avr - Does the internal 8Mhz oscillator in an ATTINY run even if no clocks are connected to it?

If I set up the fuses on an ATTINY so that the system clock uses the internal 128Khz oscillator (sometimes called the "Watchdog oscillator"), does the internal 8Mhz oscillator still run?

It does not seem to say either way in the data sheet, but I would guess that the primary motivation for running off the 128KHz oscillator would be power savings - in which case it would make sense to shut down the unused 8Mhz oscillator.

Can you think of a way you could even tell? Maybe a super-sensitive antenna coil near the chip looking for a tiny 8Mhz signal while the chip is running?

Answer

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

Hello Josh,

If you choose the internal 128KHz oscillator, 8Mhz oscillator won’t run. It is a separate internal low power on-chip oscillator.

Best Regards, Abirami Sivakumar

---

07/11/2016 08:11:39 AM PDT [josh]

To: support@atmel.com

Thank you. If I do change the system clock to the 128KHz oscillator, will the 8Mhz oscillator still run or will it be disabled?

Thanks, josh

---

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.

Link: http://www.atmel.com/images/atmel-2586-avr-8-bit-microcontroller-attiny25-attiny45-attiny85_datasheet.pdf

Best Regards, Abirami Sivakumar

---

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.

Thanks!