Software PWM on a microcontroller: the timer-interrupt method

Hardware PWM is best, but microcontrollers have only a few PWM pins. When you need ten dimmable LEDs or several slow heater outputs, software PWM from a timer interrupt does the job with nothing but ordinary I/O pins.

The idea

A timer interrupt fires at a fixed rate. Each time, a phase counter advances by one step and wraps around after N steps. Each output is on while the counter is below that channel's duty value and off for the rest of the cycle.

volatile uint8_t duty[4];          /* 0..255, set from the main loop */

void timer_isr(void) {             /* called at a fixed rate */
    static uint8_t phase;          /* wraps at 256 */
    uint8_t out = 0;
    phase++;
    if (phase < duty[0]) out |= 1 << 0;
    if (phase < duty[1]) out |= 1 << 1;
    if (phase < duty[2]) out |= 1 << 2;
    if (phase < duty[3]) out |= 1 << 3;
    PORTB = (PORTB & 0xF0) | out;  /* update all pins at once */
}

Resolution vs frequency

The PWM frequency is the interrupt rate divided by the number of steps: f_PWM = f_ISR / N. For flicker-free LEDs you want at least a few hundred hertz. With 256 steps at 200 Hz the interrupt must run at 51.2 kHz.

• On a 16 MHz AVR that leaves about 310 clock cycles per interrupt — comfortable for a handful of channels.
• On a PIC16 at 4 MHz (1 million instructions per second) it leaves under 20 instructions — impossible. Drop to 64 steps at 200 Hz (12.8 kHz interrupts, about 78 instructions each) and it fits.

Rule of thumb: pick the lowest resolution your application can live with. LEDs look smooth with 64 levels if you apply a gamma curve; heaters and slow loads can run at 1–10 Hz with any resolution you like.

Details that matter

• Write the whole port at once, as above, so all channels switch at the same instant instead of drifting apart as the ISR works through them.
• Update duty values atomically. Single-byte values are safe on 8-bit MCUs; 16-bit values need interrupts disabled briefly while you change them.
• 0 % and 100 %: with phase < duty and 8-bit values, duty 0 is fully off but 255 is only 255/256 on. Special-case full-on if it matters.
• Servos need a 1–2 ms pulse every 20 ms with fine resolution — better served by a timer-compare approach (one interrupt per edge) than by a fixed-rate counter.
• Motors and anything audible need PWM above about 20 kHz; that's a job for hardware PWM.

Bit-angle modulation

If the interrupt load is too high, bit-angle modulation (BAM) produces the same average brightness with far fewer interrupts: for 8-bit resolution you use 8 time slots per cycle with lengths 1, 2, 4 … 128 units, and in each slot you output the corresponding bit of every channel's duty value. Eight interrupts per cycle instead of 256 — ideal for driving many LEDs from a small microcontroller.