3.3v compatible LCD

[fvnktion]

I know this is a bit off topic for this forum, but hope that it may be useful to some of the rest of you who are using PIC's with an LCD application.

I am having to switch to a lower voltage 3.3 from 5 v, due to using a lower voltage sensor in the system. By dropping the VDD of the system none of my 18x2 lcd displays are working properly. The system still functions but nothing is diplaying on the LCD.

Can anyone recommend an 18x2 character LCD display that will function @ 3.3 Volts. I am also curious if I could switch out the 5v regulator on the PICdem2 plus board to a 3.3v and still have functionality.

Any suggestion for what any of you are doing would be great.

Thanks

[treitmey]

Just for giggles.. you might get the 5V to work with a different Vee
look at page 2 on how they setup Vee.
If you have a TC7662 you could get the -3.3V and give it a try.
http://www.lcd-module.de/eng/pdf/doma/dip162-de.pdf

Good luck!

[fvnktion]

Thanks for the info Treitmey,

It looks as if i would have negative source onboard to be able to use this technique. I simply have 3.3v and Gnd, so it seems this would not work in my case.

In case anyone else is interested, I did find the following lcd that is designed to run all functions on the 3.3 volts. NHD-0420DZ-FL-YBW-3V3-ND from digikey.

I think I'll just do a quick backplane PCB design to align this lcd to the existing thru hole pattern on the PICdem2 plus board. I should have plenty of extra boards left from the panel in case anyone else would like to do something similar.

Thanks

[Guest]

That's a solution. But if you are going to build a board anyway - you could just use something like the Linear Tech LTC1517-5.

This inductor less DC/DC takes 3.3 volts in and makes a regulated 5 volts with only one IC and three capacitors. That way you can still use the same LCD. The only problem would be if the LCD uses CMOS levels on the input pins or TTL levels. You would need to check the LCD data sheet.

My 2 cents worth.... Steve H.

[crystal_lattice]

Something nasty to watch out for is that the LCD backlight might not work from a 3.3v supply. I experienced this when designing a allround 3.3v system and could not get the backlight working. I was working from the 5v version datasheet and logically assumed that the 3.3v version would run from 3.3v - not the case. This might be different for other manufacturers.

Have a look on Ramtron's website for a app note describing the use of a 5v LCD in a 3.3v system - it uses PWM set to 50% duty cycle and feeding a diode/capacitor voltage doubler - quite nifty

[Ttelmah]

A lot depends on the colours you want.
I have had good behaviour, from the Powertip 3.3v modules. Obviously depending on what display layout you need. For example, their PG12864LRF-NRA-H-S0 module, is a graphic (128*64) module, supporting 3.3v logic operation, and 3.3v backlight (yellow/green). However the black/white modules, though available in 3.3v logic, require higher backlight voltages (see below).

You have four 'parts' to the LCD, which affect the drive:

The LCD itself This will require more than 3.3v. On 5v displays the SCI7660 can be used to give 2* the supply to feed the LCD, but on 3.3v units, the SCI7661 is needed, to give 3* the supply. This is slightly more expensive. Most manufacturers will offer this now, and the currents involved are tiny. Alternatively you can provide your own voltage for this. If (for instance), you have an RS232 driver wih inverters,this may well suit.

The controller chip. Most now are available in 3.3v versions.

The driver chip. Probably more than 50% are now available in 3.3v versions, but some of the more complex graphic drivers are still '5v only'.

The backlight. Remember that blue, and white LED's, in particular have Vf values that are normally above 3.3v. So green backlight, will usually be available, but white (either LED, or EL), _will_ require a supply inverter.

Now, for the original poster, do you really need 18*2?. 16*2, or 20*2, are much more common. With a green backlight, I can think of several modules in these sizes, that will run completely on 3.3v, but have not met an 18*2, that I have used at this voltage.

Best Wishes

[jgschmidt]

I can't recall where I saw this but I saw a circuit that included a MAX3232 RS232 tranceiver which generates a negative voltage on pin 6. This created enough of a voltage difference to drive the LCD. So, if you have RS232 in your circuit you may already have a power supply for your LCD.

If anyone has actually done this, or knows of a circuit, please let us know.
Jurgen

[fvnktion]

Thanks for the informative replies. I especially like the idea of the negative voltage from an rs232 driver. Indeed I am using RS232 so this may well work.

[jgschmidt]

I also found Newhaven Displays has several 3v LCD displays, even with 3v backlighting. They have both serial and parallel interfaces. Some are available from Mouser but it looks like you can order directly from them.
www.newhavendisplay.com