"Alan B. Pearce" <A.B.Pearce@RL.AC.UK> wrote:
> The TRIS bit is getting changed somehow while another couple of pins ar=
e
> having their tris bits changed to change direction

Let me guess, you used BSF or BCF on TRISC?  There is a problem with
read-modify-write instructions on the TRIS registers with certain
peripherals active.  TRISC<3:4> have to be inputs (1) for the SSP module =
to
work, however if you do an RMW operation to TRISC at a moment when the SS=
P
module is using one of those pins as an output, the bit will read as a 0,=

and get written back as a 0.  See section 3.3 of the 16F877 datasheet.
        Jason Harper

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