TMR0 – IRQ

This example modifies TMR0 in the IRQ to obtain a roll over every 1mS. You could use a timer with a prescaller for this, but in this example we show how it’s possible on TMR0 with no prescaller (leaving your prescaller timers available for other uses)

Using your own irq function

#include "mcc_generated_files/timer/tmr0.h"

	//----- SET ADDRESS OF OUR INTERRUPT HANDLERS -----
	Timer0_OverflowCallbackRegister(Our_TMR0_InterruptHandler);

	Timer0_Start();


//Inline assembler access bank variables
__near static unsigned char uc_asm_irq_temp;		// near qualifier specifies access RAM
__near static unsigned char uc_asm_irq_temp1;

//***********************************************
//***********************************************
//********** TIMER 0 INTERRUPT HANDLER **********
//***********************************************
//***********************************************
void Our_TMR0_InterruptHandler(void)
{
	//(Timer 0 - Heartbeat every 1mS)
	
	//Modify TMR0 value so we roll over every 1mS
	asm("GLOBAL _uc_asm_irq_temp");
	asm("GLOBAL _uc_asm_irq_temp1");
	//Reset timer for next rollover
	asm("movff	TMR0L,_uc_asm_irq_temp");		//read_current_timer_value (read low byte loads high byte)
	asm("movff	TMR0H,_uc_asm_irq_temp1");
	asm("movlw	0x6b");
	asm("addwf	_uc_asm_irq_temp,f,c");			//(1 = file register, 0 = access ram)
	asm("movlw	0xf0");
	asm("addwfc	_uc_asm_irq_temp1,f,c");		//(1 = file register, 0 = access ram)
	asm("movff	_uc_asm_irq_temp1,TMR0H");		//Store new value (high byte first)
	asm("movff	_uc_asm_irq_temp,TMR0L");

	//This code takes 10 instruction cycles
	//The timer is inhibited for 2 instruction cycles after the low byte is written
	//Therefore subtract 12 from the value to be written
	//For 4MHz clock 1mS irq = 1000 cycles.  Minus 12 = 988.  0xFFFF - 988 = 0xFC23
	//For 4MHz clock 10mS irq = 10000 cycles.  Minus 12 = 988.  0xFFFF - 988 = 0xd8fb
	//For 16MHz clock 1mS irq = 4000 cycles.  Minus 12 = 3988.  0xFFFF - 3988 = 0xF06B
	//For 16MHz clock 10mS irq = 40000 cycles.  Minus 12 = 39988.  0xFFFF - 39988 = 0x63CB
	//For 32MHz clock 1mS irq = 8000 cycles.  Minus 12 = 7988.  0xFFFF - 7988 = 0xE0CB
	//For 40MHz clock 10mS irq = 50000 cycles (1:2).  Minus 12 = 49988.  0xFFFF - 49988 = 0x3cbb
	//For 40MHz clock 1mS irq = 10000 cycles.  Minus 12 = 9988.  0xFFFF - 9988 = 0xd8fb

}