Chapter 5 PIC18 Timers
• Counter is a register that can be loaded with a binary number (count) which can be incremented per clock/instruction cycle.
• We can use this counter to measure time.
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• Time is calculated as follows:
– Find difference between beginning count and last count
– Multiply the count difference by the clock/instruction period
Counters for External Events
• The register can also be used as a counter for external events by replacing the clock with a signal from an event.
• When a signal from an event arrives, the count in the register is incremented (or decremented); thus, the total number of events can be counted.
Timer Applications
• Time delay
• Pulse wave generation
• Timer as an event counter • Frequency measurement
Application: How long you have travelled by a bicycle?
= No. of times the wheel rotates × Wheel circumference
= 𝑁𝑜.𝑜𝑓 𝑠𝑞𝑢𝑎𝑟𝑒 𝑠𝑖𝑔𝑛𝑎𝑙 𝑝𝑒𝑟𝑖𝑜𝑑𝑠 × 3
Wheel circumference http://www.cs.ou.edu/~fagg/classes/es_general/timers.pdf
PIC18 Timers
• The PIC18 microcontroller has multiple timers
• Timers are divided into two groups: 8-bit and 16- bit
• Labeled as Timer 0 to Timer 2
– Timer 0 can be set up as an 8-bit or 16-bit timer.
– Timer 1 is a 16-bit timer.
– Timer 2 is an 8-bit timer.
• EachtimerassociatedwithitsSpecialFunction Register (SFR): T0CON-T2CON
• Can be set up as an 8-bit or 16-bit timer
– Parameters are set up by bits in T0CON register
– Has eight options of pre-scale values (Bit2-Bit0)
– Can run on internal clock source (instruction cycle) or external clock connected to pin RA4/T0CK1
– Generates an interrupt or sets a flag when it overflows from FFH to 00H in the 8-bit mode and from FFFFH to 0000H in the 16-bit mode
– Can be set up on either rising edge or falling edge (Bit4) when an external clock is used
T0CON (Timer 0 Control) Register
T0CON (Timer 0 Control) Register
• TMR0ON = T0CON<7>: On (1) or Off (0)
• T08BIT = T0CON<6>: 8-bit (1) or 16-bit (0)
• T0CS = T0CON<5>: Use External (1) or Internal (0) clock
• T0SE = T0CON<4>: Increment on H-L (1) or L-H (0) transition
• PSA = T0CON<3>: Use (0) or Do not use (1) prescaling
• T0PS2:T0PS0 = T0CON<2:0>: Specify prescaler factor
Internal Clock Mode (T0CS = 0)
• Clock source is the instruction cycle clock
• The Timer 0 Interrupt Flag (TMR0IF = INTCON<2>) is raised when counter rollover from max value (FFFF in 16-bit mode, FF in 8-bit mode) to min value (0000 in 16-bit mode, 00 in 8-bit mode)
• Used as a timer to create delay
Create delay using Timer 0 16-bit Mode
• Characteristics of 16-bit mode
– 16-bit timer – allows values of 0000 to FFFF to be loaded into registers TMR0H and TMR0L
– The timer must be started by setting the TMR0ON (i.e., bsf T0CON, TMR0ON)
– The timer counts up after being started. When it rolls over from FFFF to 0000, it sets, the interrupt flag, TMR0IF.
Steps to create delay
1. Set T08BIT = T0CON<6> = 0 (use 16-bit mode)
2. Load register TMR0H followed by TMR0L with initial count values
3. Start the timer: bsf T0CON, TMR0ON
4. Keep monitoring TMR0IF to see whether it is raised.
Get out of loop if raised.
5. Stop the timer: bcf T0CON, TMR0ON
6. Clear TMR0IF for next round
7. Repeat from Step 2
Create delay using Timer 0 16-bit Mode
• TMR0 increments once each instruction cycle.
• Time delay obtained depends on the initial value.
• We stop when counter overflow, which is indicated
by the TMR0IF flag.
Simple Example
Create a square wave of 50% duty cycle on RB5 bit.
bcf TRISB, 5
bsf PORTB, 5
movlw B ‘00001000’
movwf T0CON
Here movlw 0xFF
movwf TMR0H
movlw 0xF2
movwf TMR0L
bcf INTCON, TMR0IF
bsf T0CON, TMR0ON
Again btfss INTCON, TMR0IF bra Again
bcf TOCON, TMR0ON
btg PORTB, 5
bra Here
; Set PB5 as output
; init. sq. wave to high (1)
; 16-bit, int clk, no prescaler
; [TMR0H] = 0xYY
; [TMR0L] = 0xXX
; Clear timer interrupt flag
; start Timer 0
; load TMR0H and TMR0L again
Duty Cycle
Time Delay Calculation
• Fosc = clock frequency = 4MHz
• Instruction Cycle = 1μs
• Instruction Cycle required to reach FFFF = FFFF-FFF2 = 13 (in decimal)
• Need one more instruction cycle to rollover to 0000 where TMR0IF is triggered.
• Total time delay generated by Timer 0= 14 x 1μs = 14μs
Formula to Compute Time Delay
• In Hex, total time delay generated by Timer 0 = (FFFF – YYXX + 1) x 1μs
where YY and XX are the initial values of TMR0H and TMR0L respectively.
• In decimal, convert YYXX into its decimal equivalence NNNNN first and then use: (65536 – NNNNN) x 1μs
Use of Prescaler
• Longest delay that can be produced is 65.536 ms using a 16-bit timer. What if we want longer delay?
• Prescaler can be used to increase period of internal clock by a factor of 2N, where N = 1, …, 8 and specified by T0PS2:T0PS0 = T0CON<2:0>.
• e.g., When N = 6, the period of the clock is increased by a factor of 64→64μs. Max delay = 65536 x 64 μs = 4.194 s
Assume Fosc = 4MHz and Instruction Cycle = 1MHz (FFFF – FC84 + 1) x 1μs = 892 μs
Initial TMR0
1 (no prescaler) 2
Time Delay (μs) 892
7,136 14,272 28,544 57,088 114,176
Create delay using Timer 0 8-bit Mode
8-bit Mode
16-bit Mode
Timer 0 as counter
• If driven by an external clock (T0CS = 1), Timer 0 will increment, either on every rising (T0SE = 0) or falling (T0SE = 1) edge of pin RA4/T0CKI.
Assuming clock pulses are fed into Pin T0CKI (RA4), use Timer 0 as a 8-bit counter of pulses and display the state of TMR0L count on PORTB
bsf TRISA, RA4; PORTA.4 set as input
clrf TRISB; PORTB set as output
movlw 0x68; 01101000: Timer 0, 8-bit, ext clk, no prescale movwf T0CON
HERE movlw 0x00
movwf TMR0L
bcf INTCON, TMR0IF
bsf T0CON, TMR0ON
AGAIN movff TMR0L, PORTB
btfss INTCON, TMR0IF bra AGAIN
bcf T0CON, TMR0ON goto HERE
8-bit or 16-bit timer or counter.
Supports prescaling factors of 1, 2, 4, 8, 16, 32, 64, 128, 256.
Timer 0 counts clock pulses fed into RA4
When counter rolls over, TMR0IF in INTCON SFR is raised
16-bit timer or counter.
Supports prescaling factors of 1, 2, 4 and 8.
Timer 1 counts clock pulses fed into RC0
When counter rolls over, TMR1IF in PIR1 SFR is raised
Lab 4 Task 2: Frequency Counter
Output signal
Variable Frequency Generator: Output frequency depends on the charging/discharging time of C1, which is controlled by varying the resistor value R2 through turning a knob on the circuit board
Lab 4 Task 2: Frequency Counter
Write a subroutine MeasureFrequency to perform the following operations:
• Set up Timer 0 to generate a 1-second delay
• Use Timer 1 as a counter to count the rising edges of an unknown signal (fed to RC0) within the second
• Start Timer 0 and Timer 1
• Wait for IF flag for Timer 0 to raise
• Stop Timer 0 and Timer 1
• TMR1H:TMR1L would then be the frequency of the signal.
Overall structure of Lab 4
The algorithm consists of two components:
1. Generate a time delay of 1s using Timer 0
– Lab 4 Task 1
– Prescaler = 16
– Expected total time delay = (FFFF-YYXX+1) x 16 x
– You will determine YYXX in prelab, but you know
that (FFFF-YYXX+1) = 62500 (i.e., Timer 0
increments 62500 times before overflow)
– Does the program generate a delay of exactly 1s?
2. Count total number of rising edges using
– The example code gives all information of how it
can be done.
Overall structure of Lab 4
Lab 4 Task 1
Lab 4 Task 1
Total Time Delay
bsf PORTD, RD0
bcf INTCON, TMR0IF
1,000,000 inst. cycles = 1s
bsf T0CON, TMR0ON
WaitT0done: btfss INTCON, TMR0IF
999999 + 2
• 333333 repetitions x 3 inst. cycle/ reps → TMR0
overflows →
TMR0IF = 1 • btfss skips
(additional 2 inst. cycles
bra WaitT0done
bcf PORTD, RD0
Total time LED has turned on (inst. cycles)
Lab 4 Task 1
• Need exact time delay for accurate frequency measurement
• Solution
– Determine the initial value YYXX again so that Timer 0 increments 62499 times (instead of 62500) before overflow.
– Time delay generated before TMR0IF is raised = 62499 x 16 inst. cycles
– Addafewnoptofillupthetime.
– Objective: Decide how many nops are needed to generate a time delay of exactly 1s.
Overall structure of Lab 4
• Validation of your measurement
– Use debugger mode of PICKit 3. Set a
breakpoint at the location where you exit the subroutine MeasureFrequency.
– TMR1H and TMR1L store your frequency measurement.
• Can only be used as a timer to create time delay – No external clock source can fed in.
• Prescaler and Postscaler to lengthen delays.
• Prescaler = 4
• Postscaler = 4
T2CON: Timer 2 Control Register
TMR2 and PR2 SFR
• TMR2: 8-bit register of Timer 2
• PR2 (Period Register) is set to be a fixed value.
• The input period can be lengthened by selecting the prescaling factor.
• When TMR2 == PR2, EQ signal will reset TMR2 to 0.
• The output of the comparator is divided by postscaling factor.
– i.e., If postscaler = 1:16, the EQ signal will need to be set 16 times before TMR2IF is set.
Example 1: Timer 2
• Write a program to turn on pin PORTB.4 when TMR2 reaches the value of 100 (decimal).
bcf TRISB, 4; make PORTB.4 as output bcf PORTB, 4; turn off PORTB.4 movlw 0x00
movwf T2CON; no prescale and postscale movlw 0x00
PORTB, 4 T2CON, TMR2ON Here
movwf TMR2
movlw D‘100’
bcf PIR1, TMR2IF bsf T2CON, TMR2ON btfss PIR1, TMR2IF bra Again
Example 2: Timer 2
• Use the prescaler and postscaler to create the longest possible delay by Timer 2.
• Total delay = (255+1) x 16 x 16 x 1μs = 65.536ms
Again: Here:
bcf TRISB, 4
bcf PORTB, 4
movlw B‘01111011’
movwf T2CON; prescale = 1:16, postscale = 1:16 movlw 0x00
movwf TMR2
movlw D‘255’
bcf PIR1, TMR2IF bsf T2CON, TMR2ON btfss PIR1, TMR2IF bra Again
bsf PORTB, 4
bcf T2CON, TMR2ON bra Here
Timer 0, Timer 1 and Timer 2
8-bit or 16-bit timer or counter.
Supports prescaling factors of 1, 2, 4, 8, 16, 32, 64, 128, 256.
Timer 0 counts clock pulses fed into PORTA.4
When counter rolls over, TMR0IF in INTCON SFR is raised
16-bit timer or counter.
Supports prescaling factors of 1, 2, 4 and 8.
Timer 1 counts clock pulses fed into PORTC.0
When counter rolls over, TMR1IF in PIR1 SFR is raised
8-bit timer. Cannot be used as counter
Supports prescaling and postscaling factors.
Cannot be used as counter
When counter rolls over, TMR2IF in PIR1 SFR is raised
Time Delay Equation
• Time delay are expressed in instruction cycles.
• Timer 0/1 16-bit mode
– Time Delay = (FFFF – YYXX + 1) x Prescaler
– where YY and XX are the initial values of TMRxH and TMRxL, respectively. x = 0, 1.
• Timer 0 8-bit mode
– Time Delay = (FF – XX + 1) x Prescaler
– Time Delay = (PR2+1) x Prescaler x Postscaler
You should be able to …
• List the PIC18 timers and their associated registers.
• Describe various modes of the PIC18 timer.
• Program the PIC18 timers to generate time delays
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