We have designed a SOM using i.MX287. The i.MX28 is powered by 3.3V only according to AN4199 section "DCDC_BATT source only". The i.MX28 is configured to boot from NAND. At initial power on we try to upload the firmware to the processor by changing to USB recovery (pulling PSWITCH up to 3.3V at power-on). The PC does not detect the SOM at all. We read the error codes 80508002 and 80502008 on the debud UART port. It seems that the bootloader tries the NAND flash for firmware and then changes to USB port. That there is communication with the debug UART means obviously that the ROM is running. When the processor is powered with 5V through VDD5V the pull-up on the D+ is internally enabled and the PC is able to detect and communicate with the i.MX28. Does the USB circuit requires 5V during the USB recovery mode?