sometimes missing response from bootloader (UART Mode) (STM32F205RET)

[TEber.5]

From time to time I run into a Problem that the STM-Bootloader doesn't respond when I try to activate it.

What I do:

Hold reset pin of STM low for at least 2s. Then set Boot0-pin to high (Boot1 is connected to GND with a 10k resistor).

After another 1s I release the reset pin. Then after 1.5s I send 0x7F over UART1 to set the STM into UART-Bootloader mode.

From here I wait 1s for a response from the STM.

What happens:

Approximately 4 of 5 times the STM responses with the 0x79 ACK Byte and I do my normal flashing routine.

But the other times there is none response from the STM. So I pull the reset line again and do the same procedure again.

Mostly it works the next time, but sometimes it needs several retries.

What I've already checked:

- The Slopes of the UART signal are steep enough.

- Currently I'm running it with 115200 Baud, but the Problem also occurres when I'm setting it down to 9600 Baud.

- If the STM response, the baudrate deviation is below 1,3%.

- There was no glitch measureable before the 0x7F Byte, that could have caused wrong Baud-rate detection (measured with 500 MHz Bandwidth).

For more clarity I've added some picures of the normal and the error case.

1. Overview of one try to start bootloader, that fails
2. Zoom of the uart byte that activates the STM uart bootloader mode -> That succeeds
3. Zoom of the uart byte that activates the STM uart bootloader mode -> That fails

Any hints for what can cause this issue?

[TDK]

If other interfaces are active, the bootloader will activate on those.

NRST doesn't need a full second to reset the chip. It needs to be low for 300ns per the datasheet. Bootloader starts up very quickly after that.