Power saving techniques on the Atmega32u4

Now that I have a Funky 2 prototype, I can toy with what I am most interested in: reducing power usage so that the Funky can operate on battery as long as possible. I have achieved amazing results with an Attiny84, and was slightly concerned how this much more capable and feature rich processor can handle low power applications.  It turns out we can do pretty good.

The MCU has the standard ultra low sleep modes, nothing fancy there. I was more interested in the consumption while awake. I was surprised to find out that on top of the classical power reducing stuff, we can actually switch the clock source in the software from the external 8Mhz clock to the internal RC (again 8Mhz). The datasheet explains in detail the necessary steps to achieve this, so it was pretty straight-forward. Strange enough, i didn’t notice any improvement, even the contrary… The Funky was running from regulated 3.3V power source (not via the USB plug)

Here is my test sketch, I leave some time between the settings so I can take measurement with my multimer. I noted the reading I was getting during each delay in the sketch itself:

#include <avr/power.h>

void setup(){

blip(5,35);
delay(10000);   //IMPORTANT!!! Allow time for changing firmware, or otherwise if we disable the USB too soon we are stuck and only a re-flash of the bootloader may help
//12.3mA at this point

power_adc_disable();
blip(1,50);
delay(8000); //12.00mA

power_usart0_disable();
blip(1,50);
delay(8000); //11.85mA

power_spi_disable();
blip(1,50);
delay(8000); //11.79mA

power_twi_disable();
blip(1,50);
delay(8000); //11.75mA

 //Leave timer 0 going for delay() function
power_timer1_disable();
blip(1,50);
delay(8000); // 11.7mA

power_timer2_disable();
blip(1,50);
delay(8000); //11.6mA

power_timer3_disable();
blip(1,50);
delay(8000); //11.55mA

power_usart1_disable();
blip(1,50);
delay(8000);  // 11.35mA

power_usb_disable();
blip(1,50);
delay(8000);  //9.97mA

USBCON |= (1 << FRZCLK);             // Freeze the USB Clock              
PLLCSR &= ~(1 << PLLE);              // Disable the USB Clock (PPL) 
USBCON &=  ~(1 << USBE  );           // Disable the USB  
blip(1,50);
delay(8000);  // 5.3mA

// Switch to RC Clock 
UDINT  &= ~(1 << SUSPI); // UDINT.SUSPI = 0; Usb_ack_suspend
USBCON |= ( 1 <<FRZCLK); // USBCON.FRZCLK = 1; Usb_freeze_clock
PLLCSR &= ~(1 << PLLE); // PLLCSR.PLLE = 0; Disable_pll

CLKSEL0 |= (1 << RCE); // CLKSEL0.RCE = 1; Enable_RC_clock()
while ( (CLKSTA & (1 << RCON)) == 0){}	// while (CLKSTA.RCON != 1);  while (!RC_clock_ready())
CLKSEL0 &= ~(1 << CLKS);  // CLKSEL0.CLKS = 0; Select_RC_clock()
CLKSEL0 &= ~(1 << EXTE);  // CLKSEL0.EXTE = 0; Disable_external_clock
blip(1,50);
delay(8000);  // 5.48mA .. hmmm?!
/*
/// Enable External Clock
CLKSEL0 |= (1 << EXTE);	// CKSEL0.EXTE = 1;	// Enable_external_clock(); 
while ( (CLKSTA & (1 << EXTON)) == 0 ){} // while (CLKSTA.EXTON != 1);	// while (!External_clock_ready()); 
CLKSEL0 |= (1 << CLKS);	// CLKSEL0.CLKS = 1;	//	Select_external_clock(); 
PLLCSR |= (1 << PLLE);	// PLLCSR.PLLE = 1;	// Enable_pll(); 
CLKSEL0 &= ~(1 << RCE);	// CLKSEL0.RCE = 0;	// Disable_RC_clock(); 
while ( (PLLCSR & (1 << PLOCK)) == 0){}	// while (PLLCSR.PLOCK != 1);	// while (!Pll_ready()); 
USBCON &= ~(1 << FRZCLK);	// USBCON.FRZCLK = 0;	// Usb_unfreeze_clock(); 
*/

clock_prescale_set(clock_div_2);
blip(1,50); 
}
void loop(){
// 3.9mA
}

void blip(int times, int dur) { 
  pinMode(4,OUTPUT); 
  for(int i=0;i<times;i++){
    digitalWrite(4,HIGH);
    delay(dur);
    digitalWrite(4,LOW);
    delay(dur);
  } 
}

So, the results are pretty good, the Funky v2 can run actively at 3.9mA and 4Mhz. The necessary hardware components may be enabled when needed and disabled again later to save precious power.

I am puzzled why running on the internal RC clock actually gave me slightly higher reading, maybe someone has explanation?