I'm pretty sure I had my BRD4321A plugged into my SLWSTK6121A baseboard with the power switch set to AEM when I plugged a cable into the on-board micro USB connector.
As a result, I can't power the board from that connector alone. When I check the VMCU voltage, it's only about 0.05V.
I have additional BRD4321A modules on the way to me, but I'm wondering about removing minimal components from my existing board to avoid doing further damage, but so that I could continue to do USB development when plugged into the baseboard. I'm a software guy, but I'm pretty sure I could remove U100 (the USB voltage regulator). but maybe Q1 would be a better way to isolate?
I don't want to risk disconnecting R100 and R101 and break the data connection. I also don't think I can just remove L100 because I noticed USB_VBUS going into the WGM160P and assume it uses that to detect when a USB cable is attached.
Unless someone has a better idea, I think I'll use precise snips to cut U100 out so I don't risk desoldering any nearby components in trying to remove it with my soldering iron.
The SLWSTK6121A_user_guide.pdf section 4 could help you to investigate this power supply issue.
But may be you have already read it.
Yep, I'm pretty sure I neglected to read through that part of the manual when I explored what was possible on that interface. I came across this section after the fact, and have confirmed that I can't power the module on its own with the USB interface.
"With the WGM160P radio board, the USB position of the power switch is disconnected. Instead, a transistor switch on the radio board itself connects the on-board regulator to the module power supply when the target USB cable is inserted. Please note that this happens regardless of the power switch position, so the power switch should be set to USB or BAT when using the radio board USB connector to avoid conflict. The transistor switch allows the radio board to be powered by a USB cable even when removed from the WSTK main- board."
I'm fairly certain that as a result I damaged the regulator or one of its support components. A nice modification to the board would be to disable U100 if the BRD4321A is receiving power from P200 or P201.
Update: I decided to remove the damaged U100 from the board, which would ensure the USB interface doesn't compete with the baseboard to provide power. My snips weren't precise enough to cut the pins, but a razor blade worked to cut the pins between the chip and solder pads. I'll be more careful with future boards to ensure that I can power them via the USB port when not connected to the baseboard.