Summary

This PR adds machine.PWM support to the alif port. It uses the existing common machine bindings and implements the standard set of functionality.

It uses the UTIMER peripheral and makes PWM available on all pins that have an alt function connection to a UTIMER, which is 54 pins (see below about UTIMER11).

Testing

Tested on ALIF_ENSEMBLE and OPENMV_AE3, using an oscilloscope, fading LEDs and the existing tests/extmod_hardware/machine_pwm.py test.

Sample test code that works on both alif boards:

import time, machine
p = machine.PWM("LED_RED", invert=True)
p.freq(1000)
for _ in range(10):
    for duty in range(101):
        p.duty_ns(duty * 10000)
        time.sleep_ms(10)

Trade-offs and Alternatives

This uses the UTIMER peripheral, but not UTIMER11 which is already in use by the HE core for its systick timer. So the following pins don't have PWM available because they need UTIMER11: P2_6, P2_7, P7_6, P7_7, P12_6, P12_7. For OPENMV_AE3, none of those pins are exposed to the user so they don't need PWM.

For the implementation I tried to use the Alif SDK driver, but it's quite limited so most of the code here uses direct register reads/writes.