Can i leave the 4 unused pins floating on a LCD in 4bit mode

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I have a LCD that i will be running in 4 bit mode , i have seen some schematics where they are left floating ,and some where they are pulled high.

Can i leave the 4 unused pins floating on a LCD in 4bit mode

I would like to , and have tested that it works.

I had one instance where i had some funny chars but i think it might have been to long ribbon cable , and i had connected all wires to ribbon cable , and left the unused pins unconnected at the "other" end of the ribbon cable.

What are your experiences if i dont even solder wires on the 4 unused pins ???

Its a "Standard" Hitachi LCD (Optrex 2x16)

/Bingo

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It's better to tie the unsed pins to known state, be it pullup or pulldown. Why is it desirable to leave these inputs floating??

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Most 4-bit schematics i have seen has those pins unconnected.

So i was just wondering .....

Well its desirable because i wont need 4 resistors and 4 wires.

I'll give it a go with the 4 lines NC at the display.

/Bingo

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From the Optrex Dot Matrix Character LCD Module User's Manual:

Quote:
When operating in 4 bit mode, data is transferred in two 4 bit operations using data bits DB4 - DB7. DB0 - DB3 are not
used and should be tied low.

However, I've used a number of character LCD modules all in 4-bit mode, Optrex included, and have never connected anything to DB0-DB3.

Don

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I have also never tied them low or high and have had good experience that way. I believe it would be better to tie them low since it will help with some issues on the reset of the LCD displays. To ensure that it works try this site: http://www.doc.ic.ac.uk/~ih/doc/lcd/initiali.html

Good luck, Richard

|
| Richard Hoehn
|
| Nobody told me it couldn't be done,
| so I did it!
|

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I would strongly reccomend to use pull-up / pull-down resistors for all unused input pins of all IC's.
Floating pins are very sensible to ESD and they can consume a lot of power if they float in an intermediate level.
You can tie the four unused input pins directly to GND if you never read from the LCD, but then you should also permanently tie the R/W pin to the right level to make sure the pins are never outputs.
Floating pins can be connected directly to GND, but should never be connected directly to Vcc. If you want to connect them to Vcc you should use a resistor. I can't remeber the exact reason for this, but remeber that my didgital design teacher at my engineering college told us always to use a resistor when tieing unused pins to Vcc, but he said that tieing them to GND could be done either directly or through a resistor.
I will try to find the reason why unused pins should never be connected directly to Vcc, while it can be ok to connect them directly to GND.

http://www.amp.com/products/tech...

Quote:
List all open-collector and tri-state driven nets. Ensure that a pullup or pulldown resistor is present.
Open-collector and tri-state driven nets require a pullup or pulldown resistor to ensure that inputs on the net do not see a floating input voltage level. If the net floats at the ICs switching threshold voltage, the ICs outputs may change. Ground bounce causes this input voltage to appear slightly lower and the outputs may again change. This may escalate into wild oscillations, generate drastic RF emissions, and possibly cause system failure.

http://www.freescale.com/files/m...

Quote:
Line Termination
and Pullup/Pulldown
Resistors

• Line termination and pullup/pulldown resistors may be required to
guarantee voltage levels at high impedance or unused pins (do not tie
directly to VDD/VSS)
• Floating input pins may store intermediate voltage levels that would
cause current drain on internal logic gates
• Floating pins are also prone to pick up noise and suffer electrostatic
discharge stress (ESD)

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These controllers seem to have integral pullups, so in 4 bit mode it is ok
to leave them unconnected. I looked at two different data sheets that
I have, and they say leave them unconnected. I measured the current
to pull a data line low and the effective pullup resistance seems to be about
80 k on the unit I tested. As you discovered, you don't want to have long
"antennas" hanging off the "unused" pins because the high pullup resistance
would allow noise induced bogus logic levels to be seen by the controller.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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The problem is that it's often hard to find the right datasheet for character LCD displays.
Almost every LCD controller is compatible with the Hitachi HD44780 LCD controller, but I don't know if that also means that they all have internal pull-ups.
Often you can't find out excactly which controller IC is on the board because it's a die with no numbers shown anywhere and the LCD datasheet often just say it's Hitachi HD44780 compatible. You don't know who actually made the controller die so you can't check the right datasheet. But you can do your own testing as tpappano suggested.

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Anders And wrote:

Quote:
Floating pins can be connected directly to GND, but should never be connected directly to Vcc. If you want to connect them to Vcc you should use a resistor. I can't remeber the exact reason for this, but remeber that my didgital design teacher at my engineering college told us always to use a resistor when tieing unused pins to Vcc, but he said that tieing them to GND could be done either directly or through a resistor.

I may be wrong but I believe that this is a hangover from TTL days, as I remember the same warning. I don't think it's relevant for modern (cmos?) stuff.

Four legs good, two legs bad, three legs stable.

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I had experienced problems with long flat cables to connect LCD modules. Specially in a system that we use them in 4 bit modes. We also left the whol cable connected to the LCD module, and no connection on the PCB. That was a very nice antenna and that interferes too much with the LCD. We made another flat cable that wasn't connected to the 4 unused LCD pins, and then it works better. But still some problem some time.

Only when we use a short flat cable, the problem dissapeared.

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Thanx for all the ansvers :)

I will leave the four pins unconnected on the LCD PCB

/Bingo

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Guillem Planisi wrote:
I had experienced problems with long flat cables to connect LCD modules. Specially in a system that we use them in 4 bit modes. We also left the whol cable connected to the LCD module, and no connection on the PCB. That was a very nice antenna and that interferes too much with the LCD. We made another flat cable that wasn't connected to the 4 unused LCD pins, and then it works better. But still some problem some time.

Only when we use a short flat cable, the problem dissapeared.

Guillem.


Have you tried to tie the unused pins to GND and see if it helps?

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John_A_Brown wrote:
Anders And wrote:
Quote:
Floating pins can be connected directly to GND, but should never be connected directly to Vcc. If you want to connect them to Vcc you should use a resistor. I can't remeber the exact reason for this, but remeber that my didgital design teacher at my engineering college told us always to use a resistor when tieing unused pins to Vcc, but he said that tieing them to GND could be done either directly or through a resistor.

I may be wrong but I believe that this is a hangover from TTL days, as I remember the same warning. I don't think it's relevant for modern (cmos?) stuff.

I don't think that has chaged with CMOS, as he told me this only 3 years ago. CMOS had been invented for a long time three years ago. But his information could be outdated?
Our course book "Ditigal Design: Principles & Pratices" by Wakely says input pins can be connected either directly to VCC or GND or through a resistor. But I remember our teacher specificially warned us about the practie of connecting inputs directly to VCC, while it could be ok to tie them directly to GND. He didn't mention any difference between TTL and CMOS in this pratice.
He also gave a good reason for not connecting inputs directly to VCC, but I fogot what he said. I think it was because of current spikes at the VCC rail could destroy the inputs connected directly to VCC, because of voltage differences between the input pin and the VCC pin, but I'm not sure on this one.

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Bingo600 wrote:
I will leave the four pins unconnected on the LCD PCB

I still think you should verify that the inputs actually has an internal pull-up as mentinoed.
Please let us know the result.