PWM¶
Pulse Width Modulation (PWM) is a basic technique to create repeated square waves (digital high/low voltage transitions) of user defined length and duty cycle. It can be used as a way to encode an “analog” signal on a single digital (high/low) line using the time between transitions (“pulse width”) as the variable; this technique is commonly used to send servo position and motor speed commands. Another use is to use to the ratio of “high” and “low” time to approximate a voltage output; this technique can be used to dim an LED or even (with careful filtering) generate audio waveforms.
Overview¶
Each PWM output is driven by an output channel connected to one of 4 timers. Some configuration, such as the clock rate or prescaling, must be common to the entire timer; see the timer documentation for more information. See your board’s pin mapping tables to track down the correspondence between timer channels and GPIO pins.
Background¶
In its simplest form, the device is a single counter with two variables. The counter starts at zero, and the output starts at “high”. The counter increments every clock cycle until it reaches the first variable number, at which point the output goes “low”. The counter continues incrementing until it reaches the second variable at which point the output goes “high” again and the counter resets to zero. The time spent with output high is called the pulse duration or duty; the total time before repeat is the period.
This simple functionality could be approximated in software by setting a GPIO high or low, but the beauty of PWM is that user code simply has to configure the device and set the two variables and the device will function on its own; no further microprocessor cycles will be consumed, and a repeated high/low waveform will spew out.
The Maple has 16-bit PWM resolution, which means that the counter and variables can be as large as 65535, as opposed to 255 with 8-bit resolution. With a 72MHz clock rate, a PWM output could have maximum period of about one millisecond; using a prescaler (clock divider) in front of the counter can increase this maximum period. Setting the period to something other than the maximum value gives further control over the total length of the waveform. However, this effectively limits the resolution with which the duty can be modified: the duty must be less than or equal to the period.
Here are some commonly used PWM configurations (note that servos are notoriously variable, especially the lower cost models):
| Purpose | Period (ms) | Duty (ms) | Prescaler | Period | Duty |
|---|---|---|---|---|---|
| LED throb | 0.020 | 0–0.020 | 1 (none) | 65535 (default) | 0–767 |
| Servo control | 20 | 1.25 (0°) 1.50 (90°) 1.75 (180°) |
21 21 21 |
65535 (default) 65535 (default) 65535 (default) |
4096 4915 5734 |
Function Reference¶
- pinMode()
- pwmWrite()
- Timer API (especially setOverflow(), setPrescaleFactor(), and setPeriod()).
- Timers reference.
Recommended Reading¶
- Wikipedia Article on Pulse-width modulation
- Arduino tutorial on PWM
- Secrets of Arduino PWM by Ken Shirriff
- So You Want To Use PWM, Eh? at Non-Lexical Vocables
- STMicro documentation for STM32F103RB microcontroller:
- Datasheet (pdf)
- Reference Manual (pdf)
