I tracked it down to the clock that is the issue, as the I/O works fine 
on the shared port/bit (PortD-5). If anything, you would think that's 
the one that would have problems being that it's a switching I/O port.

So, that just leaves the clock that works on PortB-0, but not on shared 
port D-6.

Well, I tried changing the timing (adding NOPs) before and after reading 
the bit, and that made no difference. I am right at the 200 nS minimum 
bit available time spec, but that same code worked fine with the 
dedicated bit (PortB-0). I also made sure I cleared port D at the 
beginning of the clock subroutine just so I start with a known state.

Then, I tried port D-0 (also shared), and it worked! That's odd because 
D-0 is also shared with the LCD display data. So, I tried D-5, and that 
worked.

So, I moved the clock to D-7 and moved the I/O to D-6 since I wanted 
them to use consecutive port bits. And that works. FYI, the RST line is 
dedicated since it controls when the clock is accepting data input.

So, the question is: Why does the clock signal not work on D-6, but it 
works on (seemingly) any other bit on the same port, and why does the 
I/O work on D-6?

Yes, I made sure there was nothing other than the LCD and this clock 
chip using ports D5, 6, and 7.

While this will work, I would like to understand what was wrong.
Any ideas?

Thanks,
Joe M.

Spehro Pefhany wrote:
> At 12:11 AM 2/15/2010, you wrote:
>> All I can tell you is that it works on separate pins.
>>
>> Are you thinking there is some capacitance in the LCD data input that is
>> "holding" one of the data lines higher for an extra uS or so?
> 
> No, that's quite unlikely.
> 
>> But, it might be a good idea to "clear" that port every time the LCD is
>> done reading rather than leave the pin in it's "previous state".
> 
> Yes, clear it every time, and delay at least 1usec after clearing it
> before bringing the CE high (assuming 5V supply).
> 
>> Best regards,
> 
> Spehro Pefhany --"it's the network..."            "The Journey is the reward"
> speff@interlog.com             Info for manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  http://www.speff.com
> 
> 
> 
> 
> 
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