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lcd (Liquid Crystal Display) IO


NEW! lcd Front Panel Set: 2x16 lcd w/HD44780 controller, 4 push buttons, 3 LEDs and a 2x12 pin header. +

Manufactures use the term Vcc-Vee to describe the lcd bias voltage. This is actually a NEGATIVE voltage! For example, from the Optrex DMF5005 data sheet: Supply Voltage (lcd Drive) Vcc-Vee is Min=8V Max=26V. Do the algebra: If Vcc=5v, for the Minimum value, Vcc-Vee=8, 5-Vee=8, 5=8+Vee, -3=Vee. The max is -21 volts.

A lot of lcd panels (with a built in controller) need up to 50ms after power up before they can accept commands. Check the data sheet.

A well written response to a question of directly driving lcds from microcontrollers:

The truth is, I suspect, that most of us haven't a clue about how to drive lcds from a micro, because the lcd display market is totally owned by the Far Eastern consumer electronics majors and for most Western  engineers the only available course of action is to buy one together with its fabulously specialised driver chips, and follow the instructions. This is not, of course, calculated to produce lowest-cost, lowest component count solutions. Not that there's any protectionism in the Pacific Rim electronics industry, of course (;->)

Driving lcds with only a few segments is easy: you apply a square wave to one face of the lcd (the "backplane") and apply to each segment electrode either the same square wave (segment off) or the anti-phase square wave (segment on). Many micros designed for consumer and instrumentation applications have built-in lcd drivers for exactly this purpose. But clearly it requires N+1 wires to support N segments. Fine for the display on a multimeter or a central heating programmer, but hopeless for graphics. [ed: see:

High segment count (i.e. graphic) lcds use a much more elaborate multiplexing scheme which in practice demands a custom chip for each family of displays. Doing it from scratch is a major project and you would probably have very great difficulty getting enough info on the raw lcd to be able to do it. You are more-or-less cornered into buying the lcd together with its driver.

And that means that you have to be in with a chance of understanding the data sheet. This probably requires not only a modest amount of experience of microcontroller projects, but also some parapsychic expertise to cope with the translation into English from Korean, Japanese, Yogistani or whatever. I think you need to find a soul-mate who's done something similar before.

Sorry to be so negative and depressing. Jonathan Bromley

If you are driving the pixels directly with DC you will destroy the lcd. You need to drive them with a perfect square wave 180 degrees out of phase so that the lcd sees an alternating polarity. Why? Because liquid crystal is a chemical that will electrolyze into its components just like water will go to hydrogen and oxygen, or other items will "plate out" components onto one electrode or the other. This destroys the ability of the "liquid crystal" to react.

lcd displays, particularly the serial input type, already have the appropriate drivers in the hardware and they are transparent to the user.

Rather than use EL or other expensive backlighting panels:

Try frosting one side of a piece of plexiglass, putting LEDs (especially Blue) around the edge, and the panel in front. If you have access to someone who can plate or vapor deposit a metal film on the frosted back, even better.

Not as great as a panel, but the frosting will cause the edge lighting to bounce around a lot and come out the front more or less uniformly. Cheap, and it does work. Made a lot of edge lighted signs (Exit etc.) using plexiglass and a router years ago. Light would hit the routed letters and make them glow with a nice blue-green. Really pretty.

Jinx says:

I've been trying the Varitronix MGLS160128 graphics unit as a replacement for an older green screen. The contrast on this new type is quite a marked improvement on even the standard black/ silver screens. It is almost black on white. The only snag so far is that the standard screen could be run fairly simply on batteries, using 7660 inverters to get the -15V supply. The MGLS160128 is quite demanding, requiring +5, -21 and -13.7 for best performance

See also:

manufacturers

lcd manufacturers

new suppliers

Surplus lcd suppliers

Questions:

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file: /Techref/lcds.htm, 19KB, , updated: 2013/7/23 07:57, local time: 2014/9/22 01:34,
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