How to trigger an interrupt when a variable has a certain value?

[virtualblack]

I'm bulding a car robot that uses an ultrasonic sensor (the HC-SR04) to detect objects around it.
The sensor measures the distance between the robot and an object and that value is stored in a variable. I want to create an interrupt that is triggered when the distance is equal or lower than a certain value, so I can use the void loop() function for other things meanwhile the sensor is looking for objects.

But all examples I have seen of attachInterrupt() are triggered with a low or high pulse on an input pin, either with rising, falling, change or low. How can it be triggered with a varaible value? Is that possible?

[anon73444976]

It's possible (requires a pin change interrupt) but not really desireable.
I suspect your software design needs a rethink

[ToddL1962]

There is no reason your loop() function can't do multiple things. Consider the following tutorials:

BlinkWithoutDelay

Arduino Multiple Things

Several Things at a Time

[J-M-L]

@virtualblack

The sensor measures the distance between the robot and an object and that value is stored in a variable

How is that stored? Just compare the value you store to your threshold and call your function when this happens…

Am I missing something here?

[johnwasser]

To avoid waiting for pulseIn() to read the length of the ECHO pulse, connect the ECHO pin to an interrupt pin and use a CHANGE interrupt. When the ECHO pin is HIGH, the ISR notes the time (startTime = micros() and then the pin is LOW, calculate the time difference (duration = micros() - startTime;). The loop() can then read the new duration and calculate distance whenever it needs to. You only need to pulse the TRIGGER pin about ever 50 milliseconds.

[anon73444976]

Better still, if you've only got one sensor, use the input capture facility.