Hi Osama,

> What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)
> of course it will be set to use a HS crystal...
> will it work?
> what will happen?
> will it work with an external clock?
> how accurate will the timing be?

Nothing
of course
No
Still nothing
within limits
about the same as you would expect normally


A typical 16C84 will start to ocillate in HS mode reliably upto
about 12 MHz I have found.  Driving it with an external ocillator
works usually upto about 16 NHz.  When you exceed the specific devices
tollerance to 'pain' it just stops doing anything usefull.

The numbers for a typical 16C5X are about 30MHz for a crystal and
50 MHz for an external occilator.

If you daisy chain PICs with one OSCout to the next OSCin you can
get to about 25MHz for a 16C5X device.  The ocillator in the 16C84 seems
able to buffer the signal at 12 or 16 MHz through to one more PIC
at least.  With multiple PICs in a chain you notice the buffering
weakness at about 25MHz for the 16C5X devices at teh 3rd device
in the sequence and if you look at the signals on a scope you can see that
the amplitude suffers above 20MHz.

You cannot decently use a PIC in /RC mode with a crystal or external ocillator.

You can typically program the same /RC PIC fuses for /HS or /XT mode to
make use of the 'fractionally' cheaper /RC device if you have
non-mission-critical uses for the PIC.  Also usefull if there is no /HS
parts in stock at your favourite supplier.

Cheers
--
Kalle Pihlajasaari   kalle@ip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer