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?

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".

Oh, I can also tell you that the LCD still works fine with no corruption 
of the data.

Joe M.

Spehro Pefhany wrote:
> At 11:45 PM 2/14/2010, you wrote:
>> Well, I thought I was doing well on pin sharing.
>>
>> Here is a problem I'm running into:
>>
>> I have a 4x16 LCD display (electrically the same as a 2x16).
>> I have a DS1302 clock.
>>
>> I should be able to share the clock and I/O signals of the clock with
>> the data of the LED since the LED has an 'enable' pin and of course
>> neither the clock nor the LED data lines are used at the same time.
>>
>> I know the RST of the clock needs a dedicated line, and it has one.
>>
>> But, when I redefine the lines of the clock to share the pins with the
>> LCD data, the clock does not work.
>>
>> There is nothing connected between the LCD display data inputs and the
>> PIC, and there is nothing connected between the clock and the PIC. Both
>> the clock and the LCD should be voltage-sensing pins, and the PIC should
>> be the only voltage provider, and I should not be exceeding the rating
>> of the PIC pin.
>>
>> Oh, and the RW pin on the LCD is low (write), so it should not be
>> outputting any voltage.
>>
>> Any ideas why this is not working?
>>
>> Joe M.
> 
> Are you sure the clock line (SCLK) on the DS1302 has reached a valid logic LOW
> level well BEFORE asserting the first positive edge on the CE line
> (formerly called /RST)?
> 
> If the LCD logic leaves the SCLK in a high state or if you switch both
> at once (maybe) you'll get one fewer clock edges than necessary and things
> could get ugly. The data sheet says a minimum of 1us at 5V and a whopping
> 4us at 2V (parameter tcc), which could be a lot of PIC instructions.
> 
>> 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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