STM32G491 USB for data only

[ErKaTech]

Hi,

I'm developing a custom STM32G491CEU6 board with USB-C and pins for external ST-LINK programmer. Board is powered from external source. I connected only D+, D- and GND from on USB-C to MCU, because I wanted to use this only for debug. Right now the USB-C communication works only if ST-LINK is connected (had no idea about VBUS sensing). Can i override it somehow? Schematics below

[MM..1]

What type of debug you mean? If USB CDC printing you require implement it in your code... USB pins isnt debug interface .

[ErKaTech]

Yes, I meant CDC printing, not classic Debug. More important for me is why it's not working without ST-LINK. I was looking for a solution, but couldn't find one.

[Chris21]

Perhaps the ST-LINK is providing a GND connection to your PC.

[TDK · Accepted solution]

There's no reason why it can't work. The reason that it isn't is going to be specific to your setup. Maybe NRST is held low, or the firmware doesn't pass a certain stage, or it is miswired.

There are no issues in the posted schematics.

[ErKaTech]

You were right, I had NRST floating, I connected it via 10k resistor to 3,3V and it works like a charm. TY all.

[mƎALLEm]

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@ErKaTech wrote:

I had NRST floating, I connected it via 10k resistor to 3,3V and it works like a charm. TY all.

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That's weird. There is an internal pull-up resistor inside the chip. Refer to the reference manual RM0440:

It should work without adding an external pull-up resistor on NRST pin!

However you need instead to add a filtering capacitor of 100nF on NRST connected to the ground:

Please refer to the reference design in AN5093 "Getting started with STM32G4 Series hardware development boards" / Figure 8.