I am using STM32L471 controller, I just want to ask that what is the maximum value of decoupling capacitor which can be used at JTAG SWDIO pin of controller, so that there would not be any issue during programing of controller using SWD 4MHz setting.

AC1 · ‎2021-09-15

Peter BENSCH · ‎2021-09-15

Welcome, @AC, to the community!

Is there a specific reason why you want to connect a decoupling capacitor to SWDIO, as far as I understand from SWDIO to GND?

Regards

/Peter

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AC1 · ‎2021-09-15

We have defined this JTAG-SWDIO pin as a input in our circuit, during mains as well as in battery mode. To ensure any false detection on input, we need to add a capacitor at this pin, now I just want to ask that what is the maximum value of capacitor that can be used at this pin.

Peter BENSCH · ‎2021-09-16

Well, even if connecting a capacitor there is not a good idea - you can roughly estimate that.

Assuming SDIO is driven by the debug interface at 2MHz and its internal resistance is around 20...50 ohms. Then you have a period of 500ns, which corresponds to 250ns pulses (assuming a duty cycle of 1: 1), which results in approx. C = 250ns / (20...50 ohm) = 5...12.5nF. By reducing the SWDIO frequency, the capacitor can certainly also be made larger. Ultimately, there can be no real recommendation for this and must be checked with the setup (connected debugger) provided by you.

Good luck!

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/Peter

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