arduino Time Management
Syntax
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unsigned long millis()
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unsigned long micros()
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void delay(unsigned long milliseconds)
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void delayMicroseconds(unsigned long microseconds)
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See the elapsedMillis header for constructors and operators of that class. In short:
- elapsedMillis elapsedMillisObject; creates an object to keep track of time since it was created or since some other explicitly set point in time
- elapsedMillisObject = 0; reset the time tracked by the object to "since now"
- unsigned long deltaT = elapsedMillisObject; lets us look at the tracked time
- elapsedMillisObject += and -= these work as expected
Remarks
Blocking vs. non-blocking code
For very simple sketches, writing blocking code using delay() and delayMicroseconds() can be appropriate. When things get more complex, using these functions can have some drawbacks. Some of these are:
- Wasting CPU time: More complex sketches might need the CPU for something else while waiting for an LED blinking period to end.
- unexpected delays: when
delay()is called in subroutines that are not obviously called, for example in libraries you include. - missing events that happen during the delay and are not handled by an interrupt handler, for example polled button presses: A button might be pressed for 100 ms, but this might be shadowed by a
delay(500).
Implementation details
millis() usually relies on a hardware timer that runs at a speed that's much higher than 1 kHz. When millis() is called, the implementation returns some value, but you don't know how old that actually is. It's possible that the "current" millisecond just started, or that it will end right after that function call. That means that, when calculating the difference between two results from millis(), you can be off by anything between almost zero and almost one millisecond. Use micros() if higher precision is needed.
Looking into the source code of elapsedMillis reveals that it indeed uses millis() internally to compare two points in time, so it suffers from this effect as well. Again, there's the alternative elapsedMicros for higher precision, from the same library.
