Workaround for the obsolete ATTINY15 1.5Mhz clock for FCC exemption?

Question

The ATTINY15's was ideal for designing FCC Part 15 exempt battery powered products since its 1.6Mhz internal RC clock ran just below the 1.705Mhz cut-off.

The ATTINY2/4/85 replacement parts come with a 1.6Mhz compatibility mode, but this clock is derived by dividing an internal 6.4Mhz clock. Since the FCC exemption requires that the device "not generate or use frequencies above 1.705 MHz", this replacement part does not appear to be eligible for the exemption.

It does seem possible to qualify for the exemption by using the ATTINY2/4/85's internal 128KHz clock, but this is much, much slower and not practical for some applications.

With the ATTINY15 dead, are there any good ways to design Part 15 exempt battery powered products that need more horsepower than 128KHz, without needing to resort to adding an external oscillator?

There's always strapping a crystal to one of the other ATtinyX5s. — Ignacio Vazquez-Abrams, Aug 06 '16 at 22:06
@IgnacioVazquez-Abrams that would certainly work, but hoping to avoid adding unnecessary parts. Question updated to reflect. Thx. — bigjosh, Aug 06 '16 at 22:14
Isn't this exemption pretty unclear anyway. I've seen a lot of questions about it, but never any answers, and the regulations seem to contradict themselves. — pipe, Aug 06 '16 at 22:30

score 11 · Answer 1 · answered Aug 06 '16 at 22:22

11

The RC oscillator of the ATtiny26L can be programmed to run at 1.0, 2.0, 4.0, or 8.0MHz directly without running through a prescaler. Its PLL will still perform a 64x though, so make sure it is disabled if the device must be eligible for the exemption.

answered Aug 06 '16 at 22:22

Ignacio Vazquez-Abrams

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Yes, that would work nicely- and point on the PLL well taken! So many AVRs, and no way to search on "highest generated frequency". :) Did you just know that from experience with the specific part?Thanks! – bigjosh Aug 06 '16 at 22:37
No, I've just dug through a lot of AVR datasheets and this particular quirk of the chip caught my eye one day. – Ignacio Vazquez-Abrams Aug 06 '16 at 22:47

score 1 · Answer 2 · answered Aug 07 '16 at 03:13

If the ATTiny26L is suitable, you are probably not space constrained. If that is the case you could easily go for something like an ATTiny84 or ATTiny841 and use an external oscillator. These have more functionality than the 26L and fewer pins. In fact you could probably fit an 841 plus a 1MHz RC oscillator in the same space as a 26L.

Given that the internal 8MHz oscillator doesn't run when the 128kHz oscillator is used, it follows that it also won't run if you select an external oscillator. Based on that, you could pretty much pick any device you want and use whatever oscillator fits your frequency requirement.

Workaround for the obsolete ATTINY15 1.5Mhz clock for FCC exemption?

2 Answers2