Select the best clock speed for a battery powered application

[davethecipo]

Hi,

I'd like to use an ATtiny1634 for a battery powered RTC. This application note from atmel explains the idea: an external 32768 Hz crystal drives a timer, which overflows causing an interrupt which wakes the CPU up. The CPU does its things and gets back to sleep.

In my case, I'd like to power the circuit from a CR2032, whose voltage varies from around 1.8 to 3.1 V. A graph at page 230 of the attiny datasheet shows the interval of safe frequencies vs Vcc. Therefore I should choose any frequency below 2 MHz.

Since I have size restraints, I won't add an external crystal and I'll just use the factory settings (internal 8 MHz oscillator scaled down to 1 MHz). First stupid question: is the graph valid for both external crystals and the internal one? Will I be fine running the internal oscillator @1.8 V?

Secondly, just for the sake of understanding, let's suppose I use an external 2 MHz crystal. Would this choice lower the power consumption? The CPU would do its job faster therefore going back to power-saving sleep mode sooner, am I right?

EDIT: I realized that the attiny1634 doesn't allow a separate oscillator for the counter, therefore I should use something like a ATmega48PB/88PB/168PB. Page 306 of the datasheet shows the upper limit for 1.8V operation to be 4MHz, apart from this my questions still hold.

[xtoffer]

Hello, afaik the graph should be valid for both with or without crystal. What would be most power saving depends on how much it's sleeping, what state it's in (idle, power down.. etc.) vs how long and how much power is consumed by it (and other things) during it's 'active' time.

On the page before the graph there's a little table with some values or current consumption for different voltages/clock frequencies.

Had a quick look at the fuses, it seems the tiny1634 has an option for using an internal "ultra low power" oscillator as well.

[davethecipo]

Thanks for the quick reply! I'll evaluate if the ultra low power oscillator is precise enough ??

[xtoffer]

Right, again, depends on the requirements of course. The internal oscillators can usually be software calibrated to some extent if required. But for best accuracy and stability you'll want a crystal of course.