STM32H743 rev V USART2 Bootloader will not respond. rev Y Does work

[FColl.1]

We have two revisions of our board.

RevA has STM32H743 RevY chip running bootloader V13.2 (confirmed using STLINK).

RevB has STM32H743 RevV chip running bootloader V9.0 (confirmed using STLINK).

On our RevA board we can connect successfully from another MCU to ROM Bootloader on USART2 and upload and flash firmware. This contradicts the AN2606 page264 which says USART2 does not work.

We are trying to get our RevB board working with STM32H743 RevV chip running bootloader V9.0 and it does not work. There are only very minor changes between the boards.

I have checked the following:

1. Reset and Boot0 pins toggled correctly (measured at chip)
2. BOOT_ADD1(optionbyte) = 0x1FF0 (checked with STLINK)
3. USART1 and USART3 RX pins not floating.
4. USB PB14/15 floating.
5. I2C pins pulled up
6. SPI pins floating but unlikely to be reliably clocking in 0x5A to sync
7. Tried 9600 and 115200 baud. Config UART to 8E1.
8. Waiting >100ms before sending to USART2
9. Tried sending 1000 times 0x7F with small gap between.

Does anyone have any suggestions on other things to try to get this working? Like I said it works on Rev Y chip so why not on Rev V?

Has anyone got the ROM bootloader on Rev V STM32H743 working with any of the ports?

Cheers,

Felix

[FColl.1]

Just in case anyone else stumbles across this, we narrowed the problem down. We need to pull PB15 up to 3V3 with a 10K resistor to get UART2 to connect properly.