Timer Programming

Timer Programming

• Steps for Programming PIC18F458 Timer

1. Configure the Timer Control Register.

2. Clear Timer interrupt flag.

3. Load the count in Timer register.

4. Set Timer ON bit to start the Timer operation.

5. Wait for Timer interrupt flag to become 1. Timer interrupt flag bit = 1, indicates the occurrence of the timer overflow.

 

Example 18.15.1 Write a PIC18 assembly program to generate a square wave with period of 10 ms on pin PORTB.5. Assume XTAL = 10 MHz.

Solution: Given: Fosc = 10 MHz, Period = 10 ms.

BCF TRISB, 5 ; Configure PB5 as an output

MOVLW 08H ; Timer0, 16-bit, int clk, no prescalar

MOVWF T0COM ; Load bit pattern in T0CON register

BACK MOVLW CFH ; W = CFH (Higher Byte)

MOVWF TMR0H ; Load Higher byte in TMR0H

MOVLW 2CH   ; W = 2CH (Lower Byte)

MOVWF TMR0L  ; Load Lower byte in TMR0L

BCF INTCON, TMR0IF ; Clear Timer interrupt flag bit

BSF TOCON, TMR0ON ; Start Timer0

HERE   BTFSS INTCON, TMROIF BRA HERE ; Check Timer0 flag until

BRA HERE ; it rolls over

BCF TOCON, TMR0ON ; Stop Timer0

BTG PORTB, 5 ; Toggle PB5

BRA BACK ; Repeat

 

Example 18.15.2 Write a PIC18 assembly program to generate a square wave of 1 Hz on pin PORTB.5. Assume XTAL = 10 MHz. Use Timer0, 16-bit mode, with prescalar = 256.

Solution:

The value to be loaded in 16-bit Timer register is :

65536-Count = 65536 - 4883 = 60653 = ECEDH

TOCON Configuration

BCF TRISB, 5 ; Configure PB5 as an output

MOVLW 07H  ; Timer0, 16-bit, int clk, prescalar 1: 256

MOVWF TOCOM ; Load bit pattern in T0CON register

BACK   MOVLW ECH ; W = ECH (Higher Byte)

MOVWF TMR0H ; Load Higher byte in TMR0H

MOVLW EDH  ; W EDH (Lower Byte)

MOVWF TMR0L ; Load Lower byte in TMR0L

BCF INTCON, TMR0IF  ; Clear Timer interrupt flag bit

BSF T0CON, TMR0ON  ; Start Timer0

HERE  BTFSS  INTCON, TMR0IF ; Check Timer0 flag until

BRA HERE  ; it rolls over

BCF T0CON, TMR0ON  ; Stop Timer0

BTG PORTB, 5  ; Toggle PB5

BRA BACK  ; Repeat

 

Example 18.15.3 Write a C18 program to toggle all bits of Port B continuously with delay of 20 ms using Timer 0, 16 bit mode and 1:8 prescaler. Assume XTAL = 10 MHz.

Solution: Given: F_OSC = 10 MHz. Prescaler = 1:8

#include <P18F458.h>

Void T0Delay(void);

{

TRISB = 0;    // configure Port B as output

While(1)

{

PORTB = 0x55;    // Load bit patterns

T0Delay ();

PORTB = 0xAA;

T0Delay ();

}

}

void T0Delay ( )

{

T0CON=0x02;   // Timer0, 16 bit mode, 1:8 prescaler

TMR0H=0xE7;  // Load Higher byte in TMROH

TMR0L= 0x96;  // Load Lower byte in TMR0L

T0CONbits.TMR0ON=1;  // Start the Timer0

while(INTCONbits. TMR0IF==0); // Check for overflow

T0CONbits.TMR0ON=0;  // Turn off Timer0

INTCONbits.TMR0IF==0;  // Clear the Timer0 interrupt flag

}

 

Example 18.15.4 Write a C18 program to toggle only the RC5 bit continuously every 50 ms using Timer 0, 16 bit mode and 1: 4 prescaler. Assume XTAL = 10 MHz.

Solution: Given: F_OSC = 10 MHz. Prescaler =  1 : 4

#define PORTBit PORTCbits.RC5

void main(void)

{

TRISCbits.TRISC5=0;  // configure pin 5 of Port C as output

while(1)

{

PORTBit ^ = 1;   // Toggle the bit RC5

T0Delay ();        // Wait for 50 ms

}

}

void T0Delay ()

{

T0CON=0x01;  // Timer0, 16 bit mode, 1:4 prescaler

TMR0H=0x85;  // Load Higher byte in TMR0H

TMR0L= 0xEE;  // Load Lower byte in TMR0L

T0CONbits. TMR0ON=1;  // Start the Timer0

while(INTCONbits. TMR0IF==0);  // Check for overflow

T0CONbits.TMR0ON=0;   // Turn off Timer0

INTCONbits.TMR0IF==0;   // Clear the Timer0 interrupt flag

}

 

Example 18.15.5 Write a C18 program to toggle only the RB4 bit continuously every 10 ms using Timer0, 8-bit mode. Assume XTAL = 10 MHz.

Solution: Given: F_OSC = 10 MHz. Assume: Prescaler = 1 : 1

F_TIMER = F_OSC  / 4 × Prescaler ratio = 10 / 4 1 × 1 / 1 = 2.5 MHz

Period = 1 / F_TIMER = 1 / 2.5 MHz = 0.4 μs

Count = Desired Delay / Timer Period = 10 ms / 0.4 µs = 25000

For 8-bit timer, prescaler ratio > = 25000/256 = 97.66

ஃ Prescaler = 1 : 128

Count (with Prescaler = 1 : 128) 1: 128) = 25000/128 25000/128 = 195.3125 ≈ 195

The value to be loaded in 8-bit Timer register is :

256 - Count = 256 - 195 = 61 3DH

T0CON Configuration

#include <P18F458.h>

void T0Delay(void);

#define PORTBit PORTBbits.RB4

void main(void)

{

TRISBbits.TRISB4=0;   // configure pin 4 of Port B as output

while(1)

{

PORTBit ^ = 1;   // Toggle the bit RB4

T0Delay();        // Wait for 10 ms

}

}

void TODelay ( )

{

T0CON=0x46;    // Timer0, 8 bit mode, 1:128 prescaler

TMR0L= 0x3D;   // Load byte in TMROL

TOCONbits.TMR0ON=1;   // Start the Timer0

while(INTCONbits. TMR0IF==0); // Check for overflow

T0CONbits.TMR0ON=0;   // Turn off Timer0

INTCONbits.TMR0IF==0;  // Clear the Timero interrupt flag

}

Note: When maximum prescalar count is not sufficient to produce desired delay we can call Delay subroutine multiple times. See following example.

 

Example 18.15.6 Write a C18 program to toggle only the RB4 bit continuously every 100 ms using Timer0, 8-bit mode. Assume XTAL = 10 MHz.

Solution: Given: F_OSC = 10 MHz. Assume: Prescaler = 1:1

F_TIMER = F_OSC / 4 × Prescaler ratio = 10/4 × 1/1 = 2.5 MHz

Period = 1 / F_TIMER = 1 / 2.5 MHz = 0.4 µs

Count = Desired Delay / Timer Period = 100 ms / 0.4 µs = 250000

For 8-bit timer, prescaler ratio >= 250000 / 256 = 970.66

Here, we choose maximum prescaler

ஃ Prescaler = 1: 256

Thus, the delay subroutine must be called 4 times so that 4 × 256 > 970.66.

Count (with Prescaler = 1: 256) = 250000/(256 × 4) = 244.14 = 244

The value to be loaded in 8-bit Timer register is :

256 - Count = 256 - 244 = 12 = 0CH

T0CON Configuration

#include <P18F458.h>

void T0Delay(void);

#define PORTBit PORTBbits.RB4

void main(void)

{

TRISBbits.TRISB4=0;  // configure pin 4 of Port B as output

while(1)

{

PORTBit ^ = 1;  // Toggle the bit RB4

for (i = 0; i < 4; i++)

T0Delay();   // Wait for 25 ms

}

}

void T0Delay()

{

T0CON=0x47; // Timer0, 8 bit mode, 1:256 prescaler

TMR0L= 0x0C;  // Load byte in TMR0L

T0CONbits.TMR0ON=1; // Start the Timer0

while(INTCONbits.TMR0IF==0); // Check for overflow

T0CONbits.TMR0ON=0;  // Turn off Timero

INTCONbits.TMR0IF==0;  // Clear the Timero interrupt flag

}

Actual Delay :

F_TIMER = F_OSC / 4 × Prescaler ratio = 10 / 4 × = 9.765625 KHz

Period = 1 / F_TIMER = 1 / 9.765625 KHz = 102.4 µs

Delay subroutine execution time = 244 × 102.4 µs = 24.9856 ms

Since delay subroutine is called 4 times we get delay of 99.9424 ms

 

Example 18.15.7 Write a C18 program to create frequency of 5KHz on pin RC5 using Timer1. Assume XTAL 10 MHz.

Solution: Given: F_OSC = 10 MHz. No Prescaler

The value to be loaded in 16-bit Timer register is :

65536-Count = 65536 - 250 = 65286 = FF06H

#include <P18F458.h>

void T1Delay(void);

#define PORTBit PORTCbits.RC5

void main(void)

{

TRISCbits.TRISC5=0;  // configure pin 5 of Port C as output

while(1)

{

PORTBit ^  = 1; // Toggle the bit RC5

T1Delay (); // Wait for 50 ms

}

}

void T1Delay()

{

T1CON=0x00;  // Timer1, 16 bit mode, no prescaler

TMR1H=0xFF;  // Load Higher byte in TMR1H

TMR1L= 0x06;  // Load Lower byte in TMR1L

T1CONbits.TMR1ON=1  // Start the Timer1

; while(PIR1bits.TMR1IF==0);  // Check for overflow

T1CONbits.TMR1ON=0;  // Turn off Timer1

PIR1bits.TMR1IF==0;  // Clear the Timer1 interrupt flag

}

 

Example 18.15.8 Write a program to generate 100 msec delay using Timer1. What are the values to be loaded in T1CON, TMR1H, and TMR1L. Assume XTAL = 8 MHz.

Solution: Given: F_OSC = 8 MHz. Assume: Prescaler = 1 : 4

F_TIMER = F_OSC / 4 × Prescaler ratio = 8 / 1 × 1 / 4 = 500 KHz

Period = 1 /  F_TIMER = 1 / 500 KHz = 2 µs

Count = Desired Delay / Timer Period = 2 µs = 50000

The value to be loaded in 16-bit Timer register is :

65536 - Count = 65536-50000 = 15536= 3CB0H

#include <P18F458.h>

void T1Delay(void);

void T1Delay ()

{

T1CON=0x20; // Timer1, 16 bit mode, 1 : 4 prescaler

TMR1H=0x3C; // Load Higher byte in TMR1H

TMR1L= 0xB0; // Load Lower byte in TMR1L

T1CONbits. TMR1ON=1;  // Start the Timer1

while(PIR1bits.TMR1IF==0); // Check for overflow

T1CONbits.TMR1ON=0;  // Turn off Timer1

PIR1bits.TMR1IF==0;  // Clear the Timer1 interrupt flag