STM32F407 Discovery External Power + ST-Link

[TDonh.1]

Hello,

I‘m trying to power up my development board with a external 5V power source. Is it possible to use the on-board ST-Link via USB at the same time? Thank you for your help.

Best regards.

[TDK]

It should work, but you may need to adjust solder bridges and/or jumpers if you want the power to be pulled from the 5V source and not USB when both are connected. May need to consult the schematic for exact routing details.

[TDonh.1]

Thank you!

Might be a dumb question, but I'm trying to connect a 5V display with UART communication to the controller which needs a litte bit more power than the board can provide. Therefore my idea was to power up everything together through an external power source...

Would it be also possible to just power the display externally, the board by USB and connect both grounds?

[TDK · Accepted solution]

In general, you can power everything independently provided the grounds are connected. However, you need to ensure devices are not backpowered if they cannot tolerate it. For example, with USB, a device is not allowed to send signal on the data lines unless the VBUS voltage is present. If UART is the only connection between the devices, and they're being driven in open drain mode with an external pullup, it will generally work fine.

[TDonh.1]

Understood, thank you!

But why do I need an external pullup? Isn't it possible to just configure the GPIO-Pins for UART with pullups?

[TDK]

I didn't say external pullups are required. Generally, internal pullup will work.

[TDonh.1]

Thanks!