Predko 2-Wire LCD circuit at 3.3V

[futz]

I'm trying to use Myke Predko's 2-Wire LCD circuit at 3.3V with my ARM7 board. But the (5V) LCD isn't happy with the E line being strobed at only about 3.28V. The LCD and "shift register" is 5V powered.

I tried using a PNP transistor to switch 5V to E, but the Predko circuit's E line sits at about 8mV when low, so the transistor never shuts off.

Electronics experts, what should I do with this thing? I've Googled hard and read here and there that 44780 LCDs should be happy with 3.3V logic. Other people say that some will work at 3.3V and some won't.

 

[Gordz]

Most LCD's of that type will work at 3.3 volts or so. The problem is more with the contrast voltage, I have used a simple voltage pump to get a negative supply from a spare I/O pin and the LCD works fine. Anything similar will do.

**broken link removed**

 

[parts-man73]

It should work fine, I've used a 44780 LCD with Parallax's Propeller, which is a 3.3V chip. I even sell a LCD adapter board for it. I've never had a problem running the LCD, nor have I heard from any customers that have had problems.

I power the LCD and backlight with 5V, but all the logic is 3.3V. shouldn't need any inline resistor or level shifting circuitry if you are just writing to the LCD, unless you plan to read from the LCD (like checking the busy bit)

Brian

 

[futz]

It should work fine

Yes it should. I put a PIC on the breadboard and put some known-good code in it, and it works perfect.

Then I removed that and reconnected the ARM. Connected the meter to both the data and clock lines (not at the same time) and stepped through the LCD init code. Everything seems normal. The delays in the code are ridiculously long for testing purposes, so I don't think it's timing.

Guess I'll go through it all again tonight and see if I've missed some stupid little thing...

I have a proper ARM board in transit that might even be here today. With more than two free pins to work with things will be a lot simpler.

 

[futz]

Well, I get my new Olimex LPC-P2148 board, plugged in the same 2-wire LCD circuit to the same two pins (P0.28 and 29) and made the few trivial code changes needed... And it works perfect!

**broken link removed**

That fan controller board I was tinkering with is a fine fan/peltier controller, but is useless as a general purpose experimenter board.

I'm relieved to know that my code was correct all along.