Micropython pyboard code for cailbrating and sensing with an ultrasonic sensor

ping.py

from pyb import Pin, ExtInt, LED
import time
import utime

import micropython
micropython.alloc_emergency_exception_buf(100)


global last_ping, delta, LAG

ROUNDTRIP = 58.774  # speed of sound per cm in microseconds * 2 for round trip
LAG = 500           # totally hacky lag compensation

last_ping = utime.ticks_us()
delta = utime.ticks_diff(utime.ticks_us(), last_ping)

echo = Pin('Y7', Pin.IN)
trigger = Pin('Y8', Pin.OUT)


def print_delta(d):
    # this is called by the interrupt handler
    print (delta, " == ", ((d - LAG) // ROUNDTRIP), " cm")


def cb(_):
    global last_ping, delta
    # remember : no floating poing inside an interrupt!
    delta = utime.ticks_diff(utime.ticks_us(), last_ping) - 20
    micropython.schedule(print_delta, delta)


ext = ExtInt(echo, ExtInt.IRQ_FALLING, Pin.PULL_UP, cb)


def ping():
    global last_ping, delta
    trigger.low()
    time.sleep_ms(100)
    trigger.high()
    last_ping = utime.ticks_us()
    time.sleep_us(100)
    trigger.low()
    time.sleep_ms(2)


def calibrate(measured_distance_in_cm):

    COUNT = 64

    global last_ping, delta, LAG

    avg = 0

    for n in range(COUNT):
        ping()
        time.sleep_ms(16)
        if n > 0:
            avg += delta // (COUNT - 1)

    print ("average roundtrip = ", avg)
    print ("expected roundtrip = ", measured_distance_in_cm * ROUNDTRIP)
    print ("---------------------")
    LAG = avg - (measured_distance_in_cm * ROUNDTRIP)
    print ("calculated lag = ", LAG)