wiring.c

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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
*/

#include "wiring_private.h"

// the prescaler is set so that timer0 ticks every 64 clock cycles, and the
// the overflow handler is called every 256 ticks.
#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))

// the whole number of milliseconds per timer0 overflow
#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)

// the fractional number of milliseconds per timer0 overflow. we shift right
// by three to fit these numbers into a byte. (for the clock speeds we care
// about - 8 and 16 MHz - this doesn't lose precision.)
#define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) >> 3)
#define FRACT_MAX (1000 >> 3)

volatile unsigned long timer0_overflow_count = 0;
volatile unsigned long timer0_millis = 0;
static unsigned char timer0_fract = 0;

#if defined(__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__)
ISR(TIM0_OVF_vect)
#else
ISR(TIMER0_OVF_vect)
#endif
{
        // copy these to local variables so they can be stored in registers
        // (volatile variables must be read from memory on every access)
        unsigned long m = timer0_millis;
        unsigned char f = timer0_fract;

        m += MILLIS_INC;
        f += FRACT_INC;
        if (f >= FRACT_MAX) {
                f -= FRACT_MAX;
                m += 1;
        }

        timer0_fract = f;
        timer0_millis = m;
        timer0_overflow_count++;
}

unsigned long millis()
{
        unsigned long m;
        uint8_t oldSREG = SREG;

        // disable interrupts while we read timer0_millis or we might get an
        // inconsistent value (e.g. in the middle of a write to timer0_millis)
        cli();
        m = timer0_millis;
        SREG = oldSREG;

        return m;
}

unsigned long micros() {
        unsigned long m;
        uint8_t oldSREG = SREG, t;
        
        cli();
        m = timer0_overflow_count;
#if defined(TCNT0)
        t = TCNT0;
#elif defined(TCNT0L)
        t = TCNT0L;
#else
        #error TIMER 0 not defined
#endif

#ifdef TIFR0
        if ((TIFR0 & _BV(TOV0)) && (t < 255))
                m++;
#else
        if ((TIFR & _BV(TOV0)) && (t < 255))
                m++;
#endif

        SREG = oldSREG;
        
        return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

void delay(unsigned long ms)
{
        uint32_t start = micros();

        while (ms > 0) {
                yield();
                while ( ms > 0 && (micros() - start) >= 1000) {
                        ms--;
                        start += 1000;
                }
        }
}

/* Delay for the given number of microseconds.  Assumes a 1, 8, 12, 16, 20 or 24 MHz clock. */
void delayMicroseconds(unsigned int us)
{
        // call = 4 cycles + 2 to 4 cycles to init us(2 for constant delay, 4 for variable)

        // 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 >= 24000000L
        // for the 24 MHz clock for the aventurous ones, trying to overclock

        // zero delay fix
        if (!us) return; //  = 3 cycles, (4 when true)

        // the following loop takes a 1/6 of a microsecond (4 cycles)
        // per iteration, so execute it six times for each microsecond of
        // delay requested.
        us *= 6; // x6 us, = 7 cycles

        // account for the time taken in the preceeding commands.
        // we just burned 22 (24) cycles above, remove 5, (5*4=20)
        // us is at least 6 so we can substract 5
        us -= 5; //=2 cycles

#elif F_CPU >= 20000000L
        // for the 20 MHz clock on rare Arduino boards

        // for a one-microsecond delay, simply return.  the overhead
        // of the function call takes 18 (20) cycles, which is 1us
        __asm__ __volatile__ (
                "nop" "\n\t"
                "nop" "\n\t"
                "nop" "\n\t"
                "nop"); //just waiting 4 cycles
        if (us <= 1) return; //  = 3 cycles, (4 when true)

        // the following loop takes a 1/5 of a microsecond (4 cycles)
        // per iteration, so execute it five times for each microsecond of
        // delay requested.
        us = (us << 2) + us; // x5 us, = 7 cycles

        // account for the time taken in the preceeding commands.
        // we just burned 26 (28) cycles above, remove 7, (7*4=28)
        // us is at least 10 so we can substract 7
        us -= 7; // 2 cycles

#elif 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 takes 14 (16) cycles, which is 1us
        if (us <= 1) return; //  = 3 cycles, (4 when true)

        // the following loop takes 1/4 of a microsecond (4 cycles)
        // per iteration, so execute it four times for each microsecond of
        // delay requested.
        us <<= 2; // x4 us, = 4 cycles

        // account for the time taken in the preceeding commands.
        // we just burned 19 (21) cycles above, remove 5, (5*4=20)
        // us is at least 8 so we can substract 5
        us -= 5; // = 2 cycles,

#elif F_CPU >= 12000000L
        // for the 12 MHz clock if somebody is working with USB

        // for a 1 microsecond delay, simply return.  the overhead
        // of the function call takes 14 (16) cycles, which is 1.5us
        if (us <= 1) return; //  = 3 cycles, (4 when true)

        // the following loop takes 1/3 of a microsecond (4 cycles)
        // per iteration, so execute it three times for each microsecond of
        // delay requested.
        us = (us << 1) + us; // x3 us, = 5 cycles

        // account for the time taken in the preceeding commands.
        // we just burned 20 (22) cycles above, remove 5, (5*4=20)
        // us is at least 6 so we can substract 5
        us -= 5; //2 cycles

#elif F_CPU >= 8000000L
        // for the 8 MHz internal clock

        // for a 1 and 2 microsecond delay, simply return.  the overhead
        // of the function call takes 14 (16) cycles, which is 2us
        if (us <= 2) return; //  = 3 cycles, (4 when true)

        // the following loop takes 1/2 of a microsecond (4 cycles)
        // per iteration, so execute it twice for each microsecond of
        // delay requested.
        us <<= 1; //x2 us, = 2 cycles

        // account for the time taken in the preceeding commands.
        // we just burned 17 (19) cycles above, remove 4, (4*4=16)
        // us is at least 6 so we can substract 4
        us -= 4; // = 2 cycles

#else
        // for the 1 MHz internal clock (default settings for common Atmega microcontrollers)

        // the overhead of the function calls is 14 (16) cycles
        if (us <= 16) return; //= 3 cycles, (4 when true)
        if (us <= 25) return; //= 3 cycles, (4 when true), (must be at least 25 if we want to substract 22)

        // compensate for the time taken by the preceeding and next commands (about 22 cycles)
        us -= 22; // = 2 cycles
        // the following loop takes 4 microseconds (4 cycles)
        // per iteration, so execute it us/4 times
        // us is at least 4, divided by 4 gives us 1 (no zero delay bug)
        us >>= 2; // us div 4, = 4 cycles
        

#endif

        // busy wait
        __asm__ __volatile__ (
                "1: sbiw %0,1" "\n\t" // 2 cycles
                "brne 1b" : "=w" (us) : "0" (us) // 2 cycles
        );
        // return = 4 cycles
}

void init()
{
        // this needs to be called before setup() or some functions won't
        // work there
        sei();
        
        // 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(TCCR0A) && defined(WGM01)
        sbi(TCCR0A, WGM01);
        sbi(TCCR0A, WGM00);
#endif

        // set timer 0 prescale factor to 64
#if defined(__AVR_ATmega128__)
        // CPU specific: different values for the ATmega128
        sbi(TCCR0, CS02);
#elif defined(TCCR0) && defined(CS01) && defined(CS00)
        // this combination is for the standard atmega8
        sbi(TCCR0, CS01);
        sbi(TCCR0, CS00);
#elif defined(TCCR0B) && defined(CS01) && defined(CS00)
        // this combination is for the standard 168/328/1280/2560
        sbi(TCCR0B, CS01);
        sbi(TCCR0B, CS00);
#elif defined(TCCR0A) && defined(CS01) && defined(CS00)
        // this combination is for the __AVR_ATmega645__ series
        sbi(TCCR0A, CS01);
        sbi(TCCR0A, CS00);
#else
        #error Timer 0 prescale factor 64 not set correctly
#endif

        // enable timer 0 overflow interrupt
#if defined(TIMSK) && defined(TOIE0)
        sbi(TIMSK, TOIE0);
#elif defined(TIMSK0) && defined(TOIE0)
        sbi(TIMSK0, TOIE0);
#else
        #error  Timer 0 overflow interrupt not set correctly
#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

#if defined(TCCR1B) && defined(CS11) && defined(CS10)
        TCCR1B = 0;

        // set timer 1 prescale factor to 64
        sbi(TCCR1B, CS11);
#if F_CPU >= 8000000L
        sbi(TCCR1B, CS10);
#endif
#elif defined(TCCR1) && defined(CS11) && defined(CS10)
        sbi(TCCR1, CS11);
#if F_CPU >= 8000000L
        sbi(TCCR1, CS10);
#endif
#endif
        // put timer 1 in 8-bit phase correct pwm mode
#if defined(TCCR1A) && defined(WGM10)
        sbi(TCCR1A, WGM10);
#endif

        // set timer 2 prescale factor to 64
#if defined(TCCR2) && defined(CS22)
        sbi(TCCR2, CS22);
#elif defined(TCCR2B) && defined(CS22)
        sbi(TCCR2B, CS22);
//#else
        // Timer 2 not finished (may not be present on this CPU)
#endif

        // configure timer 2 for phase correct pwm (8-bit)
#if defined(TCCR2) && defined(WGM20)
        sbi(TCCR2, WGM20);
#elif defined(TCCR2A) && defined(WGM20)
        sbi(TCCR2A, WGM20);
//#else
        // Timer 2 not finished (may not be present on this CPU)
#endif

#if defined(TCCR3B) && defined(CS31) && defined(WGM30)
        sbi(TCCR3B, CS31);              // set timer 3 prescale factor to 64
        sbi(TCCR3B, CS30);
        sbi(TCCR3A, WGM30);             // put timer 3 in 8-bit phase correct pwm mode
#endif

#if defined(TCCR4A) && defined(TCCR4B) && defined(TCCR4D) /* beginning of timer4 block for 32U4 and similar */
        sbi(TCCR4B, CS42);              // set timer4 prescale factor to 64
        sbi(TCCR4B, CS41);
        sbi(TCCR4B, CS40);
        sbi(TCCR4D, WGM40);             // put timer 4 in phase- and frequency-correct PWM mode 
        sbi(TCCR4A, PWM4A);             // enable PWM mode for comparator OCR4A
        sbi(TCCR4C, PWM4D);             // enable PWM mode for comparator OCR4D
#else /* beginning of timer4 block for ATMEGA1280 and ATMEGA2560 */
#if defined(TCCR4B) && defined(CS41) && defined(WGM40)
        sbi(TCCR4B, CS41);              // set timer 4 prescale factor to 64
        sbi(TCCR4B, CS40);
        sbi(TCCR4A, WGM40);             // put timer 4 in 8-bit phase correct pwm mode
#endif
#endif /* end timer4 block for ATMEGA1280/2560 and similar */   

#if defined(TCCR5B) && defined(CS51) && defined(WGM50)
        sbi(TCCR5B, CS51);              // set timer 5 prescale factor to 64
        sbi(TCCR5B, CS50);
        sbi(TCCR5A, WGM50);             // put timer 5 in 8-bit phase correct pwm mode
#endif

#if defined(ADCSRA)
        // set a2d prescaler so we are inside the desired 50-200 KHz range.
        #if F_CPU >= 16000000 // 16 MHz / 128 = 125 KHz
                sbi(ADCSRA, ADPS2);
                sbi(ADCSRA, ADPS1);
                sbi(ADCSRA, ADPS0);
        #elif F_CPU >= 8000000 // 8 MHz / 64 = 125 KHz
                sbi(ADCSRA, ADPS2);
                sbi(ADCSRA, ADPS1);
                cbi(ADCSRA, ADPS0);
        #elif F_CPU >= 4000000 // 4 MHz / 32 = 125 KHz
                sbi(ADCSRA, ADPS2);
                cbi(ADCSRA, ADPS1);
                sbi(ADCSRA, ADPS0);
        #elif F_CPU >= 2000000 // 2 MHz / 16 = 125 KHz
                sbi(ADCSRA, ADPS2);
                cbi(ADCSRA, ADPS1);
                cbi(ADCSRA, ADPS0);
        #elif F_CPU >= 1000000 // 1 MHz / 8 = 125 KHz
                cbi(ADCSRA, ADPS2);
                sbi(ADCSRA, ADPS1);
                sbi(ADCSRA, ADPS0);
        #else // 128 kHz / 2 = 64 KHz -> This is the closest you can get, the prescaler is 2
                cbi(ADCSRA, ADPS2);
                cbi(ADCSRA, ADPS1);
                sbi(ADCSRA, ADPS0);
        #endif
        // enable a2d conversions
        sbi(ADCSRA, ADEN);
#endif

        // 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(UCSRB)
        UCSRB = 0;
#elif defined(UCSR0B)
        UCSR0B = 0;
#endif
}