I designed a system with RaspberryPi and STM32F407's, communicating with each other via SPI interfaces.
For the last 3 months, tests were OK but yesterday a RaspberryPi and 2 of STM32F407's SPI ports were burn out. I don't know why. Everything was working, I went to sleep, everything was burned out in the morning.
Here is the implementation sketch:
PCB's are 15 cm long. SPI lines are parallel. No chip-select lines are used. No termination resistance used. SPI frequency is 200 kHz. Power supplies are 5V 1A usb charger for RaspberryPi, LM2576 3.3V circuits (implemented by myself) converting 24V to 3.3V for MCU's.
I hot plugged my modules around 10 times with no problem (why hot plugging would be a problem with SPI?) although according to Wikipedia article, SPI is not hot pluggible.
- So, what could possibly caused that damage? Standing waves? Hot plugging? Or maybe power supply? How can I find the root of this issue?
- Do I need to use optic isolators (or cmos buffers?) for each of MCU unit?