R&D 2xHC-SR04 by Interrupts

Distance measurement by ultrasonic sensors with replacement of the pulseIn() blocking function by interrupt management on Arduino Mega.

Datasheets

To start a new measurement, the TRIGGER pin should be turned HIGH for 10 µs
The sensor will generate 8 ultra-sound wave trains at 40 Khz
Then the sensor will pass the ECHO pin to the HIGH state thus marking the start of the measurement
On receiving the ultrasound return, the sensor will switch the ECHO pin to the LOW state. So the time between the transition from the HIGH state to the LOW state will correspond to the measured distance

Time = Width of Echo pulse, in uS (micro second)
- Distance in centimeters = Time / 58
- Distance in inches = Time / 148
- Or you can utilize the speed of sound, which is 340m/s

The principle is

We will therefore detect a change of state on the ECHO pins of the Left sensor or the Right sensor by interrupt.

RIGHT ECHO on pin15 -> Port J.0 -> PCINT09
LEFT ECHO on pin14 -> Port J.1 -> PCINT10
PCINT09 and PCINT10 raise the interrupt vector PCINT1

To activate the interrupt on port J, change the PCIE1 bit of the PCICR register to 1

And activate the interrupt only for pin 14 & 15 (PJ1 & PJ0) with in the register PCMSK1