lecture 12 counter_microcontroller2.ppt
- 2. 2 Timers /Counters Programming • The 8051 has 2 timers/counters: • timer/counter 0 • timer/counter 1 They can be used as 1. The timer is used as a time delay generator. – The clock source is the internal crystal frequency of the 8051. 2. An event counter. – External input from input pin to count the number of events on registers. – These clock pulses cold 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. Baud Rate Generator. 4/4/2011 microcontroller
- 3. 3 Registers Used in Timer/Counter TH0, TL0, TH1, TL1 TMOD (Timer mode register) TCON (Timer control register) microcontroller 4/4/2011
- 4. 4 Timer Special function register BIT-ADDRESSABLE ADDRESS PURPOSE Timer SFR No 89H Mode TMOD Yes 88H Control TCON No 8AH Timer0 Low-Byte TL0 No 8BH Timer1 Low-Byte TL1 No 8CH Timer0 High-Byte TH0 No 8DH Timer1 High-Byte TH1 Yes C8H Timer2 control T2CON No CAH Timer2 Low-Byte capture RCAP2L No CBH Timer2 High-Byte capture RCAP2H No CCH Timer2 Low-Byte TL2 No CDH Timer2 High-Byte TH2 8032/8052 4/4/2011
- 5. TMOD : Counter/Timer MODe Register GATE C/T M1 M0 GATE C/T M1 M0 Timer 1 Timer 0 microcontroller 5 MOV TMOD,#21H – An 8-bit register – Set the usage mode for two timers • Set lower 4 bits for Timer 0 (Set to 0000 if not used) • Set upper 4 bits for Timer 1 (Set to 0000 if not used) – Not bit-addressable 4/4/2011
- 6. 6 • 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). Timer or counter selected cleared for timer operation (input from internal system clock). Set for counter operation (input from Tx input pin). microcontroller 4/4/2011
- 7. 7 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 microcontroller 4/4/2011
- 8. Timer Modes • Timer Mode 0 (13-bit Timer): – Timer high-byte (THx) is cascaded with the 5 least- significant bits of the timer low-byte (TLx) to form a 13- bit timer, where x = 0 or 1. – Upper 3-bits of TLx are not used. – Overflow occurs on the 1FFFH-to-0000H and sets the timer overflow flag. – MSB is THx bit 7, and LSB is TLx bit 0. – MOV TMOD, #00H ; setting both timers to mode 0 Timer clock Overflow flag TLx 5-bits THx 8 -bits TFx Interrupt 7 0 4 0 microcontroller 8 4/4/2011
- 9. • Timer Mode 1 (16-bit Timer): – Same as mode 0 except that it is 16-bit. Timer high-byte (THx) is cascaded the timer low-byte (TLx) to form a 16- bit timer, where x = 0 or 1. – Clock is applied to the combined high and low-byte timer registers. – Overflow occurs on the FFFFH-to-0000H and sets the timer overflow flag. – MSB is THx bit 7, and LSB is TLx bit 0. – LSB toggles at clock frequency/21 and MSB at clock frequency/216 Timer clock Overflow flag THx 8-bits TFx Interrupt TLx 8-bits 7 0 7 0 microcontroller 9 4/4/2011
- 10. Timer clock Overflow flag THx 8-bits TFx Interrupt TLx 8-bits Trigger Reload • Timer Mode 2 (Auto-Reload): – Timer low -byte (TLx) operates as an 8-bit timer while the timer high -byte (THx) holds a reload value. – When the count overflows from FFH-to-00H, not only the timer flag set, but also the value in THx is loaded into TLx, and counting continues from this value up to next FFH-to-00H, so on. microcontroller 10 4/4/2011
- 11. • Timer Mode 3: – Timer 0 Splits into two 8-bit counter/timers. TL0 and TH0 act as two separate timers with overflows setting the TF0 and TF1 respectively. – Timer 1 (when timer 0 is in mode 3 ): Counter stopped if in mode 3 Can be used in mode 0, 1, or 2 Has gate (INT1) and external input (T1), but no flag or interrupt. May be used as a baud rate generator. Timer clock TH1 8-bits TL1 8-bits Overflow flag Timer clock TF0 Interrupt TL0 8-bits Overflow flag ÷12 Fosc. TF1 Interrupt TH0 8-bits microcontroller 11 4/4/2011
- 12. TCON : Counter/Timer CONtrol Register TR (run control bit) TR0 for Timer/counter 0; TR1 for Timer/counter 1. TR is set by programmer to turn timer/counter on/off. TR=0: off (stop) TR=1: on (start) TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 Timers Interrupts microcontroller 12 TF (timer flag, control flag) TF0 for timer/counter 0; TF1 for timer/counter 1. TF is like a carry. Originally, TF=0. When TH-TL rollover to 0000 from FFFFH, the TF is set to 1. TF=0 : not reach TF=1: reach If we enable interrupt, TF=1 will trigger ISR. 4/4/2011
- 13. 13 Equivalent Instructions for the Timer Control Register For timer 0 SETB 08CH= SETB TR0 = SETB TCON.4(start/enable timer) CLR TR0 = CLR TCON.4(stop/disable timer) SETB TF0 = SETB TCON.5 CLR TF0 = CLR TCON.5 For timer 1 SETB TR1 = SETB TCON.6 CLR TR1 = CLR TCON.6 SETB TF1 = SETB TCON.7 CLR TF1 = CLR TCON.7 TF1 IT0 IE0 IT1 IE1 TR0 TF0 TR1 TCON: Timer/Counter Control Register microcontroller 4/4/2011
- 14. Clocking Sources 1. Interval Timing: • If C/T = 0 (in TMOD), timer operation is selected and timer is clocked from on-chip oscillator. A divide-by-12 clock frequency is applied. • Timer registers (TLx/THx) increment at a rate of 1/12th the frequency of on-chip oscillator. • 12 MHz crystal would yield a clock rate of 1 MHz. • Timer overflows occur after a fixed number of clocks, depending on the initial value loaded into the timer registers. microcontroller 14 4/4/2011
- 15. Clocking Sources (contd.) 2. Event Counting: • If C/T = 1 (in TMOD), counter operation is selected and timer is clocked from external source(500KHZ). Usually, external source supplies the timer with a pulse upon the occurrence of an event. Timer counts those events. • External clock source comes through P3.4 (for Timer 0) and P3.5 (for Timer 1). • Timer registers are incremented in response to a 1-to- 0 transition at the external input. • Number of external events is determined in software by reading the timer registers TLx/THx. microcontroller 15 4/4/2011
- 16. Timer 1 Operating in 16-bit (Mode 1) Osc. ÷12 Osc. TL1 8-bits TF1 TH1 8-bits Interrupt Control T1 (P3.5) TR1 Gate INT1 (P3.3) C/T 8051 12MHz 0=Up, 1=Down Gate Disable/Enable INTx 0=Disable, 1=Enable 0=Open 1=Close microcontroller 16 4/4/2011
- 17. 17 1. Choose mode 1 timer 0 – MOV TMOD,#01H 2. Set the original value to TH0 and TL0. – MOV TH0,#FFH – MOV TL0,#FCH 3. You had better to clear the flag to monitor: TF0=0. – CLR TF0 4. Start the timer. – SETB TR0 Timer Mode 1 4/4/2011
- 18. 5. The 8051 starts to count up by incrementing the TH0-TL0. – TH0-TL0= FFCH,FFFDH,FFFEH,FFFFH,0000H FFFC FFFD FFFE FFFF 0000 TF = 0 TF = 0 TF = 0 TF = 0 TF = 1 TH0 TL0 Start timer Stop timer Monitor TF until TF=1 TR0=1 TR0=0 TF 6. When TH0-TL0 rolls over from FFFFH to 0000, the 8051 set TF0=1. TH0-TL0= FFFEH, FFFFH, 0000H (Now TF0=1) 7. Keep monitoring the timer flag (TF) to see if it is raised. AGAIN: JNB TF0, AGAIN 8. Clear TR0 to stop the process. CLR TR0 9. Clear the TF flag for the next round. CLR TF0
- 19. Reading a Timer “on The Fly” Two Timer register must be read a “phase error” may occur if the low-byte overflows into the high-byte between the two read operations. 1. Read the high-byte first 2. Read the low-byte 3. Read the high byte again, if the high-byte has changed, repeat the read operations AGAIN : MOV A, TH1 MOV R6, TL1 CJNE A ,TH1,AGAIN MOV R7,A 18 microcontroller 4/4/2011
- 20. Short and Long Intervals • Shortest possible interval is one machine cycle i.e., 1μs (using 12 MHz crystal frequency). • An 8-bit counter can give maximum delay of 256μs because 28=256. • A 13-bit counter can give maximum delay of 8192μs because 213=8192. • A 16-bit counter can give maximum delay of 65536μs because 216=65536. 65536μs = 0.065536 sec = 0.066 sec (approx.) microcontroller 19 • For more than 0.066 sec delay, there are two methods: 1. Cascade Timer 0 and Timer 1 but in this way both timers will be tied up. 2. Use one timer in 16-bit mode with a software loop counting overflows. 4/4/2011
- 21. Write a program using Timer 0 to create a 10 kHz square wave on P1.0.(crystal ferq=12MHZ). ORG 0000H MOV TMOD, #02H ;auto-reload mode MOV TH0, # -50 ;T= 1/10kHz=100µs SETB TR0 ;start timer LOOP: JNB TF0, LOOP ;wait for overflow CLR TF0 ;clear overflow flag CPL P1.0 ;toggle port bit SJMP LOOP ;repeat END 20 Example: 1 microcontroller 4/4/2011
- 22. Write a program using Timer 0 to create a 1 kHz square wave on P1.0. ORG 0000H MOV TMOD, #01H ;16-bit mode LOOP: MOV TH0, # FEH ;T= 1/1kHz=1000µs MOV TL0, # 0CH ;-500=FE0CH SETB TR0 ;start timer WAIT: JNB TF0, WAIT ;wait for overflow CLR TR0 ;stop timer CLR TF0 ;clear overflow flag CPL P1.0 ;toggle port bit SJMP LOOP ;repeat END microcontroller 21 Example: 2 4/4/2011
- 23. Ex4-4: Buzzer Interface Write a program that sounds the buzzer connected at P1.7 for 1 sec for each 1-to-0 transition detected at P1.6. ORG 0000H MOV TMOD, #01H ;16-bit mode LOOP: JNB P1.6, LOOP ;wait for 1 input LOOP1: JB P1.6, LOOP1 ;wait for 0 input SETB P1.7 ;turn buzzer ON CALL DELAY ;wait for 1 sec CLR P1.7 ;turn buzzer OFF SJMP LOOP ;repeat DELAY: MOV R7, #100 ;for 100 loops AGAIN: MOV TH0, #D8H ;10000µsX100=1sec MOV TL0, #F0H ;-10000=D8F0H SETB TR0 WAIT: JNB TF0, WAIT CLR TF0 CLR TR0 DJNZ R7, AGAIN RET END microcontroller 22 4/4/2011