Arduino/hardware/cores/arduino/wiring.c

/*
  wiring.c - Partial implementation of the Wiring API for the ATmega8.
  Part of Arduino - http://www.arduino.cc/

  Copyright (c) 2005-2006 David A. Mellis

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General
  Public License along with this library; if not, write to the
  Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  Boston, MA  02111-1307  USA

  $Id$
*/

#include "wiring_private.h"

// The number of times timer 0 has overflowed since the program started.
// Must be volatile or gcc will optimize away some uses of it.
volatile unsigned long timer0_overflow_count;

SIGNAL(SIG_OVERFLOW0)
{
	timer0_overflow_count++;
}

unsigned long millis()
{
	// timer 0 increments every 64 cycles, and overflows when it reaches
	// 256.  we would calculate the total number of clock cycles, then
	// divide by the number of clock cycles per millisecond, but this
	// overflows too often.
	//return timer0_overflow_count * 64UL * 256UL / (F_CPU / 1000UL);
	
	// instead find 1/128th the number of clock cycles and divide by
	// 1/128th the number of clock cycles per millisecond
	return timer0_overflow_count * 64UL * 2UL / (F_CPU / 128000UL);
}

void delay(unsigned long ms)
{
	unsigned long start = millis();
	
	while (millis() - start < ms)
		;
}

/* Delay for the given number of microseconds.  Assumes a 16 MHz clock. 
 * Disables interrupts, which will disrupt the millis() function if used
 * too frequently. */
void delayMicroseconds(unsigned int us)
{
	uint8_t oldSREG;

	// calling avrlib's delay_us() function with low values (e.g. 1 or
	// 2 microseconds) gives delays longer than desired.
	//delay_us(us);

#if F_CPU >= 16000000L
	// for the 16 MHz clock on most Arduino boards

	// for a one-microsecond delay, simply return.  the overhead
	// of the function call yields a delay of approximately 1 1/8 us.
	if (--us == 0)
		return;

	// the following loop takes a quarter of a microsecond (4 cycles)
	// per iteration, so execute it four times for each microsecond of
	// delay requested.
	us <<= 2;

	// account for the time taken in the preceeding commands.
	us -= 2;
#else
	// for the 8 MHz internal clock on the ATmega168

	// for a one- or two-microsecond delay, simply return.  the overhead of
	// the function calls takes more than two microseconds.  can't just
	// subtract two, since us is unsigned; we'd overflow.
	if (--us == 0)
		return;
	if (--us == 0)
		return;

	// the following loop takes half of a microsecond (4 cycles)
	// per iteration, so execute it twice for each microsecond of
	// delay requested.
	us <<= 1;
    
	// partially compensate for the time taken by the preceeding commands.
	// we can't subtract any more than this or we'd overflow w/ small delays.
	us--;
#endif

	// disable interrupts, otherwise the timer 0 overflow interrupt that
	// tracks milliseconds will make us delay longer than we want.
	oldSREG = SREG;
	cli();

	// busy wait
	__asm__ __volatile__ (
		"1: sbiw %0,1" "\n\t" // 2 cycles
		"brne 1b" : "=w" (us) : "0" (us) // 2 cycles
	);

	// reenable interrupts.
	SREG = oldSREG;
}

void init()
{
	// this needs to be called before setup() or some functions won't
	// work there
	sei();
	
	// timer 0 is used for millis() and delay()
	timer0_overflow_count = 0;
	// on the ATmega168, timer 0 is also used for fast hardware pwm
	// (using phase-correct PWM would mean that timer 0 overflowed half as often
	// resulting in different millis() behavior on the ATmega8 and ATmega168)
#if defined(__AVR_ATmega168__)
	sbi(TCCR0A, WGM01);
	sbi(TCCR0A, WGM00);
#endif  
	// set timer 0 prescale factor to 64
#if defined(__AVR_ATmega168__)
	sbi(TCCR0B, CS01);
	sbi(TCCR0B, CS00);
#else
	sbi(TCCR0, CS01);
	sbi(TCCR0, CS00);
#endif
	// enable timer 0 overflow interrupt
#if defined(__AVR_ATmega168__)
	sbi(TIMSK0, TOIE0);
#else
	sbi(TIMSK, TOIE0);
#endif

	// timers 1 and 2 are used for phase-correct hardware pwm
	// this is better for motors as it ensures an even waveform
	// note, however, that fast pwm mode can achieve a frequency of up
	// 8 MHz (with a 16 MHz clock) at 50% duty cycle

	// set timer 1 prescale factor to 64
	sbi(TCCR1B, CS11);
	sbi(TCCR1B, CS10);
	// put timer 1 in 8-bit phase correct pwm mode
	sbi(TCCR1A, WGM10);

	// set timer 2 prescale factor to 64
#if defined(__AVR_ATmega168__)
	sbi(TCCR2B, CS22);
#else
	sbi(TCCR2, CS22);
#endif
	// configure timer 2 for phase correct pwm (8-bit)
#if defined(__AVR_ATmega168__)
	sbi(TCCR2A, WGM20);
#else
	sbi(TCCR2, WGM20);
#endif

	// set a2d prescale factor to 128
	// 16 MHz / 128 = 125 KHz, inside the desired 50-200 KHz range.
	// XXX: this will not work properly for other clock speeds, and
	// this code should use F_CPU to determine the prescale factor.
	sbi(ADCSRA, ADPS2);
	sbi(ADCSRA, ADPS1);
	sbi(ADCSRA, ADPS0);

	// enable a2d conversions
	sbi(ADCSRA, ADEN);

	// the bootloader connects pins 0 and 1 to the USART; disconnect them
	// here so they can be used as normal digital i/o; they will be
	// reconnected in Serial.begin()
#if defined(__AVR_ATmega168__)
	UCSR0B = 0;
#else
	UCSRB = 0;
#endif
}