Using timers TMR0, TMR1 and TMR2. Using interrupts, declaring new function...
If you have read the previous example, you probably have noticed a disadvantage of providing delays using loops. In all those cases, the microcontroller is ‘captive’ and does nothing. It simply waits for some time to pass. Such waste of time is an unacceptable luxury and some other method should be applied therefore. Do you remember the story about timers? Interrupts? This example makes links between them in a practical way. The schematic is still the same as well as the challenge. It is necessary to provide a delay long enough to notice changes on a port. Timer TMR0 with assigned prescaler is used for this purpose. An interrupt is generated on every timer register overflow and every interrupt routine automatically increments the cnt variable by 1. When it’s value reaches 400, the port B is incremented by 1. The whole procedure is performed ‘behind the scenes’, which enables the microcontroller to do something else.
/*Header******************************************************/
unsigned cnt; // Define variable cnt
void interrupt() {
cnt++; // Interrupt causes cnt to be incremented by 1
TMR0 = 96; // Timer TMR0 is returned its initial value
INTCON = 0x20; // Bit T0IE is set, bit T0IF is cleared
}
void main() {
OPTION_REG = 0x84; // Prescaler is assigned to timer TMR0
ANSEL = 0; // All I/O pins are configured as digital
ANSELH = 0;
TRISB = 0; // All port B pins are configured as outputs
PORTB = 0x0; // Reset port B
TMR0 = 96; // Timer T0 counts from 96 to 255
INTCON = 0xA0; // Enable interrupt TMR0
cnt = 0; // Variable cnt is assigned a 0
do { // Endless loop
if (cnt == 400) { // Increment port B after 400 interrupts
PORTB = PORTB++; // Increment number on port B by 1
cnt = 0; // Reset variable cnt
}
} while(1);
}
/*Header******************************************************/
unsigned short cnt; // Define variable cnt
void interrupt() {
cnt++ ; // Interrupt causes cnt to be incremented by 1
PIR1.TMR1IF = 0; // Reset bit TMR1IF
TMR1H = 0x80; // TMR1H and TMR1L timer registers are returned
TMR1L = 0x00; // their initial values
}
void main() {
ANSEL = 0; // All I/O pins are configured as digital
ANSELH = 0;
PORTB = 0xF0; // Initial value of port B bits
TRISB = 0; // Port B pins are configured as outputs
T1CON = 1; // Set timer TMR1
PIR1.TMR1IF = 0; // Reset bit TMR1IF
TMR1H = 0x80; // Set initial value for timer TMR1
TMR1L = 0x00;
PIE1.TMR1IE = 1; // Enable interrupt on overflow
cnt = 0; // Reset variable cnt
INTCON = 0xC0; // Enable interrupt (bits GIE and PEIE)
do { // Endless loop
if (cnt == 76) { // Change port B state after 76 interrupts
PORTB = ~PORTB; // Number in port B is inverted
cnt = 0; // Reset variable cnt
}
} while (1);
}
/*Header******************************************************/
unsigned short cnt; // Define variable cnt
void Replace() {
PORTB = ~PORTB; // Define new function ‘Replace’
} // Function inverts port state
void interrupt() {
if (PIR1.TMR2IF) { // If bit TMR2IF = 1,
cnt++ ; // Increment variable cnt by 1
PIR1.TMR2IF = 0;// Reset bit and
TMR2 = 0; // reset register TMR2
}
}
// main
void main() {
cnt = 0; // Reset variable cnt
ANSEL = 0; // All I/O pins are configured as digital
ANSELH = 0;
PORTB = 0b10101010; // Logic state on port B pins
TRISB = 0; // All port B pins are configured as outputs
T2CON = 0xFF; // Set timer T2
TMR2 = 0; // Initial value of timer register TMR2
PIE1.TMR2IE = 1; // Enable interrupt
INTCON = 0xC0; // Set bits GIE and PEIE
while (1) { // Endless loop
if (cnt > 30) { // Change PORTB after more than 30 interrupts
Replace(); // Function Replace inverts the port B state
cnt = 0; // Reset variable cnt
}
}
}
