Batsocks - LED dimming using Binary Code Modulation

LED dimming using Binary Code Modulation

Summary:

This article has given a brief overview of just a couple of methods of dimming LEDs using microcontrollers. In the lists below, I have summarised points about each method, not to find a 'winner', but to help guide you to which method might be suitable for your application. If you're just dimming a single LED then hardware PWM is probably your best bet. If you want to control five or six RGB LEDs from a single AVR, then I'd suggest Binary Code Modulation as a solution (each colour is a separate channel of output). If you're playing with several hobby servo motors, then Software PWM is probably for you.

Hardware PWM

Very well known method.
Easy to use.
Zero processor overhead.
Limited number of channels.
Accurate output of a single frequency.
Suitable for all types of output; speakers, motors, bulbs, LEDs, servo motors (obviously with other driver hardware between the uC and the device)
Only a 16-bit PWM is really suitable for controlling servo motors, as the duty cycle is restricted to 10% to 20%.
Can have a very high frequency
A good solution if you only need to control a few LEDs and the appropriate pins are available for use.

Software PWM

Well known
several implementations available
Multiple channels possible
Processor overhead is dependent on the number of channels (but not dependent on the resolution)
Accurate, single-frequency output.
Also suitable for LEDs, motors, hobby-servos, bulbs etc. (again, obviously with driver hardware, depending on the peripheral in question)
Can be modified to make it especially useful for hobby servo motors

Binary Code Modulation

Less well known
Not many implementations 'in the wild'.
Can run as many channels as you like
Processor overhead is dependent on the resolution (but not dependent on the number of channels)
Suitable for any frequency-instensitive outputs, such as LEDs, bulbs and DC motors.
Not suitable for servo motors.
Can easily run at around 500hz, and with some work up to 15khz for 8-bit resolution on an 8Mhz processor.
Fixed timings and overheads regardless of the duty cycle
An excellent solution for LEDs.
Relatively easy to adapt for charlieplexing

It's always difficult/dangerous to simplify and generalise decisions. The correct choice for a given situation will inevitably involve many more factors than I could possibly cover here, but in order to offer some guidance (however mistakenly), here is what I'd choose for controlling LEDs with 8-bit resolution;

0-4 channels: H/W PWM (if they're available)
0-7 channels: S/W PWM
6+ channels: BCM

Note: Changes:
8th January 2009 -
Modified some details about Software PWM as I think I'd been slightly mislead into believing that S/W PWM is more processor intensive than it is.

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The following files are available for download:

	Version	Date	File	Size
"LED dimming using Binary Code Modulation" article	2.0	2009-01-08	art_bcm_02.pdf	174.3 kB
	1.0	2008-10-25	art_bcm_01.pdf	146.5 kB
example code	1.0	2008-10-25	code_art_bcm_10.c	4.2 kB