> want the 32kHz to act as a system clock, then the scenario
> would be something completely different (and pointless)

That's a little debatable. Say you have both the PIC and T1OSC
running at 32kHz. As I mentioned, the PIC can be put to sleep
and TMR1 will continue to interrupt. The interesting part comes
when you calculate how much power the PIC uses when it wakes
up to do what it's gotta do. A slow core clock (32kHz) uses
less power but the PIC is awake longer. A fast core clock uses
more power but is active for a much shorter time. For example
the F877 manual says

20MHz (HS) / 5.5V 7mA
32kHz (LP) / 3.0V 20uA

Extrapolating the XT figure of 1.6mA (5.5V) and 0.6mA (3.0V)
you might expect LP to be 53uA @ 5.5V + 20uA for TMR1

If the PIC woke for 500 instructions once per hour

32kHz => 61ms @ 73uA => 0.073/(3600*16.4) = 1.24nAh
20MHz => 0.1ms @ 7mA => 7/(3600*10000) = 0.19nAh

 + the SLEEP current, which is a heck more at 20,000nAh

I think nAh is the correct unit. Time * current = 0.0000044 and
0.0000007 respectively

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist