counter.py

import machine
from machine import Pin

class PinInterruptCounter():
    """Implementation of machine.Counter using standard pin interrupts"""
    def __init__(self, pin, edge=Pin.IRQ_RISING, callback=None):
        self._count = 0
        self._pin = pin
        self._callback = callback
    
        # Setup interrupt handler
        self._pin.irq(trigger=edge, handler=self._handle_interrupt)

    def _handle_interrupt(self, pin):
        self._count += 1
        if self._callback:
            self._callback(self)

    def reset(self):
        irq_state = machine.disable_irq() # Start critical section
        self._count = 0        
        machine.enable_irq(irq_state) # End of critical section
        
    def value(self):
        irq_state = machine.disable_irq() # Start critical section
        out = self._count        
        machine.enable_irq(irq_state) # End of critical section
        return out
    

hall_test.py

import machine
import time

from counter import PinInterruptCounter

def on_tacho(c):
    status_led.value(1)
    status_led.value(0)

tacho_pin = machine.Pin(26, machine.Pin.IN, machine.Pin.PULL_UP)
tacho = PinInterruptCounter(pin=tacho_pin, callback=on_tacho)
status_led = machine.Pin(19, machine.Pin.OUT)

measure_time = 2000 # ms
start = time.ticks_ms()

print("start")

while True:    
    # measure speed

    tacho.reset()
    time.sleep_ms(measure_time)
    count = tacho.value()
    hz = count / (measure_time/1000.0)

    t = time.ticks_ms() - start
    print('rpm-measure', t, count, hz)