Add PWM Support

[liamdiprose]

Opening this issue to discuss the addition of PWM support.

I'm a computer engineering student that has been allowed to use Rust for a project in one of my embedded software courses. Being an audio-processing project, I will probably need PWM support.

I am happy to add support for PWM. I have managed to get a very basic demonstration working with the tm4c123x library. My next step is to implement the PwmPin trait for every supported pin.

Are there any other points we should take into consideration for PWM pins?
Keen to hear your input.

[thejpster]

So I use the pins in PWM mode in github.com/thejpster/monotron. I've been meaning to put together a simple API but for now I do it through raw svd2rust calls.

Whatever we do should implement the embedded-hal traits. What state are they in for PWM? I haven't checked lately.

[thejpster]

As an aside, Monotron is scheduled to receive a dual-6-bit PWM audio output at some stage, so any help getting that running would be excellent. It has to be six bit because I think the PWM cycles need to divide down nicely into the video line timing, where on Monotron a video line is precisely 2112 clock cycles long (and 2112 = 33 x 2^6). I think that gives 10 effective bits in count-up/down mode.

[thejpster]

Just to follow up on the above - two points of clarification:

• The TM4C123 has two peripherals which can do PWM - the Timers and the PWM peripheral.
  • The Timers only do count-up (which is fine for Monotron's horizontal sync signal).
  • The PWM module does count-up-down as well (which is ideal for Monotron's audio output), but doesn't exist on the older LM4F120, only the TM4C12x - limiting backwards compatibility.
• The Monotron audio doesn't need to be dual-6-bit, 8-bit audio works just fine as long as you accept that you can't go full rail (i.e. maximum 256 out of 264) and set your DC bias appropriately.

[mokomull]

I took a stab in #28, but I can only test on an LM4F120 (= TM4C1233, it seems). As far as I can tell, none of the tm4c123x have the dedicated PWM peripheral; that appears to be specific to the 129x series.

[thejpster]

Nice, thanks! Just to note that the TM4C123GH6PM definitely has PWM: https://docs.rs/tm4c123x/0.9.0/tm4c123x/struct.PWM0.html.

The LM4F120 series was canned due to an alarming number of errata. I think the TM4C123G is the closest to the LM4F120, but they added a quadrature decoder and the aforementioned PWM peripheral.