AVR Freaks

YES AND NO. YES, as shown above. But NO on the basis of energy per instruction; that remains unchanged.

If you have some time where the MCU does not need to do anything, then run it at max clock speed and sleep the rest of the time. THAT will give you the lowest power consumption.

Jim

Is there any possibility of the processor sleeping in the meantime?

That's the CORE of any design that is battery powered. You should be thinking all the time "how can I get back to sleep". Don't throw this in as an afterthought at the end - make it one of your primary design decisions at the start.

So why isn't there a fuse setting that can multiply the internal clock by 2?

Clock multiplication is done by a PLL. A lot of modern micros have PLLs. On the whole AVR8's don't though some model like AT90PWM models do have a PLL for specialist purposes. If you want to play the clock multiplication game look at Xmega or SAM chips. (generally all 3.3V too)

 

But the internal oscillator for ATMega328 has only one frequency to be set: 8 MHz. (While working with ATMega16, I've seen a host of clock speed options: 1 MHz, 2 MHz, 4 MHz and 8 MHz.)

 

There is another option for the 328: Divide the clock by 8.

It seems that you are not aware of the clock prescaler register that is on [all?] recent AVR8 models, CLKPR.

The "option" to divide by 8, the CKDIV8 fuse, does nothing but jam a corresponding value into CLKPR.

And, re your initial question, what does the datasheet say about CLKPR?

l9.11 System Clock Prescaler
The ATmega48A/PA/88A/PA/168A/PA/328/P has a system clock prescaler, and the system clock can be divided by setting the ”CLKPR – Clock Prescale Register” on page 387. This feature can be used to decrease the system clock frequency and the power consumption when the requirement for processing power is low. This can be used with all clock source options, and it will affect the clock frequency of the CPU and all synchronous peripherals. clkI/O, clkADC, clkCPU, and clkFLASH are divided by a factor as shown in Table 29-12 on page 324.
 

With a 328P, the general approach I would take would be to run from the internal calibrated RC oscillator, while using a 32.768 kHz watch crystal to clock TIMER2 for timekeeping.  This will give you speed when you need it (to poll and process data while awake), and accurate timekeeping even while asleep.  Current consumption while asleep can be as low at 750 nA.

Can you tell more about the specific sensors you will be using?  Part numbers would be best.

Also, what will you be doing with all of this data?  Display locally (LCD, etc.)?  Store locally (EEPROM, SD, etc.)?  Transmit (wireless, serial, etc.)?

You still haven't said what kind of "display" or "output" this system has ...

So I wouldn't sweat the thunder showers...