python 树莓派 超声波测距离

#! /usr/bin/python
# -*- coding:utf-8 -*-

import RPi.GPIO as GPIO
import time
import datetime

def checkdist():
    GPIO.output(2, GPIO.HIGH)
    time.sleep(0.000015)
    GPIO.output(2, GPIO.LOW)
    while not GPIO.input(3):
        pass
    t1 = time.time()
    print "t1:", t1
    while GPIO.input(3):
        pass
    t2 = time.time()
    print "t2:", t2
    t3 = t2 - t1
    print t3
    return t3*340/2
GPIO.setmode(GPIO.BCM)
GPIO.setup(2,  GPIO.OUT, initial=GPIO.LOW)
GPIO.setup(3,GPIO.IN)

time.sleep(2)
try:
    while True:
        print 'Distance: %0.2f m \n' %checkdist()
        time.sleep(1)
except KeyboardInterrupt:
    GPIO.cleanup()