Quote Originally Posted by BrianT View Post
The processor is an 18LF4620 TQFP. It spends most of its time asleep where it draws 2.6 uA with a 4 MHz xtal. It runs at about 4 mA plus the external signal conditioners on a 4 MHz xtal. I am hoping to reduce the 'on' current to much below 1 mA with the slower clock.

I have to accurately measure the frequency/rate of some events so RC oscillators are out of the question during the measure phase.

I will have ICSP programming brought out to a 6 pin header. I can load different code for the logging phase and for the data dump phase. The ICSP has access to the configuration fuses so the XT/HS changeover should be possible. I figure to push a high level (~1 Vpp)20 MHz clock over the top of the 100 KHz xtal through this header. Real estate is ultra cramped so extra gates for a clean clock changeover are out.
Well there ya go. Add a jumper on the header to swap OSC1 between the crystal and an external X Mhz signal source (or whatever......you've got the 4xPLL to play with) and reprogram the unit to dump the data. Done dumping, reprogram for recording. At 100khz, a bit of sloppiness on the OSC lines shouldn't hurt (I've had much uglier on a 10Mhz crystal mounted roughly 2 inches from an '877A on lines that weren't so straight, still worked).

But, in the end, I like using the internal oscillator better. 31.25khz at the slowest and you can feed the 8Mhz intosc into the 4xPLL for some high speed data dumping.