Is SysTick Accurate Enough for Microsecond Timing?
What SysTick gets right at microsecond resolution, what bites you, and when to reach for a hardware timer instead.
Firmware
Why Does My Code Work in Debug But Break in Release?
The four usual suspects when code works at -O0 and breaks at -O2, in the order to check them.
What Actually Happens Before main() Runs?
The six things that happen between reset and main() on a typical Cortex-M, in order, and what goes wrong at each stage.