Have a look at RMT https://docs.pycom.io/chapter/tutorials/all/rmt.html allows you to send/receive pulses. - i haven't tried this.

Alternately is used pin iterrupts to measure RPM (hall effect) this would be accurate for RPM, but I doubt it would be for AC Freq

anyway

class RPM:
    
    def __init__(self, pin):
        self._pin = Pin(pin,mode=Pin.IN)
        self._intvl = 0
        self._pin.callback(Pin.IRQ_RISING,handler=self._callback,arg=self)
        self._timer = Timer.Chrono()
        self._timer.start()

    def _callback(self,obj):
        self._intvl = self._timer.read_ms()
        self._timer.reset()   

    def rpm(self):
        return 60000 / self._intvl 

I can simulate the RPM within reason by using PWM output on one of the other PINS.
That was accurate enough for my needs.

from machine import RMT, Timer, Pin
def kill_rmt(x):
    pin = Pin("P10",mode=Pin.OUT)
    pin(0)

def run():
    alarm = Timer.Alarm(kill_rmt, 2, periodic=False)
    rmt = RMT(channel=4)
    rmt.init(gpio="P10", rx_idle_threshold=20000)

    data = rmt.pulses_get()
    sum = 0
    for _ in data[2:]:
        print(_[0], _[1])
        sum += _[1]
    print(sum, len(data), ((len(data) - 2) * 1.e6)/sum/2)

run()

The question then is, how precise you want to get the result. Obviously it is limited by two factors:

• the precision of the xxPy clock. If its deviation is more or less stable, you can compensate
• the length of the pulses_get sequence. You can take 63 periods. At 50Hz this is 1.260 s. And so the error is about 10E-6.

In my test set-up here with a waveform generator set to 50Hz the script returns something like 50.0002x Hz. But I do not know the error of either component.