Hi.

I've lost the start of this discussion, which PIC did you intend using,
I've an idea that would work on 16C73 or up possibly.

Use the RC osc, but have a way to trim the frequency in software. A DAC i=
s
probably too expensive but filtering a PWM output might do. Fit a 32KHz
xtal on the timer1 osc. Use the crystal to 'calibrate' the main osc. A
switched resistor would provide the main frequency switch function, keep
the trim down to 10%

Never tried it myself, however I've had the luxury of working on mains
powered projects so far.

Oliver.