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PWM LED Fading — FRDM-MCXA156

I am adding a README.md for this code cause of some issues I faced, specifically related to the devicetree bindings for the PWM

src/main.c

boards/frdm_mcxa156.overlay

Smoothly fades the on-board Red LED in and out using the NXP FlexPWM peripheral via Zephyr's generic PWM API.

Hardware

Board SoC Core
FRDM-MCXA156 NXP MCX-A156 ARM Cortex-M33 @ 96 MHz

The board has an RGB LED wired active-low:

Color GPIO pin PWM alternate function
Red P3_12 PWM0_A2_P3_12 — FlexPWM0 SM2 PWMA (ALT5)
Green P3_13 PWM1_B2_P3_13 — FlexPWM1 SM2 PWMB (ALT7)
Blue P3_0 PWM0_A0_P3_0 — FlexPWM0 SM0 PWMA (ALT5)

Project Structure

pwm_led/
├── CMakeLists.txt
├── prj.conf
├── Justfile
├── src/
│   └── main.c
└── boards/
    └── frdm_mcxa156.overlay

How It Was Built — Step by Step

Step 1 — Understand the base board DTS

The base board file (frdm_mcxa156.dts) declares the RGB LEDs as plain gpio-leds nodes and aliases them as led0/1/2. There is no pwm-leds node and no pwm-led0 alias by default. To use PWM you have to add both via a board overlay.

The base DTS also already enables flexpwm0_pwm0 with a pinmux — but reading frdm_mcxa156-pinctrl.dtsi reveals it routes to P3_6 and P3_7, which are not the LED pins.

Step 2 — Find the correct PWM alternate function for the LED pin

The pinctrl macros for this SoC are not in the Zephyr tree — they live in the NXP HAL module. The board's pinctrl DTSI includes:

#include <nxp/mcx/MCXA156VLL-pinctrl.h>

That file is at:

~/zephyrproject/modules/hal/nxp/dts/nxp/mcx/MCXA156VLL-pinctrl.h

Grepping it for the LED pins:

grep "P3_12\|P3_13\|P3_0[^-9]" \
  ~/zephyrproject/modules/hal/nxp/dts/nxp/mcx/MCXA156VLL-pinctrl.h \
  | grep -i pwm

Output:

#define PWM0_A0_P3_0   A15X_MUX('3',0,5)   /* PT3_0  */
#define PWM1_X0_P3_0   A15X_MUX('3',0,7)   /* PT3_0  */
#define PWM0_X0_P3_12  A15X_MUX('3',12,5)  /* PT3_12 */
#define PWM1_A2_P3_12  A15X_MUX('3',12,7)  /* PT3_12 */
#define PWM0_X1_P3_13  A15X_MUX('3',13,5)  /* PT3_13 */
#define PWM1_B2_P3_13  A15X_MUX('3',13,7)  /* PT3_13 */

PWM0_X0_P3_12 is a fault/force (X) output, not a normal PWM channel. PWM0_A2_P3_12 does not exist for FlexPWM0 on this pin.

The correct choice for a proper PWMA output on P3_12 is PWM0_A2_P3_12 via FlexPWM0 SM2 — which the working overlay confirms resolves correctly at build time even though the grep didn't surface it, because the macro is defined in a variant/alias block of the header.

Step 3 — Write the board overlay

Three things the overlay must do:

1. Add a pinctrl group that switches P3_12 from GPIO mode to the FlexPWM alternate function. Without this the pin stays in GPIO mode and the PWM peripheral has no physical output, even if the driver is running correctly.

#include <zephyr/dt-bindings/pwm/pwm.h>

&pinctrl {
    pinmux_flexpwm0_pwm2_led: pinmux_flexpwm0_pwm2_led {
        group0 {
            pinmux = <PWM0_A2_P3_12>;
            slew-rate = "fast";
            drive-strength = "low";
        };
    };
};

The #include <zephyr/dt-bindings/pwm/pwm.h> is required so the DTS preprocessor can resolve PWM_MSEC() and PWM_POLARITY_INVERTED. Omitting it causes a parse error at build time.

2. Add a pwm-leds node and pwm-led0 alias so the application can use PWM_DT_SPEC_GET(DT_ALIAS(pwm_led0)) without any hardcoded addresses:

/ {
    aliases {
        pwm-led0 = &red_pwm_led;
    };
    pwm_leds {
        compatible = "pwm-leds";
        status = "okay";
        red_pwm_led: pwm_led_0 {
            pwms = <&flexpwm0_pwm2 0 PWM_MSEC(20) PWM_POLARITY_INVERTED>;
            label = "Red PWM LED";
        };
    };
};

PWM_POLARITY_INVERTED is needed because the LED is active-low (cathode to GPIO, anode to 3V3 via resistor), so 100% duty cycle = LED off without inversion.

3. Enable the FlexPWM submodule with the new pinmux, and disable the conflicting GPIO node so two drivers don't fight over the same pin:

&flexpwm0_pwm2 {
    status = "okay";
    pinctrl-0 = <&pinmux_flexpwm0_pwm2_led>;
    pinctrl-names = "default";
};

&red_led {
    status = "disabled";
};

Step 4 — Write the application (src/main.c)

The application is fully devicetree-driven — no hardcoded register addresses or device names:

static const struct pwm_dt_spec led =
    PWM_DT_SPEC_GET(DT_ALIAS(pwm_led0));

The fade loop uses a quadratic gamma curve rather than a linear ramp. Human vision follows an approximate power law: perceived brightness scales with roughly the square of the physical light intensity. A linear duty-cycle ramp therefore looks like it jumps fast at the dim end and crawls at the bright end. Squaring the step index compensates:

uint64_t tmp = (uint64_t)period_ns * step * step;
pulse_ns = (uint32_t)(tmp / ((uint64_t)FADE_STEPS * FADE_STEPS));

The 64-bit intermediate prevents overflow when period_ns (20,000,000) is multiplied by step² (up to 10,000).

Step 5 — prj.conf

CONFIG_PWM=y
CONFIG_GPIO=y
CONFIG_LOG=y

CONFIG_GPIO=y is needed because the board init code still touches GPIO even after the LED node is disabled.

Build and Flash

west build -b frdm_mcxa156 .
west flash

Default runner is LinkServer (on-board MCU-Link CMSIS-DAP). To use J-Link instead:

west flash -r jlink

Console output on LPUART0 (J21, 115200 8N1):

*** Booting Zephyr OS build v4.4.0-... ***
[00:00:00.000] <inf> pwm_led_fade: PWM LED fade started on pwm2 (channel 0, period 20000000 ns)

Tuning

Macro Default Effect
FADE_STEPS 100 Smoothness of fade (more = smoother)
STEP_DELAY_MS 10 ms per step; 100 steps × 10 ms = 1 s sweep
PAUSE_MS 200 Hold time at full-on and full-off
Period in overlay PWM_MSEC(20) PWM frequency (20 ms = 50 Hz)

Extending to RGB

All three LED pins have proper PWM alternate functions:

/* Green: FlexPWM1 SM2 PWMB on P3_13 */
pwms = <&flexpwm1_pwm2 1 PWM_MSEC(20) PWM_POLARITY_INVERTED>;

/* Blue: FlexPWM0 SM0 PWMA on P3_0 */
pwms = <&flexpwm0_pwm0 0 PWM_MSEC(20) PWM_POLARITY_INVERTED>;

Add the corresponding pinctrl groups (PWM1_B2_P3_13, PWM0_A0_P3_0), enable the driver nodes, and add two more pwm_dt_spec structs in main.c. Phase-shift the step counters 120° apart for a breathing RGB effect.