Official word back from Atmel is that the 8Mhz oscillator is disabled when the 128KHz oscillator is selected as the clock source.
This could be very useful complying with the FCC Part 15 exemption for products that do not generate any frequency higher than 1.705Mhz.
Atmel's response is pasted below.
Commented by Abirami Sivakumar (Atmel) 2016-07-12 02:31 PDT
If you choose the internal 128KHz oscillator, 8Mhz oscillator won’t run. It is a separate internal low power on-chip oscillator.
07/11/2016 08:11:39 AM PDT [josh]
Thank you. If I do change the system clock to the 128KHz oscillator, will the 8Mhz oscillator still run or will it be disabled?
07/11/2016 05:05:40 AM PDT [Abirami Sivakumar]
[Recipients: Josh Levine]
Hello Josh Levine,
Thanks for contacting Atmel Technical Support Team.
The default clock source setting is the Internal RC Oscillator running at 8 MHz with longest start-up time and an initial system clock prescaling of 8, resulting in 1.0 MHz system clock
If you are changing the fuses, system clock will change according to the new fuse value.
You can output the system clock on the CLKO pin. Please refer to the section “6.4 Clock Output Buffer” in the datasheet.
07/08/2016 02:11:25 PM PDT [Josh Levine]
On an ATTINY2/4/85, if I configure CKSEL fuses to select the 128kHz internal oscillator as trhe clock source, will the 8MHz internal oscillator still run?
I am asking specifically trying to use an ATTINY25 in a design that must fall under FCC Part 15 47 CFR 15.103 (h), which requires that the "the highest frequency generated and the highest frequency used are less than 1.705 MHz", so I am looking to confirm that the 8MHz oscillator is not active when the chip is configured to not use it.