PWM#

The kite pwm command provides two modes of operation: raw PWM channel control (set a period and duty cycle on any channel) and an RGB LED brightness mode that drives the onboard LED via PWM instead of simple on/off GPIO.

Source: lib/kite/pwm/kite_pwm.c

Commands#

Command	Description
`kite pwm set <channel> <period_us> <duty_us>`	Set PWM output on a channel
`kite pwm stop <channel>`	Stop PWM on a channel
`kite pwm led <r> <g> <b>`	Set RGB LED brightness (0-255 per channel)

The set/stop commands are available when kite-pwm is defined in the device tree. The led command is available when the kite_pwm_red, kite_pwm_green, and kite_pwm_blue nodes exist.

Examples:

uart:~$ kite pwm set 0 1000 500
PWM ch0: period=1000us duty=500us

uart:~$ kite pwm stop 0
PWM ch0 stopped

uart:~$ kite pwm led 255 0 128
LED: R=255 G=0 B=128

Kconfig#

CONFIG_CSSE4011_SHELL_PWM=y  # default y

Selects the PWM driver.

Device Tree Setup#

The overlay configures PWM for the RGB LED using the pwm-leds compatible binding:

kite_pwm_leds {
    compatible = "pwm-leds";
    kite_pwm_red: pwm_red {
        pwms = <&pwm1 0 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
    };
    kite_pwm_green: pwm_green {
        pwms = <&pwm1 1 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
    };
    kite_pwm_blue: pwm_blue {
        pwms = <&pwm1 2 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
    };
};

Three things to notice:

PWM_MSEC(20) -- the period is 20 ms (50 Hz).
PWM_POLARITY_INVERTED -- the XIAO BLE's RGB LED is common-anode, meaning the LED is on when the pin is driven low. Inverted polarity tells the PWM driver to flip the signal.
Pin control -- the &pinctrl section maps PWM1 channels 0-2 to the physical LED pins (P0.26, P0.30, P0.6) with nordic,invert set.

Code Walkthrough#

Compile-time Feature Detection#

The code uses device tree macros to determine which features are available at compile time:

#define HAS_PWM_DEV  DT_NODE_EXISTS(DT_ALIAS(kite_pwm))
#define HAS_PWM_LEDS DT_NODE_EXISTS(DT_NODELABEL(kite_pwm_red))

The raw channel commands (set/stop) are compiled under #if HAS_PWM_DEV; the LED command under #if HAS_PWM_LEDS. The shell subcommand set uses the same guards, so only the available commands show up in tab completion:

SHELL_STATIC_SUBCMD_SET_CREATE(
    kite_pwm_cmds,
#if HAS_PWM_DEV
    SHELL_CMD_ARG(set, NULL, ..., cmd_pwm_set, 4, 0),
    SHELL_CMD_ARG(stop, NULL, ..., cmd_pwm_stop, 2, 0),
#endif
#if HAS_PWM_LEDS
    SHELL_CMD_ARG(led, NULL, ..., cmd_pwm_led, 4, 0),
#endif
    SHELL_SUBCMD_SET_END);

LED Brightness Scaling#

The led command maps a 0-255 user value to the PWM duty cycle:

uint32_t duty_r = ((uint64_t)pwm_red.period * r) / 255;

pwm_red.period comes from the device tree spec (20 ms in nanoseconds). The cast to uint64_t prevents overflow -- the period is 20,000,000 ns, and 20000000 * 255 exceeds UINT32_MAX. When r = 255, the duty cycle equals the full period (LED fully on). When r = 0, the duty is zero (LED fully off). Because the polarity is inverted in the device tree, the PWM driver handles the inversion transparently -- pwm_set_pulse_dt sends the correct inverted waveform without the C code needing to know about common-anode wiring.

Raw Channel Control#

The set command uses the lower-level pwm_set() API directly, which takes the device pointer, channel number, and period/duty in microseconds:

ret = pwm_set(dev, channel, PWM_USEC(period_us), PWM_USEC(duty_us), 0);

The stop command simply sets period and duty to zero:

ret = pwm_set(dev, channel, 0, 0, 0);

Tip

The raw set command is useful for driving external devices like servos or buzzers. A typical hobby servo expects a 20 ms period (20000 us) with a pulse width between 1000 us (0 degrees) and 2000 us (180 degrees):

kite pwm set 0 20000 1500