cpu/efm32: improve PWM driver

[basilfx]

Contribution description

Trying to fix the real problem on why this driver marks EFM32 as unsupported (the comment is wrong), I stumbled upon two things:

• PWM_CENTER is actually supported (counting up-down-up)
• PWM_RIGHT was inverted.

I guess that me from ten years ago did not understand this.

Testing procedure

I used the tests/periph/pwm application to test this, together with an STK3600 that maps a single PWM channel to the onboard LED. I then used a scope to verify the period and frequency, measured at the onboard LED.

> init 0 0 1000 4096
The pwm frequency is set to 1464
>
> set 0 0 1024
>
> init 0 1 1000 4096
The pwm frequency is set to 1464
>
> set 0 0 1024
>
> init 0 2 1000 4096
The pwm frequency is set to 1464
>
> set 0 0 1024
>

Without this PR, you will notice that the onboard led is brighter in PWM_RIGHT mode, because it is inverted. The scope screenshot for the unfixed PWM_RIGHT shows this.

The scope screenshots below will not tell you if left, right or center alignment is working correctly (I don't know how to measure this easily). But it will show you that the frequency and period remains correct (requested 1000 Hz, got ~1500 Hz).

PWM_LEFT @ 25%

PWM_RIGHT (UNFIXED) @ 25%

PWM_RIGHT (FIXED) @ 25%

PWM_CENTER @ 25%

Issues/PRs references

#22651 lead to this.

Declaration of AI-Tools / LLMs usage:

AI-Tools / LLMs that were used are:

• Claude Sonnet 5 helped me to understand, then I fixed it myself.

[crasbe]

why this driver marks EFM32 as unsupported (the comment is wrong)

Wouldn't it make sense to correct this in this PR too? 🤔

[basilfx]

why this driver marks EFM32 as unsupported (the comment is wrong)

Wouldn't it make sense to correct this in this PR too? 🤔

I have another PR incoming to clean this up, and for cryptoauthlib. That keeps this PR clean, the arch_efm32 PR and the incoming one.

[riot-ci]

Murdock results

✔️ PASSED

5edac95 cpu/efm32: fix PWM_RIGHT being inverted

Success	Failures	Total	Runtime
11285	0	11285	15m:05s

Artifacts

• Documentation preview