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This module loads the Pulse Width Modulation (pwm) driver of the embedded device.

function write

write(pin, period, pulse, time_unit=MILLIS)

Activate PWM (Pulse Width Modulation) on pin pin. Up to 4 PWM can be active at the same time. Max frequency is 40 MHz (25 ns period). The state of pin is periodically switched between LOW and HIGH according to parameters:

  • period is the duration of a pwm square wave.
  • pulse is the time the pwm square wave stays in the HIGH state
  • time_unit is the unit of time period and pulse are expressed in. Can be NANOS, MICROS or MILLIS.

To disable a pwm pass 0 asperiod and 0 as pulse. A PWM wave can be depicted as a train of elements made like this:

        HIGH  _________________          _________________
             |                 |        |                 |
             |                 |        |                 |
        _____|                 |________|                 |____ LOW


The PWM driver automatically chooses the right setup to allow the highest possible resolution of the duty cycle over the desired period. The number of bits (1 to 20) of resolution for a given period T (in seconds) is calculated with the following formula, truncated to the integer result:

N_bits = log(APB_CLOCK*T)/log(2)
with APB_CLOCK = 80000000 Hz.

The relation between period and frequency is:

T = 1/FREQ

In the table below are reported some examples of PWM resolutions for some given periods.

Period Frequency N Bits Resolution
25 ns 40 MHz 1 ½
50 ns 20 MHz 2 ¼
10 ns 10 MHz 3
25 µs 40 kHz 10 1/1024
50 µs 20 kHz 11 1/2048
100 µs 10 kHz 12 1/4096
1 ms 1 kHz 16 1/65536
10 ms 100 Hz 19 1/524288
100 ms 10 Hz 20 1/1048576
500 ms 2 Hz 20 1/1048576

note: To ensure that the PWM works correctly on a specific board, please select pins labelled as PWM on the board pin-map. The correct behavior of the PWM is not guaranteed on pins not labeled as PWM.

Here are some examples:

#Remember to import the pwm module
import pwm

# A 1000 milliseconds wave that stays HIGH for 100 milliseconds and LOW for 900

# A 500 microseconds wave that stays HIGH for 10 microseconds and LOW for 490

# Disable pwm