Bernd Grunwald wrote:

> Does anyone know if and how a pic 16F84, 16C84 or any other can be
> programmed while it is working?

  Well, no, but ...

> Is there a better and cheaper way to do what I want?

  Much better.  Use a tokenised programming language stored in data
EEPROM which threads basic routines in main program space. This is what
Stamp basic does (and the 16C84 can support a quarter-size version of
this).  If more space is required, use an external EEPROM.

  Under these constraints, you can re-program "on the fly" without re-
setting at all.  Even better, you only need a one-wire (serial)
interface to do it, easy if you don't require an acknowledgement,
still possible if you do, or a two-wire (plus ground) interface like the
Stamp is even easier.

  Cheers,
        Paul B.