LCD

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I want to program the text to a 2 line 5 x 7 LCD module. But before I do
that in a language I am experimental with just the LCD module with a push
button for the clock or enable signal and some DIP switches for the data
signals. When I finished setting up the LCD and proceed to input letters to
the LCD, some letters are fine however some letters come out incorrectly. I
adjust the DIP switches for the appropriate hi's and lo's for that letter
however, when I clock the data into the LCD, a different character comes up
and when I clock it again and again, I get different letters. I notice
however that the letters that are different sometimes have similarities in
the binary number either in the high nibble or low nibble. Is my LCD busted?
It's a new one and took me ages to wire this thing up. I am using breadboard
to hold it all together. What could be the source to this problem?



Re: LCD



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Two possiblities:

1) Noise interference on the breadboard

2) Problem with your clock rate. Is it correct for the LCD?

Al

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With regards to the clock rate, I'm manually clocking the data into the LCD
with a pushbutton that has some hardware deboucing with it.

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">>> and when I clock it again and again, I get different letters. I notice
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Do you have pull up resistors on the data lines where the dil switch is
connected?



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Yes but I'm actually using pull down resistors, does that make a difference?
The configuration is a 10k resistor connects the data pin to ground and a
switch is placed between the data pin and +5V.


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Re: LCD


You will definitely get a valid HI when the switch is ON, but what is
the voltage at the input, when the switch is OFF? Try to measure that
with a meter.
Usually you put the resistor to +5V and the switch to ground because the
input may have a pull up resistor built in or worse if the input is a
"genuine" TTL input the is an inherent pull up resistor. A floating TTL
input behave usually as a HI input.
Cheers


wt skrev:
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I'd agree, change the resistors to pull-ups and the switch bits to ground.



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Should we always use pullups instead of pulldowns in all digital
electronics? Also, since the datasheet for this particular LCD is rather
crappy, can we check to see if there are internal pullups in the LCD but
using a logic probe?
Thanks to all who have contributed to this post. You have all been moste
helpful.

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99% yes, pull-ups.



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According to this datasheet http://web.media.mit.edu/~ayah/documents/hd44780u.pdf
all the input pins except for E have a max of 250uA internal pullup
current.  250uA across 10k is 2.5V.  Even the typical value of 125uA
gives 1.25V.  So the problem is almost certainly that your '0' input
value is too high.  So on all the inputs except the E line, either
lower the value of the pulldowns or switch to pullups (or just use the
built-in pullups of the display).


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