AVR Freaks

Another question, why run at 32khz?  What voltage are you running teh AVR at anyway?

JIm

If operational speed is not an issue then why not just use the internal oscillator during operation, and if you are not doing anything else during sleep do you really need the 32khz clock?

JIm

Do you need accurate timekeeping while in lowest-power sleep?  If not, there's no need for the 32 kHz crystal.

As for power consumption, it is almost always better to run fast for a shorter time then sleep for longer, than it is to run slow for longer then sleep for less.

Perhaps if you explained what your app will be doing, for which parts of that it needs to be awake, and for which parts it can afford to be asleep.

If it were me, I'd run a battery operated app at 4 MHz when awake, and go into power-down or power-save whenever I could.  At 4 MHz an AVR can run at 1.8V.  It doesn't get better with lower speed.

The only exception might be when running off of something like a CR2032 which has a significant internal resistance.  Higher cpu clock speeds will increase instantaneous load on the battery and result in a greater voltage drop.  That is, while the AVR can run at 4 MHz at 1.8V, if the CR2032 is near the end of its useful life, drawing the higher current associated with a higher clock speed a 2.0V CR2032 might dip below 1.8V under that load, whereas the lower draw from running at 1 MHz might keep it above 1.8V.  But this is 'blood-from-a-stone' territory, and can be managed in software.

I have changed my PCB to use the external 32kHz crystal now and 3.3v.

Hang on.  Are you using the LF crystal as a system clock?  Or for the asynchronous TIMER2?  For the latter, place the crystal on TOSC1/2 instead of XTAL1/2.  The rest is only fuse settings and software.  MUCH better to run from the 8 MHz internal oscillator (divided down to 4 MHz if low-voltage operation is desired), but derive an accurate clock from the 32.768 kHz LF crystal.

EDIT: typo

Is your 3.6V battery a LiPo?  It will drop as low as 2.7V while discharging.  Is your 3.3V reg an LDO reg?  What's the dropout at your designed current?  What else is drawing power?  How much is that?

The 324 has a 'speed grade' i.e. a chart which describes the fastest it can run for a given Vcc.  At 2.7V, the fastest it can run is 8 MHz.  With your reg's dropout, the 2.7V from your battery may result in 2.5V or less.  You must manage that, and the speed of your AVR.
 

EDIT: typo

Take a deep breath.  ;)  Make a target/goal:  "I want this app to run off an XYZ n.mV battery as the power source.  That has a capacity of 1234mAH.  I want to battery to last abc days."

Now there is a target, and a simple calculation gives the allowed average power draw.

1)  I'll wager that your regulator loss over time will be larger than the AVR draw.  So even if you make your AVR draw no power at all, the target/goal might not be reachable.

2)  As pointed out, your chosen model draws multiples of the power that compatible and less expensive models do.

3)  From general experience:  If at all possible, ditch the regulator and run directly from the battery.  I'd guess none of your devices are going to drop dead at 3.6V or 3.65V.  If that is really a problem then use a Shottky diode drop.

4)  From general experience, ditch the crystal.  discussed above:  More power and slow wakeup.

5)  IT TAKES NO BOARD CHANGES TO RUN FROM THE INTERNAL OSCILLATOR AND TRY STUFF OUT.

Once the gross stuff above is out of the way, then you can start tuning the power budget.  It isn't too interesting to me to continue and try to achieve e.g. 25% savings, when using the A version gives 50% savings.

For details of what I mean by "power budget", this post https://www.avrfreaks.net/comment... has links to prior threads with a "worked example".  I suggest you do the same.