My GPS was generating extremely short PPS pulses (only 1 us long!). This was fine while I used the PPS pulses to excite the interrupt inputs on the EVM board, but when Peter changed to using the second audio input to interpolate more precisely, things simply didn't work.
Even longer pulses (up to some 100 us) can cause problems with the interpolation algorithm, so it won't hurt to apply the stretcher there too.
The stretcher is simply a monostable that is designed to generate a pulse of about 500ms long. The circuit is very simple, and you can select either positive or negative going pulses from the output. Be careful to select the correct one, as the trailing edge is, of course, not as stable as necessary.
The circuit is also available as a PDF document.
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| (c) John Coppens ON6JC/LW3HAZ |