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In my Pro/Max board, I wanted to connect a mini USB printer. To do this, I enabled USB Printer support under Device Drivers → USB support → USB Printer support in the kernel configuration, and enabled USB in HOST mode through the DTS.

My board is powered by an external 5 V supply through the VSYS pin, but in this configuration the printer is never detected. After several tests, I switched the power input to the VBUS pin, and the printer started working correctly.

However, when powering through VBUS, the SD card is no longer mounted; when I switch back to VSYS, the SD card mounts properly again.

Could you tell me what is happening?

Hello. There is a Schottky diode between VBUS and VSYS to prevent power reverse flow. There is a 0.3V voltage difference between VBUS and VSYS. Therefore, when powered by VBUS, there is a possibility that the SD card may malfunction due to the lower voltage of VSYS. It is recommended to use a power supply with more stable power capacity.