STM32F405RGT6 fails to enter bootloader sometimes

[SKana.5]

I'm using STM32F405RGT6 as a Main controller on the board for our product.

When we flash the FW to STM32F405RGT6 via FT232RL, it fails to enter bootloader mode sometimes. Failling rate is around 40-50%. I observed signal behaviour about Reset(DTR) and BOOT0(RTS) with logic analyzer. In success case, its behaivor looks fine but we saw the steep voltage drop on Reset(DTR) in fail case. I tried putting pull-up resistor(10K) on DTR line but it didn't work. Actually, I have no idea about that voltage drop and what is going on the STM32. Is it not hadware related issue?

I'm using stm32flash v0.7 to flash FW.

GPIO sequence is set to “-rts,dtr,-dtr:rts�?

baud rate is set to 230400

Basically, following the application note(AN3155 USART protocol used in the STM32 bootloader) below.

[waclawek.jan]

Oh, so it's this. Nice catch.

I was confused by the reset pulse mentioned in your initial post.

You should review signals on all pins mentioned in AN2606, regardless of whether such signal is intended to be in the "protocol" for given pin, e.g. if there's some "randomly changing" signal on say PB11, under adverse conditions the bootloader may evaluate it as valid UART3 bootloader entry, and then your UART1 bootloader wouldn't work either.

It may be better to use the built-in bootloader only for the initial programming, and then use a custom bootloader which works exactly as you intended.

JW