(Atmel) Using "External Low-Freq Crystal": What clock rate is chip at?

If one were to use an ATtiny/ATmega chip, with the "External Low-Freq Crystal" option selected in the fuses, and a 32.768khz watch crystal installed per the datasheet...

What speed will the chip run at?

Does it run at 32.768 khz? I see nothing in the datasheet to suggest otherwise, but this sounds like it would be uselessly slow.

Or does it run at 8mhz or 1mhz (depending on the clock divider settings) using the internal oscillator, with the crystal being used to keep track of time (like how you can install a 32.768 khz crystal on STM32 mcu's to keep the RTC accurate)

Thanks

Super low power, fast enough for a basic clock, what's not to like?

Does it run at 32.768 khz?

Yep. Some microcontrollers contain a clock multiplier ("PLL") that will produce a high-frequency clock from a low-frequency crystal, but not the AVRs used by Arduino (Hmm. The tiny85 has an 8x multiplier, intended to produce a higher-speed clock for timer peripherals, and I think it can be used for the system clock as well. But 8*32Khz is still very slow.)

You could theoretically use the 32kHz crystal to calibrate the internal RC clock.

But in general, it's just "really slow." "Useless" is in the mind of the beholder, of course.

DrAzzy: Does it run at 32.768 khz?

Of course!

DrAzzy: I see nothing in the datasheet to suggest otherwise, but this sounds like it would be uselessly slow.

Useless for who?

It's obviously useful for somebody...

westfw: Yep. Some microcontrollers contain a clock multiplier ("PLL") that will produce a high-frequency clock from a low-frequency crystal, but not the AVRs used by Arduino

Yes they do. See the "PLL" section of the chapter "System Clock and Clock Options".

The PLL is what enables the Tiny85 to run at 16MHz with the internal clock, to run Timer1 at 64MHz, etc.

I did mention the 8x multiplier in tiny85. (there are no "official" m85-based Arduinos, right? But there's digispark, Trinket, and Gemma.) The m8, m168, m328, m1280, and m2560 have no PLL. 32u4 has a PLL that needs an 8MHz input.