tiny15 FM modulator

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This is just something I've had running through my head for a while. Has anyone ever thought of using a tiny15's high speed PWM output at it's maximum clock rate and then using the OSCCAL register to do direct FM modulatation?

12.8mhz is the frequency of the PWM output at it's 'flat out' rate, asuming a calibrated 1.6mhz clock. No modulation is possible at that frequency/duty cycle (excepting ASK) using normal methods.

The 7th order harmonic of 12.8mhz is 89.6mhz and the 8th order harmonic is 102.4mhz, within the FM broadcast range, so a filter stage would reduce out of band harmonics a bit and give at least a few micro watts of power.

Using a 2nd timer to toggle osscal between two values should give you some kind of frequency modulation, although I have no idea what this will do to the internal PLL that generates the peripheral clock. Will it cause a frequency slew as the MCU's clock changes? Will that slew rate be predictible enough to be used by the second timer to generate semi analog frequency shifting? Or even just fsk style modulation?

If the RC clock is changed by altering the osccal register the PLL frequency is going to change, but I don't know how the tiny15's pll works, or even how fast the internal RC changes.

I'd like freaks comments and experiances if they're similar. I'd like to be shot down but I'd like to know why as well =)

-Curiosity may have killed the cat
-But that's why they have nine lives

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I think the frequency step would be too coarse in this case. But, it might work

There are some cautions (somewhere) about not changing the AVR clock too much in a single step. Probably has to do with pipeline timing. Of course, the timing of the second timer would also change as OSCAL changes!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Worlds simplest fm transmitter

If this works...then maybe also a tiny15 version :)

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Quote:
Worlds simplest fm transmitter
Maybe, but it probably qualifies as the world's dirtiest FM transmitter.