Using Binary Code Modulation to control LED brightness

Pulse Width Modulation is definitely the preferred method of dimming an LED with a microcontroller, but we were interested in hearing about a different method called Binary Code Modulation. BCM does the same thing as PWM, it turns the LED on and off very rapidly so that your eye cannot detect a flicker. The brightness level is a result of the average amount of time the LED is on versus when it is off. This is called duty cycle and although it can be the same percentage for both PWM and BCD, there is a fundamental difference.

While PWM usually uses a cyclical on/off cycle (30% on, 70% off, repeat) BCD uses a cumulative cycle. As you can see above, each successive bit of binary code carries double significance compared to the previous bit. Now just assign a duty cycle based on your precision, and have an interrupt fire for each bit of the counter. The graph above shows some highs and some lows combining to reach the target duty cycle. An interrupt is used for each bit, and pin changes are made in the service routine.

The benefit of this system is that it is scaleable without adding overhead. You’re already running the interrupts so servicing 8 or 128 LEDs doesn’t have vastly different needs as it would with PWM. The big downside is that the more bits of precision you use, the faster your processor must run so that the eye doesn’t detect the lengthy on or off cycles of the higher bits as visible flickering.

Thank you [Yetihehe] for tipping us off about a link that [Tomas Martinsen] left when commenting about an Arduino library for up to 768 PWM outputs.


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