To Level Translate or not to

[be80be]

Here one I have link

I'll try to find the bubble rapping they came in there from china have a nice back light on them

 

[AtomSoft]

Cool! nice and bright. Here is a picture of mines running from 2 old AAA batteries.

 

[AtomSoft]

got bored.. so tested it on NXP (LPC2103)

I HAVE TO LEARN HOW TO USE THIS CAMERA LOL

 

[rynius]

Hey, I've been using a diode and pull-up resistor for my lcds to go between 5v and 3.3v. **broken link removed**
Works fine so far. I saw it somewhere else on the net.

 

[AtomSoft]

Cool heh . Still takes up too much space but thanks for the addition.

 

[Mike - K8LH]

Hey Jason, I just wanted to report back that my 3.6v TI LaunchPad outputs drive a 5v LCD without any problem.

I know you were concerned about destroying 3.3v µc outputs by hooking them up to 5v LCD inputs but that shouldn't be a problem because the LCD pins are "inputs" (no voltage to come back onto the µc lines).

My concern was that a 3.6v µc output level wouldn't be enough to register as a '1' on the LCD inputs but that doesn't seem to be a problem, at least on the LCDs I have.

Cheerful regards, Mike

 

[AtomSoft]

awesome!! thanks heh.. im working on making a new PCB right now for a nice HUGE LPC2378 so big! Biggest i ever had. So im sending to get the board made... dont want to mess up a $10 MCU

 

[be80be]

Hey Jason, I just wanted to report back that my 3.6v TI LaunchPad outputs drive a 5v LCD without any problem.

I know you were concerned about destroying 3.3v µc outputs by hooking them up to 5v LCD inputs but that shouldn't be a problem because the LCD pins are "inputs" (no voltage to come back onto the µc lines).

My concern was that a 3.6v µc output level wouldn't be enough to register as a '1' on the LCD inputs but that doesn't seem to be a problem, at least on the LCDs I have.

Cheerful regards, Mike

I have had all my Lcd's work with 3.6 volts and they would run even at 3 volts but you couldn't see them good till you raised it to 3.6.
I posted this over at digtal diy and it started a big argument about 3.3 volt chips.

But the bottom line is if it's a tll I/O pin it will work fine. When I started we had tubes and I still remember my first transistor it was a little tin can looking thing with 3 wire LOL

 

[Mike - K8LH]

Hi Burt. That's great but I'm running the LCD at 5 volts and feeding it from the LaunchPad board which is running at 3.6 volts.

 

[Mr RB]

The LCDs use TTL input voltage levels, 0.5v low and 2.2v high. Even that "3.3v LCD" you posted the link to on ebay is just a typical 5v one (based on the photos and specs).

There's no need for level translators.

 

[AtomSoft]

what about the RW Line ? It turns DB0-DB7 into output then what ? Some people still use that

 

[Mike - K8LH]

what about the RW Line ? It turns DB0-DB7 into output then what ? Some people still use that

That's a very good point. If the LCD was running on 5 volts then wouldn't it output signals close to that level, and couldn't that be a potential hazard for 3.3v µc inputs?

... Even that "3.3v LCD" you posted the link to on ebay is just a typical 5v one (based on the photos and specs) ...

You're probably right although I assumed the picture in that ebay listing was incorrect because advertising a 3.3 volt LCD implies (to me) that it will actually run at 3.3 volts and not just accept 3.3 volt inputs while running at 5 volts. Do you think it's possible that the LCD in that ebay listing will actually run at 5 volts or 3.3 volts? It would be nice if the seller would clarify that point.

 

[be80be]

If you looked at a Hitachi 44780 controller or compatible data sheet they all most have a operating voltage of 3.3 to 5 volts.

things that clause problems is back lights and not being able to drive the contrast negative.

Most need to be -5 volts on the contrast pin to be viewable.

Over at microchips forum there was a post that shows a simple way to get negative voltage for the contrast pin.

Just add two diodes to the common of your 3 volt regulator

 

[AtomSoft]

Ok OK but as a SAFETY MEASURE.. Since MCUs arent that cheap especially for me i would rather have protection... also if i burn out a MCU it would suck since its SMT most likely and hard to reflow with other parts on board. Also some PICs are XLP meaning they run below the 3.3v and also some other MCUs run at 1.8v so just to be safe for a LCD backpack i think a translator wouldnt hurt

 

[Mr RB]

...
You're probably right although I assumed the picture in that ebay listing was incorrect because advertising a 3.3 volt LCD implies (to me) that it will actually run at 3.3 volts and not just accept 3.3 volt inputs while running at 5 volts. Do you think it's possible that the LCD in that ebay listing will actually run at 5 volts or 3.3 volts? It would be nice if the seller would clarify that point.

The photo is definitely a 5v one, even the 150 ohm LED resistor is the value they use in the 5v units. China ebay are known for just using photos of one product on auctions to sell another product, so it might be a matter of that.

My GUESS on the matter is that they know the LCD will work fine on 3.3v as well as 5v so they are just selling the same LCD as "3.3v version". It's interesting as that backlight LED is probably about 3v Vf.

Atom, some people will use R/W and get data from the LCD but it's really common not to and just run the LCD in Write mode with the W/R pin tied to ground. If you did want to support reading the LCD I suppose you could put 22k resistors on the data lines, the PIC pin diodes will provide the protection.