Reducing the Clock Frequency

Now let’s look at how we can reduce the power consumption of the application, and verify that it is improved.

A first thought might be to get rid of the delay loop and run the LED blinker of a timer interrupt. In addition, a simple example like this does not need to run at high-speed, so we can use the clock prescaler to run slower. The code below is included in project low_power_102.

#include<avr/io.h>#include<avr/interrupt.h>// Timer 0 ISR
ISR (TIMER0_OVF_vect){if(PORTB ==0x00){// LED is OFF, turn it on 
        PORTB =(1<<5);// Shortened timeout for on cycle
        TCNT0 =0x102;}else{// LED is ON, turn it off
        PORTB =0x00;}}intmain(void){// Change the clock prescaler
    CLKPR =(1<< CLKPCE);// Scale by DIV64
    CLKPR =(1<< CLKPS2)|(1<< CLKPS1)|(0<< CLKPS0);// Port B5 to output
    DDRB =(1<<5);// Timer0 DIV 1024 
    TCCR0B =(1<< CS02)|(1<< CS00);// Overflow interrupt enable
    TIMSK0 =(1<< TOIE0);// Interrupts onsei();// Do nothingwhile(1);}

Todo:

Build the project/solution (F7)
Program the application into the target device using Start Without Debugging (Ctrl+Alt+F5)
Switch to Data Visualizer to see the results

The plot above was captured while reprogramming the target. What can we see from this plot?

Two negative power pulses are seen (the device is pulled into Reset)
Four positive pulses are seen (read ID, erase, program Flash, and verify Flash)
The new application starts to execute

Leaving the application to run for a few seconds should give you a plot similar to this one:

What can we see from this plot?

The kit now draws about 6 mA with the LED OFF
Current draw increases to about 9 mA when the LED is pulsed ON
The 1% duty cycle still seems approximately correct

Result: Power consumption has been significantly improved by clocking the device slower.

Important: Because this example prescales the clock to 8 MHz/64, the ISP programming clock must be set to less than 32 kHz to be below 1/4 of the main clock when attempting to reprogram the device!