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Interrupt and Counter

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A decoder from a motor has 2 square wave outputs that deliver 256 pulses/revolution (duty cycle 1:1). The two outputs are 90 degree out of phase for direction information.

I want to count pulse, in one direction up, in the other down. Towards this aim I attach to the first decoder output following routine:


void countStep(){

     if(digitalRead(_CIN2)){_pos++;} else {_pos--;}



"_CIN2" is the second decoder output and "pos" the counter variable.

This works well until up to approx. 16kHz.

But for higher frequencies it counts in the wrong direction.

I first guessed, that the phase relation between the 2 signals is not constant, but an independent measurement showed it to be rock stable.

Another guess is, that the interrupt is too slow (maybe because of the "digitalRead"). I am running a MSP430G2553 with 16MHz. Can somebody verify or falsify this guess?


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I do not use Energia, but i doubt that digitalRead function is interrupt based. Just try to whack up a real ISR for both pins incrementing or decrementing your variable depending on previous state variables of respective pins.

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In between I found the excellent post from this forum :


digitalReadFast and speed of digitalRead

Started by altinellerAug 19 2014 05:41 PM


and you bet, the problem was the "digitalRead". I replaced




P1IN & 0b00100000)


and it works. It is astonishing that for a CPU running with 16MHz, digitalRead is so slow.

According to the above postdigitalRead takes approx. 18 uS what explains my problem.

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