Hi Charles,
Hearty +1 for everything everyone has said so far. Trampas in
particular for suggesting you use a scope on your power rails. Most
DMMs do quite a bit of filtering (even if it's in the form of a very
slow sample rate) and can't tell you about high-frequency trash on the
supply rails (or other lines) that can wreak havoc with sensitive logic
systems. If your DMM has a DC+AC mode, try turning that on and seeing
what it says is the AC component of the power supply as an intermediate
step.
Definitely make sure your power is good first; if your power isn't
stable, all other bets are off. You want to see a nice flat line with
minimal ripple. Make sure you zoom right in on the time axis to see if
there's any high-frequency garbage. Also measure as close to the
consumers of power as possible (i.e., at IC pins, not at the regulator).
Note that DC-DC converters tend to have relatively high amounts of
ripple; the 3v3 regulator will filter a lot of that out, but if anything
is being fed directly by the 5V supply, you might need more filtering on
that rail.
Following that, I'd check your ARM firmware for things like polling
loops without adequate timeouts when talking to off-chip peripherals.
The fact that everything is copacetic until the nano is in the picture
makes me wonder if perhaps the ARM core is getting stuck waiting for
communication with the nano to finish (assuming they are talking to each
other). If you're doing time-critical tasks on the ARM and aren't
already, I suggest picking an RTOS (I'm partial to FreeRTOS mainly
because of its use on the ESP32 product family; several are available
directly in the STM Cube IDE) and learning how to prioritize
time-critical tasks so that unpredictable asynchronous communication
issues won't get in the way.
This is, of course, all very generic advice based on knowing absolutely
nothing about your design. :-D Just stuff I've run into on my own travels.
Cheers,
-Brian
On 3/23/24 07:07, Trampas Stern via TriEmbed wrote:
Charles,
The problem could be several things, the best thing to do is put an
oscilloscope on the power rail and see if that is the problem. I have
an oscilloscope you can borrow if you need one.
The problem could be many things from noise on ground, to noise on a pin
on the micro, or noise getting to crystal for the micro, again an
oscilloscope will help. However good diagnostic techniques can be
used. For example if you suspect it is the power supply, remove the
3.3V linear and power the 3.3V from a bench supply and see if the
problem goes away. This does not tell you what is wrong with the 3.3V
rail but lets you isolate the problem for more investigation.
Trampas
<snippysnip>
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