1. Timers
Chapter 2

2. Objectives Upon completion of this chapter, you will be able to:
List the timers of the PIC18 and their associated registers
Describe the various modes of the PIC18 timers
Program the PIC18 counters in C as event counters

3. Timer0
16-bit wide
Consists of a low-byte (TMR0L) and a high-byte (TMR0H) register
Can be used as 8-bit or 16-bit timer
Low byte (8-bit)
High byte (8-bit)

4. Timer0 (cont’d) Important Registers:
i) T0CON (Timer0 Control Register)
To start & stop Timer0 and other configurations
ii) TMR0H:TMR0L (for counting purposes)
Act as counting buffer
iii) INTCON (Interrupt Control Register) D0 D7

5. T0CON (Timer0 Control) Register

6. Example 1
What is the value of T0CON if the Timer0 settings are as below ?
16-bit timer
No pre-scaler
Internal clock (from oscillator) source
Increment on positive-edge
Answer:
T0CON =

7. Example 2
Calculate the amount of time delay generated by the Timer0 with the following specification:
XTAL = 10MHz
TMR0H:TMR0L = FFF2H
Solution:
TCY = 4/10MHz = 0.4us (Each tick consume 0.4us)
How many tick? (FFFF-FFF2) + 1 = 14 ticks
Time delay = 14 x 0.4us = 5.6us
FFF2
FFF3
FFF4
FFFE
FFFF
0000
TMR0IF=0
TMR0IF=1 1 2 14 13

8. Example 3
Write a C program to toggle all the bits of PORTB continuously
with 1ms delay. Use Timer0, 16-bit mode, no prescaler options
to generate the delay. (Assume XTAL=20MHz)
Solution:
TCY = 4/20MHz = 0.2us (Each tick consume 0.2us)
How many ticks in 1ms delay?
1ms/0.2us = 5000 ticks = 1388H ticks!
To find register value for TMR0H:TMR0L
FFFF - register value + 1 = 1388H ticks
register value = EC78H @
Simply take the negative value of the tick counts
-1388H = EC78H
This is much more easier!

9. Example 3 (cont’d)

10. Exercise 1
Write a C program to toggle only the PORTB.4 bit continuously
every 50ms. Use Timer0, 16-bit mode and the 1:4 prescaler to
create the delay. (Assume XTAL = 20MHz)
Solution:
TCY = 4/20MHz = 0.2us (Each tick consume 0.2us)
How many ticks in 50ms delay?
50ms/0.2us = ticks = 3D090H ticks! (out of range!!)
With 1:4 prescaller
250000/4 = ticks = F424H ticks!
Therefore, register counts = -F424H = 0BDCH

11. Exercise 1 (cont’d)

12. Timer1
16-bit wide
Consists of a low-byte (TMR1L) and a high-byte (TMR1H) register
Can be used as 16-bit timer only!
Low byte (8-bit)
High byte (8-bit)

13. Timer1 (cont’d) Important Registers:
i) T1CON (Timer1 Control Register)
To start & stop Timer1 and other configurations
ii) TMR1H:TMR1L (for counting purposes)
Act as counting buffer
iii) PIR1 (Peripheral Interrupt Request Register 1) D7

14. T1CON (Timer1 Control) Register

15. Exercise 2
Write a C program to create pulses with a frequency of 2500Hz
with 50% duty cycle on pin PORTB.1. Use Timer1 to create the
delay. (Assume XTAL = 20MHz)
Solution:
T = 1/2500 = 400us (HIGH: 200us; LOW: 200us)
How many ticks in 200us delay?
200us/0.2us = 1000 ticks = 03E8H ticks!
Therefore, register counts = - 03E8H = FC18H

16. Exercise 2 (cont’d)

17. Exercise 2 (cont’d)
Approx.1000 ins. cycles
Approx.1000 ins. cycles

18. Timer0 & Timer1 as Counter
Can used as Counters
Counter0 (Timer0 counter):
Count pulses on T0CKI (RA4) pin
Counter1 (Timer1 counter):
Count pulses on T13CKI (RC0) pin

19. Example - Counter
Please refer Example 9-35, 9-36 & 9-37 in the textbook

20. Timer2 8-bit wide Consists of a PR2 (Period Register 2)
TMR2 will increment from 00 until reaches PR2 value before TMR2IF flag is set
Consists of prescaler and postscaler
No counter function

21. Timer2 (cont’d) Important Registers:
i) T2CON (Timer2 Control Register)
To start & stop Timer2 and other configurations
ii) PR2 (to set the counting value)
If TMR2 = PR2; TMR2IF flag is set
iii) PIR1 (Peripheral Interrupt Request Register 1) D7

22. Timer2 (cont’d)

23. Timer3
16-bit wide
Consists of a low-byte (TMR3L) and a high-byte (TMR3H) register
Enable the CCP Mode for PWM Application

24. Timer3 (cont’d) Important Registers:
i) T3CON (Timer3 Control Register)
To start & stop Timer3 and other configurations
ii) TMR3H:TMR3L (for counting purposes)
Act as counting buffer
iii) PIR2 (Peripheral Interrupt Request Register 2)

25. Timer3 (cont’d)

26. End of Chapter 2
Assignment Timer