Low-power PWM library

The low-power pulse-width modulation (PWM) library provides functions to generate a pulse-width modulated output signal while using minimal power. The signal can be used, for example, to change the intensity or the color of LEDs.

Key features include:

• Any number of independent PWM instances.
• One output channel per instance that can control multiple pins.
• Minimal power usage.
• Configurable per-instance polarity and period.

This library uses an application timer and is therefore not very accurate. It can be used to control, for example, LEDs, but for peripherals that need a high accuracy, you should use the PWM library instead.

Generating a low-power PWM signal

Complete the following steps to generate a low-power PWM signal:

1. For each instance, create a low_power_pwm_t structure.
2. For each instance, create a low_power_pwm_config_t structure that holds the configuration parameters. If you want to use the default configuration, call the LOW_POWER_PWM_DEFAULT_CONFIG macro with the pin mask to fill the init structure.
   The following parameters must be specified:
   • active_high: Polarity.
   • period: Period of app_timer in ticks.
   • bit_mask: Mask of used pins.
3. If required, provide a callback function (see app_timer_timeout_handler_t) that will be executed in the time-out handler.
4. Initialize each instance using the low_power_pwm_init function, with pointers to the instance, the configuration structure, and optionally the callback function as parameters. The function will return NRF_SUCCESS on success or an error code on failure. Initialization will fail if the instance was already initialized or if an invalid configuration was provided.
5. Call low_power_pwm_duty_set to set the duty cycle of a given instance. You can change the duty cycle later by calling low_power_pwm_duty_set again, for example from the PWM callback function.
6. Call low_power_pwm_start to start the timers of a given instance.

The following code example shows how to initialize and start a low-power PWM instance with the default configuration and a given pin mask:

low_power_pwm_t low_power_pwm;
low_power_pwm_config_t low_power_pwm_config = LOW_POWER_PWM_DEFAULT_CONFIG(LEDS_MASK); // use default configuration with given LEDs mask
low_power_pwm_init((&low_power_pwm), &low_power_pwm_config, pwm_handler);
low_power_pwm_duty_set(&low_power_pwm, 50); // set duty cycle to 50
low_power_pwm_start(&low_power_pwm, LEDS_MASK);

As a result, a PWM signal is generated on the selected output pins.

Limitations

In the default configuration, the RTC frequency is 32768 Hz. The prescaler must not be greater than 6, therefore the timer cannot be started with less than 5 ticks. This means that the break between time-outs is at least 150 microseconds.

Example

See the Low-Power PWM Example for a full example that uses the low-power PWM library.

Usage with a SoftDevice

Low-power PWM uses RTC. Therefore, it is less accurate when used with a SoftDevice.