Embedded patterns: interrupts, timing and constraints

The super loop

int main(void) {
    hardware_init();
    for (;;) {
        read_sensors();
        update_state_machine();
        update_outputs();
    }
}

Interrupts

An interrupt is a hardware signal (a byte arrived on the UART, a timer expired, a button was pressed) that makes the CPU pause the current code, run a special function called an interrupt service routine (ISR), and then resume exactly where it was. Interrupts let the device react within microseconds without constantly polling.

static volatile uint32_t ticks;            /* shared with the ISR: volatile */
static volatile uint8_t  button_pressed;

void SysTick_Handler(void) {               /* called by hardware every 1 ms (name from the vendor) */
    ticks++;
}

void EXTI0_IRQHandler(void) {              /* button interrupt */
    button_pressed = 1;                    /* just record it; do the work in the main loop */
    /* clear the interrupt flag here, as the reference manual requires */
}

int main(void) {
    for (;;) {
        if (button_pressed) {
            button_pressed = 0;
            start_wash_cycle();            /* slow work happens outside the ISR */
        }
    }
}

Timing without blocking

uint32_t last = 0;
for (;;) {
    if (ticks - last >= 500) {     /* every 500 ms, without stopping everything else */
        last = ticks;
        toggle_led();
    }
    poll_uart();                   /* the loop keeps serving other jobs */
}

Debouncing a button

A mechanical button "bounces": for a few milliseconds after pressing, its contact opens and closes many times, which the MCU sees as several presses. Debouncing means accepting a new state only after it has been stable for, say, 20 ms (in software) or adding a small RC filter (in hardware).

Constraints that shape embedded C

• Little RAM: avoid big local arrays (small stacks) and often avoid malloc entirely, because fragmentation over months of uptime can cause failure. Allocate statically at start-up.
• Real-time deadlines: an airbag controller or a motor driver must respond within microseconds or milliseconds; code paths must have predictable timing.
• Reliability: devices run for years without restarts; watchdog timers reset the MCU if the main loop ever hangs.
• Safety and standards: automotive code follows MISRA C and processes like ISO 26262; AUTOSAR provides a standard software architecture for car ECUs.
• RTOSes such as FreeRTOS and Zephyr add tasks, queues and timers when a super loop becomes too complex.