Register

  4. Timer0 and Timer1 Registers • Accessed as lower byte and higher byte • The lower byte register is TL0 / TL1 • The higher byte register is TH0 / TH1 • Accessed like any other register • mov TL0, #4Fh • mov R5, TH0

  5. TCON register

  6. Timer control and Flag bits • TR (Timer run control bit) • TR0 for Timer/counter 0; TR1 for Timer/counter 1. • TR is set by programmer to turn timer/counter on/off. • CLR TR0 : off (stop) • SETB TR0 : on (start) • TF(timer flag bit) • TF0 for timer/counter 0; TF1 for timer/counter 1. • TF is like a carry. Originally TF=0. When TH-TL rolls over from FFFFh to 0000, the 8051 sets TF to 1. TF should be cleared by software in polling method. • TF=0 : not reach • TF=1: reach

  7. TMOD Register timer clock

  8. Gate • Every timer has a mean of starting and stopping. • GATE=0 • Internal control • The start and stop of the timer are controlled by way of software. • Set/clear the TR for start/stop timer. SETB TR0 CLR TR0 • GATE=1 • External control • The hardware way of starting and stopping the timer by software and an external source. • Timer/counter is enabled only while the INT pin is high and the TR control pin is set (TR).

  9. Mode selection Bits in the TMOD M1 M0 Mode Operation 0 0 0 13-bit timer mode 8-bit THx + 5-bit TLx (x= 0 or 1) 0 1 116-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 3Split timer mode

  10. Find Timer Register values Assume XTAL frequency = 12MHz • Clock frequency of the timer = 12MHz/12 = 1MHz • Clock period = 1/1MHz = 1uS • Devide the desired time delay by 1uS = decimal value n • Perform 65536 - n (for 16bit mode) • convert the result to hex , where yyxx is the initial value to be loaded into the timer’s register • Set TH = yy and TL = xx.

  11. Timer Register Values Example • Generate 500us time delay. Let crystal frequency = 12MHz • Timer clock period ( 12 / Xtal freq. ) = 1us • The required Delay time = 500 us • No. of counts to be counted by timer = 500us/1us = 500 • The value in TH0 & TL0 = 65536-500 = 65036 = FE0Ch • TH0 = FEh, TL0 = 0Ch

  12. Timer mode 1 programming • Set the timer & mode in TMOD register • Load 16 bit register with the initial count value • Start the timer SETB TRX • Keep monitoring TFxJNB TFx, step4 • TFX = 0 until timer reaches FFFFh • TFX = 1 when timer rolls over from FFFFh to 0000h • Stop the process CLR TRX • Clear the TFx flag for the next round • Go back to step 2 to load TH and TL

  13. Programming timer in mode1 Example • Assuming XTAL = 12MHz write a program to generate a square wave of 1khz at P2.0 with 50% duty cycle • The required ON/OFF time or Delay time = ( 1 /1KHz)/2 = 500 µS • Machine cycle period = 12MHz / 12 = 1 µS • No. of counts to be counted by timer = 500us/1 µS = 500 • The value in TH0 & TL0 = 64536 - 500 = 64036 = FE0Ch

  14. Programming timer in mode1 Example cont.. ORG 0 ;Reset entry point ljmp MAIN ;Jump above interrupt ORG 0030H;Main Program entry point after vector table space MAIN: mov TMOD,#02H ;Timer 0, mode 1 Again: mov TH0, #0FAh ; TH0 = FEh mov TL0, #024h ; TL0 = 0Ch setb TR0 ;Start timer Here: JNB TF0, Here ;keep monitoring TF0 clr tr0 ;Stop timer cpl p2.0 ;toggle bit 0 of port2 clr TF0 ;clear timer 0 flag bit sjmp Again ;go for next round END

  15. Programming timer in mode2 Example • Assuming XTAL = 12MHz write a program to generate a square wave of 10khz at P1.0 with 50% duty cycle • The required ON/OFF time or Delay time = ( 1 /10KHz)/2 = 50 µS • Machine cycle period = 12MHz / 12 = 1 µS • No. of counts to be counted by timer = 50us/1 µS = 50 • The value in TH0 & TL0 = 256 - 50 = 206 = CEh

  16. Programming timer in mode2 Example Cont… ORG 0 ;Reset entry point sjmp MAIN ;Jump above interrupt ORG 0030H MAIN: mov TMOD,#02H ;Timer 0, mode 2 mov TH0, #0CEh ; TH0 = CEh mov TL0, #0CEh ; TL0 = CEh Again: SETB TR0 ;Start timer Here: JNB TF0, Here ;keep monitoring TF0 clr tr0 ;stop the process cpl p1.0 ;toggle bit 0 of port1 clr TF0 ;clear timer 0 flag bit sjmp Again ;go for next round END

  17. Generate a Large Time Delay • The size of the time delay depends on two factors: • The crystal frequency • The timer’s 16-bit register, TH & TL • The largest time delay is achieved by making TH=TL=0. • What if that is not enough?

  18. Large Time DelayExample Examine the following program and find the time delay in seconds. Exclude the overhead due to the instructions in the loop. org 0 mov TMOD,#10H mov R3, #200 again: mov TL1,#08 ;2 mov TH1,#01 ;2 setb TR1 ;1 back: jnb TF1,back ;65272 clr TR1 ;1 clr TF1 ;1 djnz R3, again ;2 end Solution: TH – TL = 0108H = 264 in decimal; 65536 – 264 = 65272. One of the timer delay = 65272 X 1 s = 65.272 ms Total delay = 200 X 65.272 ms = 13.054400 seconds

  19. Counter Programming • Programming is as same as timer • Only the difference is the source frequency • Timer = internal frequency • Counter = pulses from external input pins (T0/1) • C/T bit in the TMOD decides the source frequency

  20. Counter • Count the number of events • Show the number of events on registers • External input from T0 input pin (P3.4) for Counter 0 • External input from T1 input pin (P3.5) for Counter 1 • External input from Tx input pin. 8051 TH0 P1 to LCD TL0 P3.4 T0 a switch

  21. Counter Programming Example Assume that clock pulses are fed into pin T1, write a program for counter 1 in mode 2 to count and display the state of the TL1 count on P2. Org 00h Sjmp start Org 030h Start: mov TMOD, #60h ; Counter 1 mode 2, mov th1,#00h ;clear th1 setb P3.5 ;make T1 as input Count_again: setb tr1 ;start the counter Back: mov a, tl1 ;get copy of TL1 mov p2, a ;display it on port 2 jnb tf1, back ;keep monitoring timer1 flag bit clr tr1 ;stop timer clr tf1 ;make tf1 =0 (polling mode) sjmp Count_again end

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