unsigned long micros()
micros() returns the number of microseconds that have elapsed since the board was powered up.
The number of microseconds that have elapsed since powerup.
The maximum value of a 32-bit integer is 4294967295 and consequently this is the maximum number of microseconds that can be counted. After this the counter resets to 0 and the counting starts again. This period is approximately 71.5 minutes.
micros() conforms to the Arduino 1.6.x API.
uint32_t us = micros();