UART Rx Pin Input Impedance for RC Low Pass Filter Design [STM32 MCUs]

Chubs · ‎2024-06-17

Hello Community,

I am trying to confirm with you all a design decision for RC based Low Pass Filter that I plan to install on my the UART Rx pin as the STM32G0 Errata mentions this pin has to have a filtered/noise free input to avoid glitches.

I am trying to re-confirm what is the input impedance for the UART Input pin on STM32G series MCUs? Is it a fair assumption that all input pins irrespective of the peripheral (ie UART/I2C/SPI etc) have high input impedance in Mohm range?

Kindly suggest/share your experience of using RC Filter on UART Rx input.

BarryWhit · ‎2024-06-25

> while selecting shunt resistor

@Chubs, I must be missing something here. Can you post a schematic of the filter circuit you intend to use (ignoring values for now).

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Chubs · ‎2024-06-25

If noise is really an issue, consider using RS232 or RS422 or similar ...

@Andrew Neil thanks for you suggestion.

The key idea is IF things can be solved with a simple RC passives - why to indulge into another set of CMOS base solutions?

Said that, I often use RS422 - TI's AM26C31/32 pair which are extremely low cost solutions for differential signaling.

RS-485 for any EMC compliance ready system or any significant multi-MCU communication, no compromise there.

In my experience we have been able to get away with offboard single ended cables of upto 3m when the speeds are below ~4.8kHz (our I2C based Temperature sensors) when using RC filtering not only for noise but also to reduce the rise & fall times & associated high frequency harmonics on the I/O cables. Have never ventured into anything beyond 4.8kHz speed on offboard single ended communications.