This is a simple project that demonstrates Pulse Width Modulation (PWM) using GenServer.

Nerves applications produce images for hardware targets based on the MIX_TARGET environment variable. If MIX_TARGET is unset, mix builds an image that runs on the host (e.g., your laptop). This is useful for executing logic tests, running utilities, and debugging. Other targets are represented by a short name like rpi3 that maps to a Nerves system image for that platform. All of this logic is in the generated mix.exs and may be customized. For more information about targets see:

https://hexdocs.pm/nerves/targets.html#content

To start your Nerves app:

• export MIX_TARGET=my_target or prefix every command with MIX_TARGET=my_target. For example, MIX_TARGET=rpi3
• Install dependencies with mix deps.get
• Create firmware with mix firmware
• Burn to an SD card with mix firmware.burn

Here is an example of blinking an LED in various timings.

# A GPIO pin for an LED.
gpio_pin = 12

# Get a reference to the LED.
{:ok, led_ref} = Circuits.GPIO.open(gpio_pin, :output)

# Start a scheduler with on/off callback functions and initial settings for the
# period (frequency in Hz and duty cycle in percentage).
NervesHelloPwm.PwmScheduler.start_link(%{
  id: gpio_pin,
  frequency: 1,
  duty_cycle: 50,
  on_fn: fn -> Circuits.GPIO.write(led_ref, 1)  end,
  off_fn: fn -> Circuits.GPIO.write(led_ref, 0) end
})

# Change the on/off ratio to 4:1.
NervesHelloPwm.PwmScheduler.change_period(gpio_pin, 1, 80)

# Change the frequency to 2Hz (2x faster than 1Hz).
NervesHelloPwm.PwmScheduler.change_period(gpio_pin, 2, 80)

# Stop the scheduler.
NervesHelloPwm.PwmScheduler.stop(gpio_pin)
** (EXIT from #PID<0.1202.0>) shell process exited with reason: shutdown

PWM scheduler processes are registered with an ID so it is possible to start multiple PWM scheduler processes as long as IDs are unique.

Since Elixir's Process.send_after/3 function is millisecond precision, there is a limitation on the pulse precision. So NervesHelloPwm.PwmScheduler has the maximum frequency of 100Hz (10ms / period), which is probably fast enough to dim the brightness of an LED.

If you need faster and more precise PWM, please consider alternative approaches, such as accessing the target device's built-in hardware PWM, using an external PWM driver board, etc.

gpio = 12
frequency = 800
Pigpiox.Pwm.hardware_pwm(gpio, frequency, 1_000_000) # 100%
Pigpiox.Pwm.hardware_pwm(gpio, frequency, 500_000)   # 50%
Pigpiox.Pwm.hardware_pwm(gpio, frequency, 100_000)   # 10%
Pigpiox.Pwm.hardware_pwm(gpio, frequency, 10_000)    # 1%

• Official docs: https://hexdocs.pm/nerves/getting-started.html
• Official website: https://nerves-project.org/
• Forum: https://elixirforum.com/c/nerves-forum
• Discussion Slack elixir-lang #nerves (Invite)
• Source: https://github.com/nerves-project/nerves