boards: support common-cathode PWM RGB LED - #402
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The PWM LED driver assumes a common-anode LED: on nRF52 PWM (Up mode) duty == 0 leaves the pin HIGH, so "larger duty == brighter" only holds when the pin sinks current (common-anode). A common-cathode RGB LED is driven the opposite way and ends up fully lit with no state response. Add an opt-in LED_RGB_COMMON_CATHODE macro. When defined, the duty is inverted in led_pwm_duty_cycle() and the redundant LED_STATE_ON inversion in led_tick() is skipped to avoid a double inversion. When undefined (default) behavior is unchanged from upstream (common-anode). Verified on a common-cathode RGB board: previously all three channels stayed fully lit; with the fix the status color shows and the primary LED breathes correctly. Common-anode boards (e.g. particle_argon, feather_nrf52840_express) still build and behave identically.
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Description of Change
The PWM LED driver assumes a common-anode LED: on nRF52 PWM (Up mode) duty == 0 leaves the pin HIGH, so "larger duty == brighter" only holds when the pin sinks current (common-anode). A common-cathode RGB LED is driven the opposite way and ends up fully lit with no state response.
Add an opt-in LED_RGB_COMMON_CATHODE macro. When defined, the duty is inverted in led_pwm_duty_cycle() and the redundant LED_STATE_ON inversion in led_tick() is skipped to avoid a double inversion. When undefined (default) behavior is unchanged from upstream (common-anode).
Verified on a common-cathode RGB board: previously all three channels stayed fully lit; with the fix the status color shows and the primary LED breathes correctly. Common-anode boards (e.g. particle_argon, feather_nrf52840_express) still build and behave identically.