Dual clock speeds on a PIC

[skimask]

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

I don't think you can use the dual crystal idea because a low speed crystal uses XT config setting (low drive current), whereas a 20Mhz crystal would use HS. High drive current which low speed crystals don't like, might work, but might drift a bit. Conversely, you might get lucky and be able to get a 20Mhz crystal to run off the XT setting.

Now, if you used 2 separate external DIP oscillators...that's a different story...
A couple of 2 input AND gates...a 7408 or a couple of those SOT-23 package single gate logic chips, I think Fairchild calls them 'Tiny Logic'.
One AND gate uses input A as an enable along with input B from 100khz clock
2nd AND gate uses input A as the enable for the other clock (as well as power for the oscillator maybe?) along with input B from the 20Mhz clock.
If you're using a PIC with a built-in clock fail oscillator (usually runs at 37-40khz), you could switch to that while switching main clock speeds, which would basically solve the problem of start up.
Or maybe skip the AND gates all together and just power on a 20Mhz DIP oscillator and force it over the top of the 100khz clock. Might look a bit ugly on a 'scope though.

I do that on my '4620. External 10Mhz, using 4xPLL, internal @ 40Mhz. When I don't need the processing power, I go to the internal 8Mhz. When I really don't need the power, I go to the internal ~37Khz.

Don't know what kind of PIC you're going to use, so we don't know what kind of options are available...