UART Glitch On Startup Causes Hardfault in FreeRTOS

[joe (jhg)]

@joe

Okay. Let’s test the hypothesis of entry/exit stack issue. If we guard the entire call/return to CalcAuth(), we observe no hardfault. Now the next test might be to guard the entry or return instead of the entire function call and return. If it protects from the hardfault, then we are most likely getting a stack corruption from the interrupt somehow. This was also a speculated hypothesis from the ST FAE, so it is worth consideration.

Guarding the function entry and exit of CalcAuth() with NVIC_DisableIRQ(TIM2_IRQn) ... NVIC_EnableIRQ(TIM2_IRQn) no hardfault is observed.

Guarding just one of either or neither of the entry or exits of CalcAuth() results in the hardfault
Usage fault: INVPC: Attempt to load EXC_RETURN into pc illegally

At this point, it always crashes when the GPTimer interrupt fires once CalcAuth() has started getting called.