Information

PIC18F46J11

DS80000435K-page 8  2009-2013 Microchip Technology Inc.

13. Module: Timer1/Timer3

When the timer is operated in Asyncronous

External Input mode, unexpected interrupt flag

generation may occur if an external clock edge

arrives too soon following a firmware write to

TMRxH:TMRxL register. An unexpected

interrupt flag may also occur when enabling the

module or switching from Synchronous to

Asynchronous mode.

Work around

This issue only applies when operating the timer

in Asynchronous mode. Whenever possible

operate the timer module in Synchronous mode

to avoid spurious timer interrupts.

Example 3 shows a method to suppress the

spurious interrupt flag if it occurs upon writing to

the timer.

EXAMPLE 3:

ASYNCHRONOUS TIMER MODE WORK AROUND TO AVOID SPURIOUS INTERRUPT

//Timer1 update procedure in asynchronous mode

T1CONbits.TMR1ON = 0; //Stop timer from incrementing

PIE1bits.TMR1IE = 0; //Temporarily disable Timer1 interrupt vectoring

TMR1H = 0x00; //Update timer value

TMR1L = 0x00;

T1CONbits.TMR1ON = 1; //Turn on timer

//Now wait at least two full T1CKI periods before re-enabling Timer1 interrupts.

//Depending upon clock edge timing relative to TMR1H/TMR1L firmware write operation,

//a spurious TMR1IF flag event may sometimes assert. If this happens, to suppress

//the actual interrupt vectoring, the TMR1IE bit should be kept clear until

//after the "window of opportunity" (for the spurious interrupt flag event has passed).

while(TMR1L < 0x02); //Wait for 2 timer increments more than the Updated Timer

//value (indicating more than 2 full T1CKI clock periods elapsed)

PIR1bits.TMR1IF = 0; //Clear TMR1IF flag, in case it was spuriously set

PIE1bits.TMR1IE = 1; //Now re-enable interrupt vectoring for timer 1

A2 A4