Arduino/hardware/libraries/Servo/Servo.cpp

#include <avr/interrupt.h>
#include <wiring.h>
#include <Servo.h>

/*
  Servo.h - Hardware Servo Timer Library
  Author: Jim Studt, jim@federated.com
  Copyright (c) 2007 David A. Mellis.  All right reserved.

  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., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/


uint8_t Servo::attached9 = 0;
uint8_t Servo::attached10 = 0;

void Servo::seizeTimer1()
{
  uint8_t oldSREG = SREG;

  cli();
  TCCR1A = _BV(WGM11); /* Fast PWM, ICR1 is top */
  TCCR1B = _BV(WGM13) | _BV(WGM12) /* Fast PWM, ICR1 is top */
  | _BV(CS11) /* div 8 clock prescaler */
  ;
  OCR1A = 3000;
  OCR1B = 3000;
  ICR1 = clockCyclesPerMicrosecond()*(20000L/8);  // 20000 uS is a bit fast for the refresh, 20ms, but 
                                                  // it keeps us from overflowing ICR1 at 20MHz clocks
                                                  // That "/8" at the end is the prescaler.
#if defined(__AVR_ATmega8__)
  TIMSK &= ~(_BV(TICIE1) | _BV(OCIE1A) | _BV(OCIE1B) | _BV(TOIE1) );
#else
  TIMSK1 &=  ~(_BV(OCIE1A) | _BV(OCIE1B) | _BV(TOIE1) );
#endif

  SREG = oldSREG;  // undo cli()    
}

void Servo::releaseTimer1() {}

#define NO_ANGLE (0xff)

Servo::Servo() : pin(0), angle(NO_ANGLE) {}

uint8_t Servo::attach(int pinArg)
{
  return attach(pinArg, 544, 2400);
}

uint8_t Servo::attach(int pinArg, int min, int max)
{
  if (pinArg != 9 && pinArg != 10) return 0;
  
  min16 = min / 16;
  max16 = max / 16;

  pin = pinArg;
  angle = NO_ANGLE;
  digitalWrite(pin, LOW);
  pinMode(pin, OUTPUT);

  if (!attached9 && !attached10) seizeTimer1();

  if (pin == 9) {
    attached9 = 1;
    TCCR1A = (TCCR1A & ~_BV(COM1A0)) | _BV(COM1A1);
  }
  
  if (pin == 10) {
    attached10 = 1;
    TCCR1A = (TCCR1A & ~_BV(COM1B0)) | _BV(COM1B1);
  }
  return 1;
}

void Servo::detach()
{
  // muck with timer flags
  if (pin == 9) {
    attached9 = 0;
    TCCR1A = TCCR1A & ~_BV(COM1A0) & ~_BV(COM1A1);
    pinMode(pin, INPUT);
  } 
  
  if (pin == 10) {
    attached10 = 0;
    TCCR1A = TCCR1A & ~_BV(COM1B0) & ~_BV(COM1B1);
    pinMode(pin, INPUT);
  }

  if (!attached9 && !attached10) releaseTimer1();
}

void Servo::write(int angleArg)
{
  uint16_t p;

  if (angleArg < 0) angleArg = 0;
  if (angleArg > 180) angleArg = 180;
  angle = angleArg;

  // bleh, have to use longs to prevent overflow, could be tricky if always a 16MHz clock, but not true
  // That 8L on the end is the TCNT1 prescaler, it will need to change if the clock's prescaler changes,
  // but then there will likely be an overflow problem, so it will have to be handled by a human.
  p = (min16*16L*clockCyclesPerMicrosecond() + (max16-min16)*(16L*clockCyclesPerMicrosecond())*angle/180L)/8L;
  if (pin == 9) OCR1A = p;
  if (pin == 10) OCR1B = p;
}

uint8_t Servo::read()
{
  return angle;
}

uint8_t Servo::attached()
{
  if (pin == 9 && attached9) return 1;
  if (pin == 10 && attached10) return 1;
  return 0;
}