I have a B404 based product where I have seen random reset of the Particle on random systems. The issue seems related to the placement of the antenna relative to the RST/Mode pins, could the antenna be causing some EMI interference with that?
Yes. The pull-up on RST is weak (~100KΩ) and strong RF interference can cause a reset. This is even more common if there are long wires or traces connected to the pins.
How big of an external would be recommended to prevent this?
10K should be fine. 4.7K would also work. Also try to keep the antenna a few inches away from the MCU just to be safe, if possible. Orientation also matters a bit, we generally put the cellular antenna so the flat side is perpendicular to the flat side of the SoM.
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If we are still having reset issues we believe are from antenna interference would a larger pull up help or are we just risking not being able to use the button at that point?
The button should work fine with a pull-up to 3V3 with a pretty small resistance. I wouldn't go much lower than 1KΩ.
Ohm's law V = IR is also I = V/R so if V = 3.3 and R=1000, then I = 3.3 mA. You need to take into account the maximum switching current of the physical switch and the maximum draw from the 3.3V rail.
Thanks, we may need to play around with the values and see if we can get it resolved that way.