也做了一些實驗。結果就是讓板子在進到power down mode時，功耗為
18.7uA/3.3V。如果有那位大大知道如何讓功耗再往下降，請不吝指教。
在一開始我參考下面reference列的那個網址，在進到power down mode
的功耗是112uA，後來去修改register MCUCR去關掉BOD，功耗降到
106uA。後來再去修改register PRR，將Timer0~2,TWI,SPI,USART0,
ADC關掉，才讓功耗降到18uA。此外原本reference code是用函式
sleep_mode()來進到sleep mode，但這個函式會去打開一些前面關掉的
設定，所以要改用函式sleep_cpu()來進sleep mode。
為了去降低功耗，也需要硬體去配合，我將Arduino Pro Mini上的U2
LDO拆掉，還有紅色的LED1也拿掉。另外將INT0 pin接到一個button
，當系統開機後3秒，會自動進到sleep mode，這時再按下button，
就可以觸發 attachInterrupt事件，來喚醒MCU。
#include <avr/power.h>
void setup(void)
{
// the GPIO comprise 3 part, B,C,D.
DDRD &= B00000011; // set Arduino pins 2 to 7 as inputs, leaves 0 & 1
DDRB = B00000000; // (RX & TX) as is set pins 8 to 13 as inputs
PORTD = B00000100; // disable pullups on pins 3 to 7
PORTB = B00000000; // disable pullups on pins 8 to 13
pinMode(13,OUTPUT); // set pin 13 as an output so we can use LED to monitor
digitalWrite(13,HIGH); // turn pin 13 LED on
Serial.begin(9600);
}
//
void loop(void)
{
// Stay awake for 1 second, then sleep.
// LED turns off when sleeping, then back on upon wake.
delay(3000);
sleepNow();
}
//
void sleepNow(void)
{
Serial.println("Go to sleep");
// Set pin 2 as interrupt and attach handler:
attachInterrupt(0, pinInterrupt, LOW);
delay(100);
//
// Choose our preferred sleep mode:
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
//
// Set sleep enable (SE) bit:
sleep_enable();
digitalWrite(13,LOW); // turn LED off to indicate sleep
// Put the device to sleep:
MCUCR |= bit(BODSE) | bit(BODS); // timed sequence
MCUCR = (MCUCR & ~bit(BODSE)) | bit(BODS);
ADCSRA &= ~(1 << ADEN);
PRR = 0xFF;
sleep_cpu();
//sleep_mode();
// Upon waking up, sketch continues from this point.
sleep_disable();
Serial.println("sleep disable");
digitalWrite(13,HIGH); // turn LED on to indicate awake
PRR = 0x00;
// delay(10000);
}
//
void pinInterrupt(void)
{
Serial.println("Interrupt");
detachInterrupt(0);
}
下表為各個power mode，它有那些block是開著或關掉。
我們利用下面的函式就可以改變sleep mode，在各個sleep mode可以得到不同的功耗。
set_sleep_mode(SLEEP_MODE_PWR_DOWN);
ACTIVE_MODE 8.0mA
SLEEP_MODE_IDLE 1.56mA
SLEEP_MODE_ADC 1,.02mA
SLEEP_MODE_PWR_SAVE 0.65mA
SLEEP_MODE_STANDBY 0.65mA
SLEEP_MODE_PWR_DOWN 18.7uA.SLEEP_MODE_PWR_SAVE 0.65mA
SLEEP_MODE_STANDBY 0.65mA
Reference.
http://www.engblaze.com/hush-little-microprocessor-avr-and-arduino-sleep-mode-basics/
