cpu/fe310: add pwm peripheral driver

[aabadie]

Contribution description

This PR implements the PWM driver for RISC-V FE310 CPU and adds a basic configuration for Sifive HiFive1 and HiFive1b boards.

Since a valid core clock value is required, this PR is based on #12519.

Testing procedure

• Build and flash the tests/periph_pwm application on a Hifive1/b board:

$ make BOARD=hifive1b -C tests/periph_pwm flash term

• Call the osci shell command

> osci

You should see the on-board leds oscillate synchronously (they are connected to an RGD led, so the led oscillates in white). You can also plug a led on the other configured pins (Arduino D4, D16, D17) and watch them oscillate as well.

Issues/PRs references

Based on #12934

[aabadie]

rebased

[fjmolinas]

Looks good, one minor comment. Will test on monday.

[fjmolinas]

Seems like hifive1 and hhifive1b can share the config.

[fjmolinas]

This calculation seems very bad to me, how can you get a good approximation of freq without taking the cpu_freq into account? It results in very weird frequency settings that do not make much sense.

If you want full resolution for example that means you can only scale the register. But if you wan't only lets say 8 bits of resolution then you have 8 more to tune the desired frequency. For me the calculation should take into account:

• bits left over by desired resolution: res/2^16
• cpu_freq

We are basically trying to resolve:

min CPU_FREQ * (2^x)/(2^(16-scale)) - freq, where x = 1...2^rembits and scale = 0...(PWM_CMPWIDTH - 1). I don't know efficient algorithms to find this, would have to think about it more. But one way would be find a scale which gives you a PWM_FREQ higher than freq and then trim the value with the remaining resolution bits. Another option is for every scale find the best match and then compare them all.

For the first suggestion.

1. In pseudo code: find scaling that fives you a PWM_FREQ > freq:

    uint8_t scale = (PWM_CMPWIDTH - 1);
    while ((scale > 0) &&
           cpu_freq() / (uint32_t)(1 << (scale + PWM_CMPWIDTH)) << freq) {
        scale--;
    }

2. Use the leftover bits for the required resolution to trim PWM_FREQ as close to freq as possible

Then those leftover bits or rem_res give you more wiggle room to set the a freq closest to the desired one.

[aabadie]

I removed the Murdock label to get this one out of the queue.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.

[benpicco]

Needs a rebase

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.