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I'm using a dsPIC33F (Microchip) to sample a 555-timer signal. I've tried square, triangular, and "sinusoidal" signal shapes, but always have the same problem: the ADC samples the waveform correctly, but the result is always half of the expected frequency.

For example, my input is 440Hz, and when I plot what the ADC writes to the buffer, I get:

enter image description here

Without the lines it is:

enter image description here

On the oscilloscope I read it as 440Hz. My sampling frequency is 8kHz (which I checked with the oscilloscope by toggling a pin), and as I change it, the sampled values change to reflect that, so I'm almost certain I'm using the correct timer to trigger the ADC. Since each point represents 1/8000 = 0.000125s, the above graphs show a waveform with a period of 36*.000125=0.0045, or 222Hz.

I feel like the conversion time should have no effect, as long as it's shorter than 0.000125s. I'm confident that it is, and have tried different conversion times with no change. I know it's not the most efficient way of doing things, but to start I also convert only one sample at a time before triggering an interrupt and writing to the buffer. I'm not sure if that would matter.

If anyone has any ideas, I'd be grateful.

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    \$\begingroup\$ When you checked the sampling frequency by toggling a pin, did you toggle the pin at the sampling rate, and measure the output waveform frequency as 8kHz? That would mean that the sampling clock is 16kHz. \$\endgroup\$ – Justin Jul 9 '15 at 19:02
  • \$\begingroup\$ Wow, I figured it would be a dumb mistake, but I had hoped something a bit more respectable than that. Thank you. I wish I'd asked sooner. \$\endgroup\$ – MrUser Jul 9 '15 at 19:15
  • \$\begingroup\$ Glad you got it sorted out. I converted my comment to an answer so the question can be marked as resolved. \$\endgroup\$ – Justin Jul 9 '15 at 19:19
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When you toggle a pin at a certain rate, the resulting waveform will have a frequency at half of the toggle rate. So if your output waveform was a 8kHz square wave, then your ADC sample rate is actually 16kHz, which explains the factor of 2 error.

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  • \$\begingroup\$ It can be pretty easy to forget to toggle twice for a single period. \$\endgroup\$ – Nick Williams Jul 9 '15 at 19:35

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