Four built-in timers are available, numbered 0-3: PITimer0, PITimer1, PITimer2, and PITimer3. Note that PITimer3 is disabled by default because it conflicts with the existing tone() functionality. Each timer's period defaults to one second. Change this with the value(), period(), or frequency() functions. If they're called without arguments, these functions will instead return their respective values. Period is specified in seconds, and frequency is specified in hertz. Floating-point values are fine for either. You can update these values on-the-fly, while a timer is running, if you want to.
Invalid values will be silently fixed. For example, a period of 100 (seconds) will be changed to about 89.48 which is the absolute maximum. Likewise, only specific higher frequencies are available. For example, a frequency of 74000 (74 kHz) will be changed to about 73959.94 because of the granularity between cycle counts. Use period() and frequency() to check the actual values of your timers if you're setting them close to the extremes. Also keep in mind that the value() function returns/requires 1 less than the actual/desired bus cycle count.
- Value:
639to4294967294(2^32 - 2) bus clocks - Period:
0.000013312to89.478485312seconds (14 µs to 89 s) - Frequency:
0.011175871to75000hertz (12 mHz to 75 kHz)
Start a timer by calling it's start() function, and passing it the name of the function you'd like it to execute periodically (this is known as its callback function). Stop a timer by calling its stop() function. Once started, the periodic interrupts will call their specified callback functions whenever they expire. You can change a callback function simply by stopping a timer and restarting it. Your callback routines should have no return value. The reset() function will reset the timer's countdown so that one full period will elapse from when it's called, thereby delaying when the next interrupt is to be generated.
The current() function will return the remaining countdown value of the current timer cycle. This value is measured in individual bus clock cycles. The default bus speed for the Teensy 3.0 is 48 MHz (aka 48,000,000 cycles). Likewise, remains() will return the remaining time on the counter (in seconds) as a floating-point value. Calling the count() function will return the number of times the timer has executed, while calling zero() will reset this counter. Last but not least, you can use running() to check whether the timer is active or not.
- Daniel Gilbert
- loglow@gmail.com