core: arduino: analogWrite: Fix DAC output calculation

[soburi]

• Correct max value calculation
• Add guard for DAC entry is not defined case
• initialize only if DAC not initialized

[Copilot]

Pull request overview

Updates the Zephyr-based Arduino core’s DAC-backed analogWrite() to produce correct full-scale outputs, avoid setup work on every call, and compile cleanly when DAC channel configuration is missing from the devicetree.

Changes:

• Fix DAC max/output scaling by mapping to (2^resolution - 1) and clamping the input range.
• Add devicetree guard for missing dac-channels and make analogWrite(DAC…) a no-op when not defined.
• Initialize DAC channels once (per channel) and only after verifying the DAC device is ready.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cores/arduino/zephyrCommon.cpp	Adds guarded DAC channel config, one-time channel setup, input clamping, and corrected max/output scaling for DAC writes.
cores/arduino/Arduino.h	Guards DAC enum expansion so builds succeed when dac-channels isn’t defined in the devicetree.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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[github-actions]

Built 0.53.2-0.dev+a039a686

CI run PASSED 🟢

Artifact	Board	Core	Tests	RAM	Sketches	Warnings	Errors
✅ zephyr_contrib	ek_ra8d1	📗	✅	11.9%	2	-	-
	frdm_mcxn947	3 🏷️	✅	58.0%	2	-	-
	frdm_rw612	1 🏷️	✅	83.0%	2	-	-
✔️* zephyr_main	giga	4 🏷️	✅*	54.5%	44	8	-
	nano33ble	1 🏷️	✅*	78.7%	22	8	-
	nano_matter	📗	✔️*	⚠️ 85.7%	20	8	(2*)
	niclasense	2 🏷️	✅*	⚠️ 87.3%	20	8	-
	opta	4 🏷️	✔️*	46.7%	54	8	(2*)
	portentac33	3 🏷️	✔️*	⚠️ 95.1%	56	8	(8*)
	portentah7	3 🏷️	✔️*	47.3%	58	8	(2*)
✅* zephyr_unoq	unoq	📗	✅*	26.4%	60	8	-

Legend

Board	Test	Status description
🔥	🔥	Test run failed to complete.
❌	🔴	Test completed with unexpected errors.
✔️*	🚫	Test completed with errors, but all are known/expected.
✅*	🟡	Test completed with some warnings; no errors detected.
✅	🟢	Test passed successfully, with no warnings or errors.
🌑	🌑	Test was skipped.

[soburi]

@pillo79

This is the same issue as #373, so please check it if you have the time.

[CLAassistant]

All committers have signed the CLA.