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IntervalTimer

Timer library for Teensy 3.0

Overview and usage

Start a timer by creating a new IntervalTimer object and then calling it's begin() function, passing it the name of the function you'd like it to execute periodically (this is known as its callback function), as well as its period in microseconds (millionths of a second):

IntervalTimer timer0;
timer0.begin(timerCallback0, 5000000); // 5 seconds

Once started, the specified callback function will be called at a regular interval. You can change a callback function simply by stopping the timer and restarting it. Your callback routines should have no return value nor any arguments, and they should contain as little code as necessary. Any external variables they modify should be marked as volatile. For example:

volatile uint32_t timerCounter0;
void timerCallback0() {
  timerCounter0++;
}

Stop a timer by calling its end() function:

timer0.end();

Details and limitations

Timers are dynamically allocated to hardware resources. A call to the begin() function will return true if the timer was able to be allocated and started, whereas it will return false if all hardware resources are already being used. A resource in use will become available to other timers after the timer using it has been stopped. At present you can have up to three IntervalTimer objects operating at any given time. Further development will increase this number.

Currently, this library utilizes the PIT (Periodic Interrupt Timer) modules, of which there are four. However, only three are available for general use because one is being used by the built-in tone() functionality. The first timer to be allocated to a PIT module will enable the PIT clock and the last timer using a PIT module to be stopped will disable the PIT clock.

Invalid period values will result in begin() returning false. Assuming the default bus speed of 48 MHz, any period value greater than 89478485 (slightly less than 89.5 seconds) will be considered invalid. This is because the timer periods are stored internally as a quantity of bus clock cycles, and the register storing this value has 32 bits of precision. A value of 0 is also considered to be invalid.

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