> ok, i am doing a new project, u guessed it...a clock.

'noob, good h/w - s/w project to start with. Endless possibilities
for mechanical and functional imagination

> and instructions are executed at this new frequency? im using a
> pic16f84a

The f84 was the first PIC I used and all of my clocks built with it
at that time (1996 ? and similar Motorolas before that) are still out
there, working away as good as new. The f84 is fine to use if you
have one, but for future projects there are better and cheaper
alternatives (628, 88, 688, 18F, 12F etc). For a simple clock or
timer, you can get by with just TMR0

I'd suggest you not use a 4MHz time-base, but 32768Hz. This
makes counting and rollover IRQs from TMR0 very easy. A crystal
will drift though and is not a good long-term solution. Fine for power
outages though. If you see this drift, and you will, make the reset
feature set a particular time, eg 6pm

The best you can expect is that the crystal works at an average of
32768Hz. But 1Hz high will be (32769/32768) * 86400 = 86402.6
or 2.6 seconds per day fast

If you want this to keep good time, use an AC wall-wart so that you
can count mains cycles. A loaded unregulated DC one may also
have enough ripple to use for this, or you could get into the wall-
wart and remove the filter cap and bridge. These can be used in your
circuit for rectification, but you'll have the AC accessible to the PIC
before that

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