Jinx wrote:
> Am I right in thinking that PortC <4:5> on the 18F2550 can't be
> used as general purpose inputs ?
>
> My take on the datasheet is that they are dedicated USB data lines
Why, the datasheet is very explicit about what they can and can't be 
used for

 From the section on portC

"Pins RC4 and RC5 are multiplexed with the USB
module. Depending on the configuration of the module,
they can serve as the differential data lines for the onchip
USB transceiver, or the data inputs from an
external USB transceiver. Both RC4 and RC5 have
TTL input buffers instead of the Schmitt Trigger buffers
on the other pins.
Unlike other PORTC pins, RC4 and RC5 do not have
TRISC bits associated with them. As digital ports, they
can only function as digital inputs. When configured for
USB operation, the data direction is determined by the
configuration and status of the USB module at a given
time. If an external transceiver is used, RC4 and RC5
always function as inputs from the transceiver. If the
on-chip transceiver is used, the data direction is
determined by the operation being performed by the
module at that time."



>  
>
> If possible I'd like to use them for either switch or pushbutton detect,
>   
That does indeed seem to be possible if the USB module is disabled.
> but if that isn't doable then I'll just move on and try something else,
> perhaps multiplex a couple of the LCD's data lines
>   
May I ask why you are using a 2550 if you aren't using USB?
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