PWM

Low-level PWM peripheral driver.

This interface enables access to CPU peripherals generating PWM signals. On most platforms, this interface will be implemented based on hardware timers, though some CPUs provide dedicated PWM peripherals.

The characteristics of a PWM signal can be defined by three basic parameters, namely the frequency, the duty cycle, and the operational mode. This interface supports basic PWM generation in left-aligned, right-aligned, and center mode. Additionally the interface supports the definition of the used resolution, defining the granularity with which one can specify the duty cycle. This brings more flexibility to the configuration of the frequency, especially on systems with low system clocks.

Typically, a single PWM device (e.g. hardware timer) supports PWM signal generation on multiple pins in parallel. While the duty cycle is selectable for each channel individually, the frequency and resolution are shared for all channels.

The mapping/configuration of PWM devices (timers) and the used pins has to be done in the board configuration (the board’s `periph_conf.h).

When using the PWM interface, first thing you have to do is initialize the PWM device with the targeted mode, frequency, and resolution settings. Once the device is initialized, it will start the generation of PWM signals on all configured pins immediately, with an initial duty cycle of 0. Use the pwm.h::pwm_set() function to change the duty cycle for a given channel. If you want to disable the PWM generation again, simply call pwm.h::pwm_poweroff().

(Low-)Power implications

After initialization, the a PWM peripheral should be powered on and active. When manually stopped using the pwm.h::pwm_poweroff() function, the PWM generation should be stopped for all channels and the PWM peripheral should be fully power off (e.g. through peripheral clock gating). Once being re-enabled by calling the pwm.h::pwm_poweron() function, the PWM peripheral should transparently continue its previously configured operation, including the last active duty cycle values.

While a PWM device is active, some implementations might need to block certain power modes.

enum pwm_mode_t
PWM_CENTER_INV = PWM_MODE(1, 0)
not supported
PWM_LEFT = PWM_MODE(0, 1)
left aligned PWM
PWM_RIGHT = PWM_MODE(0, 0)
right aligned PWM
PWM_CENTER = PWM_MODE(1, 1)
not supported
unsigned int pwm_t

Default PWM type definition.

uint32_t pwm_init(pwm.h::pwm_t dev, nrf52/include/periph_cpu.h::pwm_mode_t mode, uint32_t freq, uint16_t res)

Initialize a PWM device.

The PWM module is based on virtual PWM devices, which can have one or more channels. The PWM devices can be configured to run with a given frequency and resolution, which are always identical for the complete device, hence for every channel on a device.

The desired frequency and resolution may not be possible on a given device when chosen too large. In this case the PWM driver will always keep the resolution and decrease the frequency if needed. To verify the correct settings compare the returned value which is the actually set frequency.

Parameters

dev:PWM device to initialize
mode:PWM mode, left, right or center aligned
freq:PWM frequency in Hz
res:PWM resolution

Return values

  • actual PWM frequency on success
  • 0 on error
uint8_t pwm_channels(pwm.h::pwm_t dev)

Get the number of available channels.

Parameters

dev:PWM device

Return values

  • Number of channels available for the given device
void pwm_set(pwm.h::pwm_t dev, uint8_t channel, uint16_t value)

Set the duty-cycle for a given channel of the given PWM device.

The duty-cycle is set in relation to the chosen resolution of the given device. If value > resolution, value is set to resolution.

Parameters

dev:the PWM device to set
channel:the channel of the given device to set
value:the desired duty-cycle to set

void pwm_poweron(pwm.h::pwm_t dev)

Resume PWM generation on the given device.

When this function is called, the given PWM device is powered on and continues its previously configured operation. The duty cycle of each channel will be the value that was last set.

This function must not be called before the PWM device was initialized.

Parameters

dev:device to start

void pwm_poweroff(pwm.h::pwm_t dev)

Stop PWM generation on the given device.

This function stops the PWM generation on all configured channels for the given device and powers down the given PWM peripheral.

Parameters

dev:device to stop

PWM_DEV( x)

Default PWM access macro. 

1
(x)
PWM_UNDEF

Default PWM undefined value. 

1
(UINT_MAX)