----- Original Message ----- 
From: "Jinx" <joecolquitt@clear.net.nz>

> > Then I call this code to read the switches on portb(4..7):
> >
> > setb    rp0
> > mov    trisb,#11110000b
> > clrb    rp0
>
> Have you tried a delay here ? Could be some residual
> charge or capacitance

Yes, I thought about that so I put delays of up to 50 msec on both ends of
the subroutine.  I also tried clearing portb both before and after
reconfiguring the port, even though this wouldn't work in my application
because I need portb(0..3) to remain unglitched outputs, if at all possible.

> Are internal pull-ups on ? I think nominally they are around
> 27k, which might marginally balance the 20k pull-downs

No, the internal pullups are off.  Anyway, if balancing was the problem it
shouldn't work when called at the beginning of the code, but it does.

Interesting thing - if I run a simple program that configures portb as
outputs but doesn't write anything to them, and then reconfigure as inputs
and read the switches, I get all 1's.  If I don't configure portb as outputs
first but rather initially as inputs, it reads the switches fine.

> enabled ?) or for some external reason the pins really do
> always have '1' on them. Have you looked at them with a
> 'scope or meter ?

I will try this to see if they're staying high for some reason, but I doubt
it.  I have removed the switches and associated pull-ups from the circuit
and replaced them with just 4.7k resistors from each pin to ground, and the
same problem occurs.  I get all 0's (no switches so they're all off) if I
call the subroutine at the beginning, and all 1's if I run it in the main
loop, even a simple loop that just configures portb as outputs, and then
reconfigures it as inputs on 4..7 and reads the pins.

I am starting to wonder if it has something to do with the read-modify-write
characteristic of the ports.  If so, I wonder if there is a software
solution to get around this - seems like there must be.

If you can, try doing this yourself and see if you get the same result.
Simple circuit - just 4.7k resistors to ground from each portb pin.  Write a
program to initally configure portb as outputs, then reconfigure (4..7) as
inputs, read the pins (should be all 0's), reconfigure as outputs, and loop.
Output the result of the portb(4..7) on porta with LEDs.

Bruce

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