Sine signal frequency measurement without input capture

[STuser2]

I have a hardware where i need to measure the input signal of the AC mains signal 50Hz, the problem is the hardware does not have provision for input capture. Is it possible to measure? I also several other tasks running like RS485, 13 channels ADC measurement etc, the input signal is given to ADC channel. 

[Andrew Neil]

1. read a free-running timer at your measurement point on the waveform (eg, zero crossing);
2. read again at the next measurement point.

Difference between the readings gives you the time between readings - whence the frequency.

You could trigger this on an interrupt...

[mbarg.1]

If the input signal is connected to ADC, use digtized waveform to measure frequency - get max and min, compute zero crossing, count number of samples between zero crossing, and you have a half period, average on many measurements, compute freq …

OK ?

[STuser2]

At present my implementation is i am triggering the ADC at every 10KHz with DMA enabled once the number of samples are received, in the receive call back i calculate the RMS of the samples. So, as per your suggestion i can check those samples get the maximum and minimum values, i need to have logic for zero crossing. Is my understanding correct? 

[TDK]

It would be more accurate to do an FFT of the data to determine frequency rather than just a zero-crossing. Otherwise, yes.

[mbarg.1]

Zero = min + (max - min) / 2 = ((2 * min - min) + max) / 2 = (min + max) / 2 - not a real logic, just some math 

unless you have huge non linear loads causing waveform distortions.

​@TDK : yes FFT is more accurate providing you have large number of samples and good math engine - it depends from what STuser2 is looking for - computational burden is several orders of magnitude greater

[MM..1]

All is based on hav many instruction you can waste for this. Based on range frequency can arrive use DMA buffer size , if memory isnt prblem use bugger and chcek max values on half and full complet etc. simply two max values distance on sine is perfect frequency value...