Calculation of accurate timing of the timer of PIC microcontroller-EEWORLD

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Here, TMR0 of 16C711 microcontroller is used as timer interrupt, hoping to achieve accurate timing. In the program, TMR0 uses the 32-frequency division of the crystal oscillator, and the initial value is #0FCH. Therefore, POPBEAR brothers calculated the timing time of each timer interrupt as (256-X)*32*4/32768=0.015625 seconds. Note that the problem is here! In fact, this time is the time from the initial value of TMR0 being placed in the next two instruction cycles (see PIC microcontroller timer/counter data) to the next interrupt. If you want to use the timer for accurate timing, you must understand this concept!

If a 32768Hz crystal oscillator is used, each instruction cycle is 122us. In the interrupt handler, there are 7 instructions until the initial value of TMR0 is placed, plus the two instruction cycles after the initial value of TMR0 is placed. If the interrupt handler is not directly placed at address 0004H but uses two cycles of GOTO instruction, there are a total of 11 instruction cycles. In other words, the interval between each timer interrupt is 0.015625 seconds + 11*122us. 64 interrupts in the program are 1 second, so the error of 1 second is 64*11*122us=85.9375ms, and the error of 1 minute is 5.156s.

So how do we get accurate timing? This requires us to make some adjustments in the initial value assignment of the timer and in the interrupt handler.

Reference address：Calculation of accurate timing of the timer of PIC microcontroller

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