Another HD44780 question

[_]

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?

 

[Jan Panteltje]

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?

It says leave D<3-0> NOT connected.

 

[James Beck]

It says leave D<3-0> 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

 

[Joel Koltner]

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.

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."

 

[Jan Panteltje]

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."

If I measure on those unused data lines I measure 4.98V.
Possibly there is some internal pullup.

 

[James Beck]

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."

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.

 

[John Devereux]

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.

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.

 

[James Beck]

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.

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

 

[Spehro Pefhany]

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.

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

 

[Anthony Fremont]

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?

I've always left them unconnected in my tinkerings. IIRC, the HD44780 has
internal pull-ups on the data lines.

 

[_]

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

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.

 

[Tom2000]

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?

I leave them open.

Tom