[seanwitte]

Based on information from IFI and Kevin Watson's sample code we figured out the value we needed to pre-load into the timer3 accumulator to make it overflow after 10ms. The formula (from IFI's whitepaper on timers) is:

number of ticks = (time elapsed) * 10^7/prescaler
number of ticks = (10/1000) * 10^7/8 = 12500

Since the counter overflows when it reaches 65535, you want to preload it with the difference. Therefore, the value is 65535 - 12500 = 53035. Converted to hex the value is 0xCF2B.

Using Kevin Watson's template we initialize timer3 using this code:

#define TIMER_LOW 0x2B
#define TIMER_HIGH 0xCF

void Initialize_Timer_3(void)  
{
	//preload the accumulator to overflow after 10ms
	TMR3L = TIMER_LOW;
	TMR3H = TIMER_HIGH;

	//1:8 prescaler (clock=1.25MHz/each tick=800ns)
	T3CONbits.T3CKPS0 = 1;
	T3CONbits.T3CKPS1 = 1;

	// 0: use the internal clock
	T3CONbits.TMR3CS = 0;

	// 1: synchronize the external clock to the PIC18F8520s internal clock, which is desirable				//
	T3CONbits.T3SYNC = 1;

	/* timer 3 register operations are done in one 16-bit access */
	T3CONbits.RD16 = 1;

	/* 1: enable timer 3 interrupt on overflow (i.e., a transition from FFFF->0) */
	PIE2bits.TMR3IE = 1;
							
	/* 0: timer 3 overflow interrupt is low priority (leave at 0 on IFI controllers) */
	IPR2bits.TMR3IP = 0;
						
	/* timer 3 is enabled */
	T3CONbits.TMR3ON = 1;
}

Don't forget to preload the accumulator for the timer again during the ISR. At the top of the function we just copied the first two lines of the initialization:

void Timer_3_Int_Handler(void)
{
	/*reload the timer offset*/
	TMR3L = TIMER_LOW;
	TMR3H = TIMER_HIGH;
etc.

Since we need to do analog conversion during the ISR we made sure to disable the global interrupts in the main program before using the ADC. Setting INTCONbits.GIEL = 0 disables the low priority interrupts, then setting it back to 1 enables them.