PWM Output

Component Type: Output

Status: Implemented

Overview

CIOPwm is a PWM output object with constant frequency.

Frequency defaults to CONFIG_DAWN_IO_PWM_DEFAULT_FREQ and may be overridden per descriptor object with the optional freq config field. Duty cycle is changed by data.

Data format: uint32_t, dimension must match PWM driver instance number of channels (for NuttX this is CONFIG_PWM_NCHANNELS). Every write updates the full duty vector for the PWM device.

Duty cycle uses 0.1% units (0 - 0%, 10 - 1%, 1000 - 100%).

Protocol and UI surfaces often expose one scalar control per output channel, while CIOPwm expects one multichannel vector write for the whole device. For those integrations, expose writable scalar virt IOs and connect them to the multichannel pwm IO with vecpack. Numeric duty-cycle controls should use analog-style scalar values; boolean or bitfield-style controls are not a good representation for variable PWM duty.

Implementation

CIOPwm opens the PWM character device at /dev/pwm<devno> during configure() and allocates a per-channel duty buffer. setData() takes duty cycles in the 0..1000 range (parts-per-mille); values out of range return -EINVAL. The number of items in the data buffer must match the configured channel count. Runtime writes to the freq config item update the cached frequency and immediately write the full cached PWM state to the driver. doStart()/doStop() map to NuttX PWMIOC_START/ PWMIOC_STOP ioctls. The IO is write-only and does not support batch operations.

Configuration

Kconfig

• CONFIG_DAWN_IO_PWM: enables PWM-backed IO objects.

• CONFIG_DAWN_IO_PWM_DEFAULT_FREQ: default PWM frequency in Hz when the descriptor does not provide freq. Default: 1000.

YAML

ios:
  - id: pwm1
    type: pwm
    config:
      device: 0
      freq: 1000

  - id: pwm_ch0
    type: virt
    dtype: uint32

  - id: pwm_freq
    type: config
    dtype: uint32
    config:
      objid_ref: pwm1
      objcfg_ref: freq

programs:
  - id: pwm_pack
    type: vecpack
    config:
      inputs: [pwm_ch0, pwm_ch1, pwm_ch2, pwm_ch3]
      output: pwm1

External Control

ConfigIO: the freq config item is read-write. Bind a config IO to objcfg_ref: freq to expose runtime frequency control. The PWM object must include a descriptor freq item for ConfigIO to read and write; if freq is omitted, the object still uses CONFIG_DAWN_IO_PWM_DEFAULT_FREQ but there is no descriptor-backed field for ConfigIO.

Constant duty with data-controlled frequency is not supported yet. It needs a new IO object because CIOPwm data writes are duty updates; frequency is configuration.

ControlIO: not supported yet.

TriggerIO: not supported.

Planned: CIOPwm should support start/stop via CIOControl.

Brainstorming & Future Ideas

Use cases:

• PWM output for capture inputs

• Control DC offset with PWM (for example for heaters)

• PWM as DAC output

• Drive induction loads

• Generate pulses

• Control motor speed

Doxygen

• dawn::CIOPwm