A Schmitt trigger GPIO input doesn't have any guarantees about the voltages at where it switches. No surprise if changing the signal frequency causes the low-going voltage to change. It's a design with hysteresis, but I do not recall any logic Schimtt trigger datasheet that has a design with stable hysteresis behaviour over a wide input frequency range.
See the RP2040 datasheet, 5.5.3. Pin Specifications. The data is mostly characterization data of the foundry process; designed and characterized and usually only the fields where there is typical data has measured for this particular chip and its datasheet. Zero guarantees for any of the logic behaviour, only max/min characterization data.
If you need a precision trigger at a very specific voltage point, you need a comparator with suitable specs. plus a precision reference voltage.
If you run the RP2040 at high frequencies, the die temp will likely have some effect as well.
Doing what you want with the GPIO Schimitt trigger will not be simple at all.
See the RP2040 datasheet, 5.5.3. Pin Specifications. The data is mostly characterization data of the foundry process; designed and characterized and usually only the fields where there is typical data has measured for this particular chip and its datasheet. Zero guarantees for any of the logic behaviour, only max/min characterization data.
If you need a precision trigger at a very specific voltage point, you need a comparator with suitable specs. plus a precision reference voltage.
If you run the RP2040 at high frequencies, the die temp will likely have some effect as well.
Doing what you want with the GPIO Schimitt trigger will not be simple at all.
Statistics: Posted by katak255 — Sat Aug 03, 2024 12:38 pm