4 years ago.
Device crashes without USB power
I have an issue. The board crashes about once per day. This is improved by supplying the 5V to the USB connector however it is not a fix.
Once the board has crashed just touching L2 seems to wake the device again. I have 3 C027 and all behave in the same way.
Question relating to:
4 years ago.
The interface chip is powered by the USB voltage rather than the main system supply. Having that part unpowered makes the entire system unstable.
This is apparently by design although I'm not quite sure why someone thought requiring both external power and USB connected at all times in order to operate was a good idea.
If you don't mind making a minor hardware mod this is easy to fix. Connect a schottky diode between the 5V IO pin and the USB VBUS pin on the USB connector (pin 1 from memory but double check).
Another gotcha on this board is that the 3.3V pin can't power an SD card reliably. On most mbeds the 3.3V pin is designed to supply a reasonable amount of power out. On this board there is a series resistor, if you try pulling any current the output voltage drops. For an SD card this creates something between random corruption when writing to files and complete failure depending on the card used. Fortunately this is also easy to fix, just replace the resistor between the 3.3V and 5V outputs with a piece of wire.
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