How can I get 200 ns or more delay?

Timer 0 Help needed

I am using PIC18F252 with 20MHz crystal, I have tried to use the following program to get the delay(ON/OFF time of LED) minimum i.e. 200ns, but the delay I am getting is 5ms.

Please let me know what is the mistake in my program.

unsigned cnt;

void interrupt() {
cnt++; // Increment value of cnt on every interrupt
TMR0L = 96;
INTCON = 0x20; // Set T0IE, clear T0IF
}

void main() {
ADCON1 = 0x0F; // Set AN pins to Digital I/O
T0CON = 0xC4; // Set TMR0 in 8bit mode, assign prescaler(1/32) to TMR0
TRISB = 0; // PORTB is output
PORTB = 0x00; // Initialize PORTB
TMR0L = 96; // Timer0 initial value
INTCON = 0xA0; // Enable TMRO interrupt, TMR0 Overflow Interrupt Enable bit(bit 5)
cnt = 0; // Initialize cnt

do {
if (cnt == 1) {
PORTB = ~PORTB; // Toggle PORTB LEDs
cnt = 0; // Reset cnt
}
} while(1);
}

Re: Timer 0 Help needed

Try declaring cnt as a volatile.
eg:

volatile unsigned cnt;

Part of the optimisation the compiler will do is read cnt once into a register and then never read it again. The compiler will also then proceed to optimise the loop then declare a lot of code dead and remove it.

Declaring the variable as volatile will force the compiler to actually access the memory location instead of caching in a register.

Another thing is you should disable interrupts before accessing a volatile variable larger than 8 bits and disabling after you have finished with it but only in non-interrupt routines and only on processors that can't do the operation with one cpu instruction. The same also applies if you want to do a read-modify-write operation that requires multiple instructions. The reason for this is to prevent an interrupt occuring part way through a read or write of the variable. Failure to do this can introduce some very interesting bugs.

Hope this helps.

Timer 0 Help needed

Only you have to do on your main is:
- Disable Timer
- Clear Timer Counter Register
- Enable Timer Interrupts and Clear Interrupt Flag
- Write your Preset Counter
- Enable Timer
- *Be sure your global interrupts are enables
- Loop forever

On your interrupt:
- Clear Interrupt Flag
- Toggle your bit


void main() {
ADCON1 = 0x0F; // Set AN pins to Digital I/O
T0CON = 0x; // Set TMR0 in 8bit mode, please assign prescaler(1/2) to TMR0
TRISB = 0; // PORTB is output
PORTB = 0x00; // Initialize PORTB
TMR0L = 1; // Timer0 initial value
INTCON = 0xA0; // Enable TMRO interrupt, TMR0 Overflow Interrupt Enable bit(bit 5)
while(1);
}

void interrupt () { // Please setup your interrupt well
PORTB = ~PORTB; // Toggle PORTB LEDs
INTCON = 0x20;
}