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How to get a STM32L4 to recognize DFU in a self powered configuration.

Kent Swan

I'm using a STM32L462 in a self powered application. I have no problem communication as a USB serial device (CDC Mode). This is a battery powered device with charger on board. No problems there either. No problems detecting Vusb via a GPIO input port. VUSB on the chip is tied to the VCC of the cpu (self powered). I've been using Cube to generate (and regenrate) code quite successfully on this and other projects. BOOT0 is pulled down with a 10K resistor and can be pulled up with a push button during reset.

Per the design guides, for the STM32L4, no external pull up resistors are required on D+.

The problem is that, no matter what I try from the collection of tricks (BOOT0, Jump to boot loader, etc causes a jump to the boot loader (verified) the STM32L4 refuses to select to USB in DFU mode.

I've even tried lifting Vusb pin on the processor and tying it to Vusb buss through a resistor divider.

I'm wondering if I havn't missed something which triggers the boot loader to properly test and select DFUsb mode.

Any clues or tricks to actually get this to work?