I am working on a custom board with STM32F411RC which can communicate over SPI and I2C to the master board and acts as a slave in both cases. The master board also needs to be able to reprogram my board over I2C. Chip select signal is used to control the BOOT0 pin on my processor by the master board via an inverter chip to put it in bootloader mode in order to reprogram it. The signal applied to the BOOT0 pin is always in a defined state. At runtime SPI communication is constantly ongoing between the two boards and the pulses on chip select line are being issued at a rate of 4 KHz. If I pull my BOOT0 to ground with an external zero ohm resistor everything works fine with of course the exception of not being able to be reprogrammed by the master board. If however I remove the pull down resistor and let the chip select signal reach my BOOT0 pin I observe that after some time of running my processor enters hard fault. When I checked the source of the hard fault in the Configurable fault status register (part of System control block (SCB)) I was not getting consistent fault sources also when I checked the stacked register I found the link register always to be zero which I found strange. So my question is, can the external signal being applied to the BOOT0 pin have souch an effect to cause a hard fault to occur during runtime?
