PIC18 Timer programming in C

The PIC18 timer is divided into 4 types

Timer 0
Timer 1
Timer 2
Timer 3

PIC18 timers can be used to generate a time delay or as a counter to count external event happening outside the microcontroller.

In this article, we will see how to generate a time delay by programming the PIC18 timer.

Timer 0

The timer 0 module has the following features

Software is scalable as an 8 bit or 16-bit timer/counter.
Readable and writable
Dedicated 8 bit software programmable Prescaler
Clock source selectable to be internal or external
Edge select for external clock

Register required for Timer 0

Control register

Each timer has a control register called TCON to set the various timer operation modes.

T0CON is an 8-bit register used for control of timer 0.

TOCON

TMR0ON (Timer0 on/off control bit)

1 = Enable timer 0

0 = Disable timer 0

T08BIT (Timer0 8-bit/16-bit control bit)

1 = Configured as an 8-bit timer/counter

0 = Configured as an 16-bit timer/counter

T0CS (Timer0 clock source select bit)

1 = Transition on T0CKL pin

0 = Internal instruction cycle clock (CLK0)

T0SE (Timer0 Source Edge select bit)

1 = High to Low transition on T0CKL pin

0 = Low to High transition on T0CKL pin

PSA (Timer0 Prescaler assignment bit)

1 = Timer 0 Prescaler is not assigned

0 = Timer 0 Prescaler is assigned

T0PS2 - T0PS0 (Timer0 prescaler select bit)

111 - 1:256 prescaler value

110 - 1:128 prescaler value

101 - 1:64 prescaler value

100 - 1:32 prescaler value

011 - 1:16 prescaler value

010 - 1:8 prescaler value

001 - 1:4 prescaler value

000 - 1:2 prescaler value

Load register

TMR0H (higher byte)

TMR0L (lower byte)

FLAG bit

TMR0IF (present in INTCON register)

Delay calculation for timer 0 without Prescaler value

The formula for the delay is

Delay = time period of 1 clock X max value X count

In PIC:

1 machine cycle = 4 clock

XTAL freq of PIC = 20MHz

So Frequency for I clock = 20/4 = 5MHz

Hence, time period for 1 clock = 1/5 MHz = 0.2us

Max. Value:

For 8 bit = 256

For 16 bit = 65536

Hence the modified formula for delay is

Delay = 0.2 X 10^-6 X 65536 or 256 X count

Delay calculation for timer 0 (with Prescaler)

Suppose the Prescaler ratio is 1:4 (obtained from T0PS2 - T0PS0)

So, the frequency for 1 clock = (20/4)/4 = 1.25 MHz

time period for 1clock = 1/1.25 MHz = 0.8us

Hence, the modified formula for the delay is

Delay = 0.8 X 10^-6 X 65536 or 256 X count

While calculating the count value if the value of count is less than 1 then

Delay = 0.25 X 10^-6 X count

After getting the count value

Initial count value = Max value (65536 or 256) - count

Then convert the initial count value into the HEX value and then load the MSB to TMR0H and LSB to TMR0L.

Example 1 -

Generate a delay on RC0 of 1 second, 16-bit operation, XTAL frequency = 20MHz (without Prescaler)

Solve

1 = 0.2 X 10^-6 X 65536 X count

count = 76.26

hence count = 77

Code

#include <xc.h>

void delay_1sec(int count);

void main()

{

TRISC0=0;

while(1)

{

delay_1sec(77);

RC0=~RC0;

}

void delay_1sec(int count)

{

T0CON = 0x08;

unsigned int i;

for(i=0;i<count;i++)

{

TMR0H=0;

TMR0L=0;

TMR0ON=1;

while(TMR0IF==0);

TMR0ON=0;

TMR0IF=0;

}

PIC18 Timer programming in C

Timer 0