Tuesday, December 18, 2012


For some time now I have had almost an obsession with LCD displays.  In the past, there was a wide range of displays with a wide range of prices, generally more characters being a little more expensive, backlit displays being very expensive.

The most common backlight was the sickly, yellow-green LED backlight, which not only looked bad, but tended to be a real current hog.  Electroluminescent backlights were available, but they tended to require unfortunate voltages; typically AC and somewhere in the 60 to 400 volt range, not generally what we have lying around on a microcontroller project.

More recently, white LED backlights which are far more efficient have become available.  And, they have become a lot less expensive.  It seems as if a white backlight is often available for close to the cost of a non-backlit display, and while a yellow-green backlight may take almost an amp, the white ones tend to be in the 20 mA neighborhood.

N8ERO picked up some Newhaven negative displays which show white letters on a blue background.  I had seen these before, or something similar, on some FDIM projects, and while they don't look as nice as their pictures, they do look a lot better than the ugly green.

LCDs exhibit two kinds of slow.  The processor on the LCD takes some time to do it's thing, so code either has to delay or read the LCD's busy signal.   Reading the busy signal is kind of a pain, so most hobbyist projects simply wait long enough.  The actual display itself is a chemical change, so compared to electronics, it is very slow.  A result of this is that there it rarely a real penalty for waiting far longer than necessary for the display's processor to do it's thing.

However, in experimenting with different displays, it appears that negative displays are slower, as are 3 volt displays.  In the case of the blue Newhaven display, part of this slowness results in some rather ugly brown artifacts when the display is changing.

I found a blue, positive Newhaven display that was very inexpensive, but 3 volts.  3 volts is pretty handy for PIC24, PIC32, or dsPIC33 projects, not so much for the more common 5 volt projects.  This display is quite small, something that is also sometimes nice.  LCDs tend to take a lot of the panel space, and depending on the project, having a 20x2 display in a small space is an advantage.  However, small also means that the connections to the part are different than most LCDs, and kind of a pain.

Well, with LEDs now available in umpteen colors, and plummeting in price, it couldn't be long before someone came up with a display with an RGB backlight.  I picked one up, the price, while a little high, wasn't crazy.    This display is, interestingly, a 3 to 5 volt display.  Unfortunately, the 3 volt part means it is slow, and the negative display, which looks a lot nicer, makes it even slower.  This is a pretty slick choice for projects that have a display that mostly isn't changing.

More recently, displays based on organic LEDs have become available.  OLEDs have the advantage of very nice contrast, high speed, and relatively low current compared to backlit displays.  This particular OLED display has both the traditional 4 or 8 bit parallel interface as well as a serial interface.  I have to say, I think this one is my favorite.  It is slightly thinner than the LCDs, and the display is extremely crisp.  And it doesn't exhibit any of the shadow artifacts that LCDs tend to show when they are changing.

The one downside is that it appears to require that you use the busy flag.  While this is annoying, especially when using the 4 bit interface, once the code is written and wrapped in a library, that detail becomes invisible.  It is a little more expensive than an LCD, but boy is it nice.

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