Using a 4-bit interface, I assume I should tie D0-D3 to ground. I have decided to enable the R/W line, and might be reading from the LCD. Should I put a resistor between D0-D3 and ground, or can they be directly connected?
Another HD44780 question
Feb 06, 2008
11 Replies
On a sunny day (Wed, 06 Feb 2008 17:49:12 GMT) it happened _ wrote in :
It says leave D NOT connected.
My Optrex manual says to ground the lower nibble to use in 4 bit mode, but I have seen others that say to just N/C them.
Jim
All the ones I've used had the unused bits unconnected.
I suppose the Optrex manual is trying to save a little current, perhaps, that might be drawn if the inputs float up to mid-Vcc or thereabouts?
If you look at their initialization protocol it's clear that the actual values the parts sees on those pins are all "don't cares."
On a sunny day (Wed, 6 Feb 2008 12:33:41 -0800) it happened "Joel Koltner" wrote in :
If I measure on those unused data lines I measure 4.98V. Possibly there is some internal pullup.
I'm not sure. Would it make a difference on a display that says the values are don't cares if they are grounded? I've never tried it.
values
If it got into "byte" mode, then you tried to read out a location with "ones" in it, the displays drivers would be running into a short circuit. So floating seems safest. Or pull-down resistors of course.
John Devereux
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I can see that as a possibility. I grabbed my Optrex "DOT MATRIX CHARACTER LCD MODULE USER'S MANUAL" (the caps are theirs) again, just to be sure, and it does say to 'ground' D0-D3 if using 4 bit mode. Maybe they have some series resistance on the lines? Maybe they hope the CMOS drivers hold up during a fault condition? Maybe it helps sell more displays. :)
Jim
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Better to use pull-ups, if deemed necessary. Pull-downs would waste current & could put the inputs into indeterminate states. There are internal pullups (10-120uA on the HD44780U DB0?DB7, RS, R/W inputs) according to the datasheet, presumably the older chips and clones have them as well. Best regards, Spehro Pefhany
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I've always left them unconnected in my tinkerings. IIRC, the HD44780 has internal pull-ups on the data lines.
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OK, I just didn't think it would be good to have them wobbling around. Internal pull-ups so if not connected they are still predictable works for me.
I leave them open.
Tom
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