Dual clock speeds on a PIC

[tenaja]

I need to have the lowest power possible while logging data for up to a year on a small 3.6 volt lithium battery but I also want a fast download to dump the very large data file of 128 MBytes after the data logger has been retrieved many months after deployment. Can I have a PIC running on a 100 KHz crystal while in data gathering mode and just force an external 20 MHz clock into CLKI/OSC1 for the data dump phase. I want to leave the 100 KHz crystal in place between OSC1 and OSC2 and just force a 1 volt peak to peak 20 MHz signal in for the data unload phase.

Has anyone tried this dual clock speed approach before? Any problems I should look out for?

BrianT

The easy answer is to use a PIC with an internal oscillator. You can run most of them at eight different speeds from 31.25khz or 125khz & doubling on up to 8MHz. Run it as slow as you want for gathering mode, and 8mhz for downloading--you can change the clock speed on the fly by changing the OSCCON register.

For even more power saving, put the PIC to sleep, and wake up periodically. A PIC and a 32khz watch crystal works well as an RTC, and is even documented on the microchip site.