Hi Steve

I have to count incoming telephone rings (certain number of rings).

The ring signal is as you know trhu opto. The negative out put of opto I have connected to Ext-0 interrupt.

At the output of the opto I have connected an LED. When ring comes the flicker of the LED can be seen and noticed fairly. Therefore you can asses the frequency of the high-low transition in the ring burst.

I have to count only a certain number of bursts. The gap duration between two bursts (two rings) is fairly long, you know.

After detecting the first low at the start of a burst (ring), I put a delay in the ISR before RETI to avoid other lows the burst (ring signal).

Therefore it is planned to count X number of bursts (rings).

But what's happening is that most of the times the count reaches before the counter value sometimes at (X-1) count or (X-2) counts.

I Suggest:
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I use level type interrupt (not the edge type) and hold the ISR longer than the burst (ring signal). So that after the ring the level at ext-0 interrupt is high before the ISR executes RETI ready for the next ring count.




Simon