[sdiy] Odd problem with LCD

[Tom Wiltshire]

On 5 Apr 2009, at 20:48, Scott Gravenhorst wrote:

> Hey Tom,
>
> I'm sure you know what the heck you're doing...  That's very odd.   
> My experience
> with PICs reminds me that outputs are configurable in many  
> different ways.  I would
> look at my own code and wonder whether I am indeed setting it as an  
> output and with
> the specific characteristics I need.  I would want to make sure  
> that the line being
> programmed _can_ be programmed exactly like the ones that work  
> correctly.
>
> I also do weird little tests sometimes. In this case, I'd write  
> what I call a "scope
> loop", a tiny bit of infinite loop code which does only the test I  
> want, like set up
> the one line in question as an output and then set it high, and  
> then low and then
> high,low, high, forever.  Run it and stick a scope probe on it and  
> watch it.
> Fiddling with that loop code will usually tell what's wrong -  
> eventually.

Hi Scott,

I've now got just the uP and the LCD on the breadboard, and I've got  
the same problem.

I set up a "scope loop" like you suggested (I like it - I could use  
some terminology for some of these tricks!). It sets all Port B pins  
high, then waits, then sets them low, then waits, then repeats.
All the port pins are fine, except for RB0, which goes high  
correctly, but doesn't go to zero when it is low. Instead, there is a  
small voltage remaining. Presumably this is enough for the LCD to  
think the pin is still high. Ever seen anything like that before?

I'm beginning to suspect that I might be playing with a chip with a  
dead or damaged I/O pin...and (of course) that's one of the ones that  
I don't have spare of, so I can't easily test that theory.

Thanks,
Tom