Another idea i=B4ve had (not tested) is to have one PIC always on, runing=
 32kHz.  This PIC also store all variables, states etc and is running a main=
 control program.

When this PIC decides more processing power is needed (detects a change on=
 pin, timer overflow, keypress, A/D ready or whatever) it starts the 20MHz=
 pic.

The PICs do not need to be the same.  They do not have to share same lines,=
 but a possibility is htat he 32kHz pic releases the shared lines when it=
 let the fast PIC go.  When the fast PIC starts it can fetch variables from=
 the slow PIC using SPI, and then report the result back.

The fast PIC may be the very cheap C505 runing on internal 4MHz clock,=
 communicating to the slow PIC through software SPI. (and hardware SPI for=
 speed on th eslow PIC.

RAM data in the fast PIC RAM is preserved during reset, but have to be=
 loaded once after power up from slow PIC. One control line, or command byte=
 through SPI should be enugh to decide.


Or we can flip it all around, having main control in fast pic, and the fast=
 PIC can command the slow PIC "Please set me in reset and wake me up again=
 if anything happens"  The fast PIC RAM is retained during reset, and the=
 slow PIC can tell the fast PIC after reset "Had a nice nap? Your RAM data=
 is OK and hardware initialized before, so now run!" or after powerup: "This=
 is initial power up, please read data from EEPROM and initialize hardware!"

I think the last idea is better.  Comments?


/Morgan

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