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/*
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 reference to AVCC (5 volts)
cbi(ADMUX, REFS1);
sbi(ADMUX, REFS0);
// 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
}
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