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Timers ELEC 330 Digital Systems Engineering Dr. Ron Hayne

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1 Timers ELEC 330 Digital Systems Engineering Dr. Ron Hayne
Images Courtesy of Ramesh Gaonkar and Delmar Learning ELEC 330

2 Hardware Counters and Timers
Counter (register) Can be decremented or incremented per clock cycle Time calculation Find difference between beginning count and last count Multiply the count difference by the clock period 330_11

3 Hardware Counters and Timers
Counter (register) Replace the clock with a signal from an event Count is incremented (or decremented) Total number of events can be counted 330_11

4 Types of Counters Up-counter
Counter is incremented at every clock cycle When count reaches the maximum count, a flag is set Counter can be reset to zero or to the initial value Down-counter Counter is decremented at every clock cycle When count reaches zero, a flag is set Counter can be reset to the maximum or the initial value Free-running counter Counter runs continuously and only readable When it reaches the maximum count, a flag is set 330_11

5 Timer Applications Time delay Pulse wave generation
Pulse width or frequency measurement Timer as an event counter 330_11

6 Capture, Compare, and PWM (CCP) Modules
CCP modules are common in recent MCUs Registers specially designed to perform the following functions (in conjunction with timers) Capture: The CCP pin can be set as an input to record the arrival time of a pulse. Compare: The CCP pin is set as an output, and at a given count, it can be driven low, high, or toggled. Pulse width modulation (PWM): The CCP pin is set as an output and the duty cycle of a pulse can be varied. 330_11

7 Pulse Width Modulation
Duty cycle Percentage ratio of on time of a pulse to its period Changing of the duty cycle is defined as PWM CCP pin is set as an output Count for period and duty cycle loaded into CCP registers Varying the duty cycle generates PWM 330_11

8 PIC18 Timers PIC18 timers Timer0 Timer1 (and Timer3)
All up-counters 8-bit and 16-bit Timer0 8-bit or 16-bit timer Timer1 (and Timer3) 16-bit timers Timer2 (and Timer4) 8-bit timer SFRs T0CON-T2CON 330_11

9 Timer0 8-bit or 16-bit timer Readable and writable
Parameters in T0CON register Eight pre-scale values (Bit2-Bit0) Clock source (Bit5) Internal (instruction cycle) External clock connected to pin RA4/T0CK1 Rising edge or falling edge (Bit4) Generates an interrupt or sets a flag when it overflows TMR0IF Flag must be cleared to start the timer again 330_11

10 Timer0 T0CON 330_11

11 Timer1 16-bit counter/timer Two 8-bit registers (TMR1H and TMR1L)
Four prescale values (Bit5-Bit4) Clock source (Bit1) Internal (instruction cycle) External (pin RC0/T13CK1) on rising edge 330_11

12 Timer1 Interrupt Resetting Timer1 using CCP module
Generates an interrupt or sets a flag when it overflows TMR1IF Flag must be cleared to start the timer again Resetting Timer1 using CCP module CCP1 in the Compare mode Timer1 and CCP1compared at every cycle When a match is found, Timer1 is reset 330_11

13 Timer2 8-bit timer (TMR2) 8-bit period register (PR2)
TMR2 and PR2 are readable and writable Three prescale values (Bit1-Bit0) 16 postscale values (Bit6-Bit3) Flag (TMR2IF) is set when TMR2 matches PR2 Can generate an interrupt 330_11

14 Timer2 Timer2 operation 8-bit number is loaded in PR2
When TMR2 and PR2 match Output pulse is generated and the timer is reset Output pulse goes through postscaler Sets the flag TMR2IF 330_11

15 CCP Modules Capture, Compare, and Pulse Width Modulation (PWM)
CCPR1H (high) and CCPR1L (low) 16-bit Capture register 16-bit Compare register Duty-cycle PWM register Timer1 used as clock for Capture and Compare Timer2 used as clock for PWM 330_11

16 CCP in the Capture Mode CCPR1 captures the 16-bit value of Timer1
When an event occurs on pin RC2/CCP1 Interrupt request flag bit CCP1IF is set Must be cleared for the next operation To capture an event Set up pin RC2/CCP1 of PORTC as the input Initialize Timer1 T1CON register Initialize CCP1 CCP1CON register 330_11

17 CCP in the Compare Mode CCPR1 constantly compared with TMR1
When a match occurs Pin RC2/CCP1 on PORTC Driven high, low, or toggled Interrupt flag bit CCP1IF is set To set up CCP1 in Compare mode Set up pin RC2/CCP1 of PORTC as output Initialize Timer1 and CCP1 Clear the flag CCP1IF 330_11

18 PWM Mode CCP module with Timer2 Duty cycle Period
Output a pulse wave form for a given frequency/duty cycle Duty cycle CCPR1 register Period PR2 register When TMR2 is equal to PR2 TMR2 is cleared Pin RC2/CCP1 of PORTC is set high PWM duty-cycle byte loaded into CCPR1 330_11

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