Problem reading UltraBoard values with threading

The reason you see values with decimals on occasion is because the readdistance thread updates the global values twice. This means the other thread can read the value after the first update, but before the second.

For example:

def readdistance():
	global act_dist1
	global act_dist2
	global act_dist3
	global act_dist4
	while True:
		act_dist1 = UB.GetDistance1()
		act_dist2 = UB.GetDistance2()
		act_dist3 = UB.GetDistance3()
		act_dist4 = UB.GetDistance4()
		# The control thread looks at the values at this point
		act_dist1 = int(act_dist1)
		act_dist2 = int(act_dist2)
		act_dist3 = int(act_dist3)
		act_dist4 = int(act_dist4)
		print("act_dist1 read ", act_dist1)
		print("act_dist2 read ", act_dist2)
		print("act_dist3 read ", act_dist3)
		print("act_dist4 read ", act_dist4)
		time.sleep(0.5)

Without the sleep this occurs more often, but it will happen occasionally even with the sleep. This can be fixed by only changing the global values once:

def readdistance():
	global act_dist1
	global act_dist2
	global act_dist3
	global act_dist4
	while True:
		read_dist1 = UB.GetDistance1()
		read_dist2 = UB.GetDistance2()
		read_dist3 = UB.GetDistance3()
		read_dist4 = UB.GetDistance4()
		act_dist1 = int(read_dist1)
		act_dist2 = int(read_dist2)
		act_dist3 = int(read_dist3)
		act_dist4 = int(read_dist4)
		print("act_dist1 read ", act_dist1)
		print("act_dist2 read ", act_dist2)
		print("act_dist3 read ", act_dist3)
		print("act_dist4 read ", act_dist4)
		time.sleep(0.5)

in this case the temporary read_dist values can only be seen by the readdistance thread, not the control thread.

In the readdistance thread you can also catch when the values failed to read:

def readdistance():
	global act_dist1
	global act_dist2
	global act_dist3
	global act_dist4
	while True:
		read_dist1 = UB.GetDistance1()
		read_dist2 = UB.GetDistance2()
		read_dist3 = UB.GetDistance3()
		read_dist4 = UB.GetDistance4()
		if read_dist1 is None:
			print("act_dist1 not updated")
		else:
			act_dist1 = int(read_dist1)
			print("act_dist1 read ", act_dist1)
		if read_dist2 is None:
			print("act_dist2 not updated")
		else:
			act_dist2 = int(read_dist2)
			print("act_dist2 read ", act_dist2)
		if read_dist3 is None:
			print("act_dist3 not updated")
		else:
			act_dist3 = int(read_dist3)
			print("act_dist3 read ", act_dist3)
		if read_dist4 is None:
			print("act_dist4 not updated")
		else:
			act_dist4 = int(read_dist4)
			print("act_dist4 read ", act_dist4)
		time.sleep(0.5)

This can happen if the UltraBorg failed to respond in time.

The HC-SR04 ultrasonic sensor has a maximum range of 4 meters. The distance returned should be 0 if the sensor did not get a valid reading.