mirror of
https://github.com/epasveer/seer.git
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Ernie Pasveer
59 lines
1.5 KiB
C
59 lines
1.5 KiB
C
/**
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* \file millis.c
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*
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* Provide an analog of the Arduino `millis()` function. This implementation
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* uses `TIMER0` with a `/64` divider, which will allow it to operate with a
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* clock frequency of up to 16Mhz.
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*/
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#include <avr/interrupt.h>
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#include <avr/io.h>
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#include <util/atomic.h>
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volatile unsigned long timer_millis = 0;
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/** Timer interrupt service routine.
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*
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* This fires each time the timer counter (`TCNT0`) reaches the
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* target value stored in `OCR0A`.
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*/
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ISR(TIMER0_COMPA_vect) {
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timer_millis++;
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}
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/** Initialize the millis service.
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*
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* Configure timer 0 in CTC mode.
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* See the [attiny85 datasheet][1] for more information.
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*
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* [1]: https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf
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*/
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void init_millis() {
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// Enable CTC mode
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TCCR0A = 1<<WGM01;
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// Select /64 scale, since this will let us operate at 16Mhz without
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// overflowing our 8-bit TCNT0 register.
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TCCR0B = 3<<CS00;
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// (F_CPU/1000) is number of clock cycles/ms, and we are using the
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// /64 scaler.
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OCR0A = (F_CPU/1000) / 64;
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// Enable timer compare interrupt
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TIMSK |= 1<<OCIE0A;
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sei();
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}
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/** Return milliseconds since mc boot. */
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unsigned long millis() {
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unsigned long _millis;
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// Updating a 16 bit value is not an atomic operation.
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ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
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_millis = timer_millis;
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}
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return _millis;
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}
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