Hi all
I have a couple of (16mhz, 5V) Pro Micro boards that will work fine when powered from USB, however do not power on when supplied with regulated +5V via the RAW or VCC pins. Inversely, there is also no voltage at the VCC pin when the boards are powered by USB. Closing/bridging J1 has no effect.
I am able to work around the issue by adding a wire from VCC to J1. Once done, supplying VCC with 5V powers the board.
Any idea why this might be happening? As far as I can tell from the schematic, supplying 5v to VCC should directly power the 32U4 and the fuse does not appear to be in that path, or have I misunderstood that?
https://cdn.sparkfun.com/datasheets/Dev … 6MHzv1.pdf
Thanks,
Tangles
Connecting to VCC bypasses everything and applies power directly to the 32U4 on these boards so that should work.
Can you provide photos of the top and bottom of your boards?
I’ll see if I can get a photo but it seems pretty clear that isn’t the case - the boards are unmistakably powering on when plugged into USB and doing nothing at all when powered with 5V at the VCC pin. There must be a fuse or other component in the path somewhere responsible for the issue
Looking at the datasheet there are four or so VCC pins on the 32U4, labelled ‘digital VCC’, ‘analogue vcc’, ‘vcc’, and something else (from memory) so without doing further study I can’t tell exactly which of those pins should see voltage in order for the board to be ‘powered on’ (you would think just ‘vcc’…). I don’t have test equipment, but I’ll try and use an LED to compare exactly which pins do and don’t have power when on USB / VCC
I honestly don’t know what’s causing the issue you’re seeing but it’s not any kind of fault in the board design.
All the VCC pins are tied together and directly connected to the VCC pad on the edge of the board. There’s no other components or fuses between the VCC pad and the chip so if it’s working over USB, it would have to work when you apply power to VCC since USB ties in further upstream.
A simplified diagram showing how power routes through the board is below. As you can see, the VCC pad is directly tied into the ATmega chip and is closest to the chip. You have to go through the VCC pad when you power via USB.
Thanks for your help Chris. I don’t really have the equipment at home to do any useful testing but will have access on tuesday to a multimeter and whatever else, so I’ll get some photos and probe voltages at the various pins then.
My research told me the same thing you just said - that the VCC pin is tied straight to the 32U4… so I just can’t compute that the board works powered by USB, or when VCC is supplied to the bridged J1, but not by the VCC pin 
I’d put it down to a faulty 32U4 but am seeing this condition on 3 separate pro micros
