Post about building a Matter light bulb by franc0is · Pull Request #626 · memfault/interrupt

franc0is · 2026-03-24T20:38:32Z

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cloudflare-workers-and-pages · 2026-03-24T20:40:27Z

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Pull request overview

Adds a long-form blog post describing how to build a Matter dimmable light on the nRF54LM20 DK, along with accompanying example firmware snippets and helper scripts for composing/generating ZAP artifacts.

Changes:

Add ZAP fragment compose/decompose utilities and a generation wrapper script.
Add a Zephyr-based light module (PWM + shell commands) and a board device-tree overlay for PWM LED pins.
Add a new Jekyll post documenting the full build and implementation walkthrough.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 13 comments.

Show a summary per file

File	Description
example/matter-light-bulb/zap_generate.sh	Adds a shell pipeline to compose ZAP fragments and run Matter ZAP codegen.
example/matter-light-bulb/zap_decompose.py	Adds a utility to split a monolithic `.zap` into reusable fragments.
example/matter-light-bulb/zap_compose.py	Adds a utility to merge fragments into a generated `.zap` and endpoints list.
example/matter-light-bulb/nrf54lm20dk_nrf54lm20a_cpuapp.overlay	Adds PWM LED + pinctrl configuration for the target DK.
example/matter-light-bulb/light.h	Declares a simple C API for light on/off + brightness control.
example/matter-light-bulb/light.c	Implements PWM-based LED driving and Zephyr shell commands.
_posts/2026-03-31-matter-light-bulb.md	Publishes the “Matter light bulb from scratch” post with code excerpts and build steps.

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Copilot · 2026-04-02T19:27:18Z

example/matter-light-bulb/light.c

+#include <zephyr/drivers/pwm.h>
+#include <zephyr/shell/shell.h>
+#include <zephyr/logging/log.h>
+


strtoul() is used below but <stdlib.h> isn’t included, which can trigger a build failure under C99/C11 (implicit declaration). Add the proper standard header.

Copilot · 2026-04-02T19:27:18Z

example/matter-light-bulb/light.c

+			/* Map level 1–254 to pulse width (use uint64 to avoid overflow) */
+			pulse = (uint32_t)(((uint64_t)pwm_leds[i].period * light_level) / 254);
+		}
+		pwm_set_pulse_dt(&pwm_leds[i], pulse);
+	}


pwm_set_pulse_dt() returns an error code, but it’s ignored here. If the driver rejects an out-of-range pulse (e.g., due to an unexpected level), failures will be silent and the light state will desync from Matter. Capture and log/handle the return value.

Copilot · 2026-04-02T19:27:19Z

_posts/2026-03-31-matter-light-bulb.md

+set(ZAP_GEN_DIR ${CMAKE_CURRENT_SOURCE_DIR}/build/zap)
+if(NOT EXISTS ${ZAP_GEN_DIR}/matter_echo.zap)
+  execute_process(
+    COMMAND bash ${CMAKE_CURRENT_SOURCE_DIR}/tools/zap_generate.sh ${ZAP_GEN_DIR}


The post’s CMake snippet references ${CMAKE_CURRENT_SOURCE_DIR}/tools/zap_generate.sh, but in this repo the script is at example/matter-light-bulb/zap_generate.sh and there is no example/matter-light-bulb/tools/ directory. Update the snippet (or the file layout) so readers can follow it verbatim.

Suggested change

COMMAND bash ${CMAKE_CURRENT_SOURCE_DIR}/tools/zap_generate.sh ${ZAP_GEN_DIR}

COMMAND bash ${CMAKE_CURRENT_SOURCE_DIR}/zap_generate.sh ${ZAP_GEN_DIR}

Copilot · 2026-04-02T19:27:19Z

_posts/2026-03-31-matter-light-bulb.md

+    /* Light endpoint ID must match the ZAP file (endpoint 2) */
+    static constexpr EndpointId kLightEndpointId = 2;


This comment/snippet says the light endpoint is "endpoint 2", but earlier the post describes the application endpoint as endpoint 1, and the provided zap_compose.py assigns endpoint IDs sequentially starting at 0 (root) then 1 (first fragment). This mismatch is likely to break copy/paste for readers; align the endpoint ID consistently.

Suggested change

/* Light endpoint ID must match the ZAP file (endpoint 2) */

static constexpr EndpointId kLightEndpointId = 2;

/* Light endpoint ID must match the ZAP file (endpoint 1) */

static constexpr EndpointId kLightEndpointId = 1;

Copilot · 2026-04-02T19:27:19Z

_posts/2026-03-31-matter-light-bulb.md

+
+Phew, I learned a lot about Matter putting this together. In the end, building a Matter light bulb is straightforward! Indeed, had I not gotten sucked into the ZAP file side-quest, it would have taken no more than an hour or two.
+
+This firmware build stands on the shoulders of giants. Below our 200 lines of application code lies tends of thousands of lines of C++ implementing the Matter spec. The resulting firmware binary is almost 1MB! So while this was relatively little work, it is a complex firmware.


Typo: “tends of thousands” should be “tens of thousands”.

Suggested change

This firmware build stands on the shoulders of giants. Below our 200 lines of application code lies tends of thousands of lines of C++ implementing the Matter spec. The resulting firmware binary is almost 1MB! So while this was relatively little work, it is a complex firmware.

This firmware build stands on the shoulders of giants. Below our 200 lines of application code lies tens of thousands of lines of C++ implementing the Matter spec. The resulting firmware binary is almost 1MB! So while this was relatively little work, it is a complex firmware.

Copilot · 2026-04-02T19:27:20Z

example/matter-light-bulb/zap_generate.sh

+python3 "$GENERATE_PY" \
+    --no-prettify-output \
+    --templates "$APP_TEMPLATES" \
+    --zcl "$ZCL_JSON" \
+    --output-dir "$ZAP_GENERATED" \
+    "$ZAP_FILE" 2>&1 | grep -E "^[✍🕐]" || true


Because this python invocation is in a pipeline and ends with "|| true", the script will report success even if generate.py fails. Consider enabling pipefail and preserving generate.py’s exit status (while still optionally filtering output) so failures stop the script.

Copilot · 2026-04-02T19:27:21Z

example/matter-light-bulb/zap_generate.sh

+python3 "$GENERATE_PY" \
+    --no-prettify-output \
+    --templates "$IDL_TEMPLATES" \
+    --zcl "$ZCL_JSON" \
+    --output-dir "$ZAP_DIR" \
+    "$ZAP_FILE" 2>&1 | grep -E "^[✍🕐]" || true


Same issue as above: piping through grep and ending with "|| true" masks generate.py failures. The second generation step should also propagate errors so ZAP generation can’t silently fail.

Copilot · 2026-04-02T19:27:21Z

example/matter-light-bulb/light.c

+
+#define NUM_LEDS ARRAY_SIZE(pwm_leds)
+


NUM_LEDS uses ARRAY_SIZE(), but this translation unit doesn’t include the header that defines it (it relies on transitive includes). Add the appropriate Zephyr util header (or define a local ARRAY_SIZE) to make this file self-contained.

Copilot · 2026-04-02T19:27:21Z

example/matter-light-bulb/light.c

+	if (level > 0) {
+		light_on = true;
+	}


When level is set to 0, apply_pwm() will turn the LEDs off, but light_on remains unchanged. That can make light_get()/status report ON while brightness is 0. Consider explicitly setting light_on = (level > 0) to keep state consistent.

Suggested change

if (level > 0) {

light_on = true;

}

light_on = (level > 0);

Copilot · 2026-04-02T19:27:21Z

example/matter-light-bulb/light.c

+	uint8_t level = (uint8_t)strtoul(argv[1], NULL, 10);
+
+	light_set_level(level);
+	return 0;


The shell input is parsed with strtoul() and then cast to uint8_t, so out-of-range values will wrap (e.g., 256 → 0) and 255 can produce an out-of-range PWM pulse. Validate/clamp the value to 0–254 (per the help text) and return an error on invalid input.

franc0is requested a review from a team as a code owner March 24, 2026 20:38

franc0is force-pushed the fbo/matter-light-bulb-post branch 2 times, most recently from c4e68e4 to 0784754 Compare March 24, 2026 20:40

noahp requested a review from Copilot April 2, 2026 19:21

Copilot started reviewing on behalf of noahp April 2, 2026 19:22 View session

franc0is and others added 2 commits April 2, 2026 15:26

New post: building a matter light bulb with the LM20A

47ed959

fix url reference

f6a5b82

noahp force-pushed the fbo/matter-light-bulb-post branch from 0784754 to f6a5b82 Compare April 2, 2026 19:27

Copilot AI reviewed Apr 2, 2026

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Post about building a Matter light bulb#626

Post about building a Matter light bulb#626
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	COMMAND bash ${CMAKE_CURRENT_SOURCE_DIR}/tools/zap_generate.sh ${ZAP_GEN_DIR}
	COMMAND bash ${CMAKE_CURRENT_SOURCE_DIR}/zap_generate.sh ${ZAP_GEN_DIR}

		/* Light endpoint ID must match the ZAP file (endpoint 2) */
		static constexpr EndpointId kLightEndpointId = 2;


		Phew, I learned a lot about Matter putting this together. In the end, building a Matter light bulb is straightforward! Indeed, had I not gotten sucked into the ZAP file side-quest, it would have taken no more than an hour or two.

		This firmware build stands on the shoulders of giants. Below our 200 lines of application code lies tends of thousands of lines of C++ implementing the Matter spec. The resulting firmware binary is almost 1MB! So while this was relatively little work, it is a complex firmware.

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franc0is commented Mar 24, 2026

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