Interrupts and ISRs

Sources include timers, GPIO edges, UART bytes received and ADC conversions complete.

volatile uint32_t ticks = 0;

void SysTick_Handler(void) {     // runs every 1 ms
    ticks++;
}

Rules for ISRs:

• keep them short — set a flag, store data, return;
• mark shared variables volatile;
• protect multi-byte shared data from being read half-updated (disable interrupts briefly or use atomics);
• never use blocking calls (delay, printf) inside them.