Am working on a smart car automation system using MicroPython, the system include various functionalities such as `obstacle detection`, `line following`, `speed control`, and `environmental monitoring`. am trying to detect obstacles in front of the car using an ultrasonic sensor `HC-SR04` and control the motor accordingly. my aim is to measure the distance to obstacles and stop the car if an obstacle is too close.
But I keep getting the error
`UnboundLocalError: local variable ‘start’ referenced before assignment`

here’s my code 👇

from machine import Pin
import time

# Ultrasonic sensor setup
trigger = Pin(5, Pin.OUT)
echo = Pin(18, Pin.IN)

def measure_distance():
    trigger.off()
    time.sleep_us(2)
    trigger.on()
    time.sleep_us(10)
    trigger.off()

    while echo.value() == 0:
        start = time.ticks_us()
    while echo.value() == 1:
        end = time.ticks_us()

    duration = time.ticks_diff(end, start)
    distance = (duration * 0.0343) / 2
    return distance

while True:
    distance = measure_distance()
    print('Distance:', distance, 'cm')
    if distance < 10:
        print("Obstacle detected! Stopping car.")
        stop()  # Add stop function from Stage 1
    time.sleep(1)