Arduinoのクロック数を落として省電力稼働させる。

Arduino Nano
Arduino IDE 1.8.12
参考サイト
　https://playground.arduino.cc/Code/Prescaler/https://playground.arduino.cc/Code/Prescaler/

上のサイトに書いてある通り、クロックをゆっくりにすれば省電力になる。
クロックは分周すれば半分、そのまた半分と、周期を長くできる。
マイコン使いならおなじみのprescalerってやつだ。

/*
 * prescaler.h
 *
 * Provides useful tools to manage the clock prescaler and issues
 * related to time and delays. Allows to easily set the prescaler
 * and get access to its value. Also provides alternative functions
 * to the millis() and delay() functions.
 *
 * (c) 2008 Sofian Audry | info(@)sofianaudry(.)com
 * https://sofianaudry.com
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/
#ifndef PRESCALER_INC
#define PRESCALER_INC

#include "Arduino.h"

/**
 * Prescaler division
*/
#define CLOCK_PRESCALER_1   (0x0)
#define CLOCK_PRESCALER_2   (0x1)
#define CLOCK_PRESCALER_4   (0x2)
#define CLOCK_PRESCALER_8   (0x3)
#define CLOCK_PRESCALER_16  (0x4)
#define CLOCK_PRESCALER_32  (0x5)
#define CLOCK_PRESCALER_64  (0x6)
#define CLOCK_PRESCALER_128 (0x7)
#define CLOCK_PRESCALER_256 (0x8)

// Initialize global variable.
static uint8_t __clock_prescaler = (CLKPR & (_BV(CLKPS0) | _BV(CLKPS1) | _BV(CLKPS2) | _BV(CLKPS3)));

inline void setClockPrescaler(uint8_t clockPrescaler) {
  if (clockPrescaler <= CLOCK_PRESCALER_256) {
    // Disable interrupts.
    uint8_t oldSREG = SREG;
    cli();

    // Enable change.
    CLKPR = _BV(CLKPCE); // write the CLKPCE bit to one and all the other to zero

    // Change clock division.
    CLKPR = clockPrescaler; // write the CLKPS0..3 bits while writing the CLKPE bit to zero

    // Copy for fast access.
    __clock_prescaler = clockPrescaler;

    // Recopy interrupt register.
    SREG = oldSREG;
  }
}

inline uint8_t getClockPrescaler() {
  return (__clock_prescaler);
}

inline uint16_t getClockDivisionFactor() {
  return ((uint16_t)(1 << __clock_prescaler));
}

/**
 * Time in milliseconds.
 *
 * NOTE: This is the equivalent of the millis() function but it readjusts it according
 * to the current clock division. As such, be careful of how you make use of it, in
 * particular remember it will be wrong if the clock division factor is changed during the
 * course of computation. Remember that you can reset the overflow counter by calling the
 * init() function from wiring.h.
 */
inline unsigned long trueMillis()
{
  return millis() * getClockDivisionFactor();
}

// Waits for #ms# milliseconds.
// NOTE: Please see comment above.
inline void trueDelay(unsigned long ms)
{
  unsigned long start = trueMillis();
  while (trueMillis() - start < ms);
}

/**
 * Rescales given delay time according to division factor. Should be called before a call
 * to delay(). Insures compatibility with function using delay().
 * Example use:
 * delay( rescaleDelay(1000) ); // equivalent to wait(1000)
 */
inline unsigned long rescaleDuration(unsigned long d) {
  return (d / getClockDivisionFactor());
}

/**
 * Rescales given time (in milliseconds or microseconds) according to division factor. Should
 * be called
 */
inline unsigned long rescaleTime(unsigned long t) {
  return (t * getClockDivisionFactor());
}

#endif

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* prescaler.h

* Provides useful tools to manage the clock prescaler and issues

* related to time and delays. Allows to easily set the prescaler

* and get access to its value. Also provides alternative functions

* to the millis() and delay() functions.

* https://sofianaudry.com

* This program is free software: you can redistribute it and/or modify

* it under the terms of the GNU General Public License as published by

* the Free Software Foundation, either version 3 of the License, or

* (at your option) any later version.

* This program 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 General Public License for more details.

* You should have received a copy of the GNU General Public License

* along with this program. If not, see <https://www.gnu.org/licenses/>.

#ifndef PRESCALER_INC

#define PRESCALER_INC

#include "Arduino.h"

/**

* Prescaler division

#define CLOCK_PRESCALER_1 (0x0)

#define CLOCK_PRESCALER_2 (0x1)

#define CLOCK_PRESCALER_4 (0x2)

#define CLOCK_PRESCALER_8 (0x3)

#define CLOCK_PRESCALER_16 (0x4)

#define CLOCK_PRESCALER_32 (0x5)

#define CLOCK_PRESCALER_64 (0x6)

#define CLOCK_PRESCALER_128 (0x7)

#define CLOCK_PRESCALER_256 (0x8)

// Initialize global variable.

static uint8_t __clock_prescaler = (CLKPR & (_BV(CLKPS0) | _BV(CLKPS1) | _BV(CLKPS2) | _BV(CLKPS3)));

inline void setClockPrescaler(uint8_t clockPrescaler) {

if (clockPrescaler <= CLOCK_PRESCALER_256) {

// Disable interrupts.

uint8_t oldSREG = SREG;

cli();

// Enable change.

CLKPR = _BV(CLKPCE); // write the CLKPCE bit to one and all the other to zero

// Change clock division.

CLKPR = clockPrescaler; // write the CLKPS0..3 bits while writing the CLKPE bit to zero

// Copy for fast access.

__clock_prescaler = clockPrescaler;

// Recopy interrupt register.

SREG = oldSREG;

}

inline uint8_t getClockPrescaler() {

return (__clock_prescaler);

}

inline uint16_t getClockDivisionFactor() {

return ((uint16_t)(1 << __clock_prescaler));

}

/**

* Time in milliseconds.

* NOTE: This is the equivalent of the millis() function but it readjusts it according

* to the current clock division. As such, be careful of how you make use of it, in

* particular remember it will be wrong if the clock division factor is changed during the

* course of computation. Remember that you can reset the overflow counter by calling the

* init() function from wiring.h.

inline unsigned long trueMillis()

{

return millis() * getClockDivisionFactor();

}

// Waits for #ms# milliseconds.

// NOTE: Please see comment above.

inline void trueDelay(unsigned long ms)

{

unsigned long start = trueMillis();

while (trueMillis() - start < ms);

}

/**

* Rescales given delay time according to division factor. Should be called before a call

* to delay(). Insures compatibility with function using delay().

* Example use:

* delay( rescaleDelay(1000) ); // equivalent to wait(1000)

inline unsigned long rescaleDuration(unsigned long d) {

return (d / getClockDivisionFactor());

}

/**

* Rescales given time (in milliseconds or microseconds) according to division factor. Should

* be called

inline unsigned long rescaleTime(unsigned long t) {

return (t * getClockDivisionFactor());

}

#endif

もらって来たコードを「prescaler.h」という名前でスケッチのあるフォルダに置く。
スケッチの先頭に
#include “prescaler.h”
を追加する。
setup()の先頭に
setClockPrescaler(CLOCK_PRESCALER_8);
などと追加する。
CLOCK_PRESCALER_# の#の数字が分周比を決める。
8なら8分の1のクロック数になる。
２の乗数で指定する。

コードの初めの方でAVRのレジスタを変更してクロックの分周用のPrescalerの値を決めていると思われるがまだ詳細は確認していない。
後半は、クロック数変更によって影響を受ける millis() と delay() のかわりに trueMillis() と trueDelay(long) を提供している。
millis() と delay() をそのまま使うとゆっくりになってしまうので置き換えるわけだ。

以下のスケッチを実行すると、

delay(100);は本来100msのところ8倍の800ms間隔になる。