Programming Help

[Gav1985]

Hi everyone, I need help

I'm working with a PIC18F4550. The idea is to start and stop a timer (timer 0) and measure the time period.

I have a Astable Schmitt trigger providing an oscillating input to pin RB7 and I need to measure the time period of this oscillation.

So, the code I have looks like this...

// Setup timer.
	T0CON = 0x04; ;				// Internal Clock src (Fosc/4), 1:8 prescaler   0.2uSx8 = 1.6uS Resolution
	INTCONbits.GIEH = 0;
	INTCONbits.TMR0IE = 1;
	INTCONbits.TMR0IF = 0;				

	x = PORTBbits.RB7; 			// Read pin state.
	while (x==PORTBbits.RB7){	// Wait until pin state change.
		_asm nop _endasm
	}

	T0CONbits.TMR0ON = 1; 		// Start the timer
	while (PORTBbits.RB7 != x){ 
		_asm nop _endasm
	}
	while (PORTBbits.RB7 == x){ 
		_asm nop _endasm
	}
	T0CONbits.TMR0ON = 0;		// Stop Timer

	// -- Workout tA - Period -- //			

	period = TMR0L;			// Read TMR0 Register
	period *=0.0000016;

The prescaler is set to a 1:8 resolution and this is more than adequate so that TM0L shouldn't overflow so I don't have to deal with interrupt flags.

While simulating this, Var period will work out to around 230 -260uS. Using breakpoints, MPLAB/proteus timestep tells me that the time elapsed between starting and stopping TIMER0 is around 330us. Roughly a 100uS discrepancy.

I should add, the digital oscilloscope doesn't produce a symmetric square wave which I put down to the limitations of the simulation software.

The Schmitt Trigger i'm using is HCC/HCF 40106b and there is a formula for working out the time period which is handy, I'm just struggling to get the code/simulator to produce a result which is even close to what the formula is telling me I should get.....

Thanks all.

 

[KrisBlueNZ]

I can't help much here, since I have no experience with PICs, but for measuring intervals, it's more usual (and more accurate) to use an input capture peripheral, as described in section 15 of the data sheet.

 

[BobK]

Try zeroing the timer just before turning it on.

bob

 

[Gav1985]

Thanks Bobk.