Monday, December 24, 2012

Older LCDs

After the previous post on LCDs, I was reminded that it only covered current displays.  Getting from there to here was, in some ways, something of an adventure.

It is probably worth mentioning that when I say LCD, I am speaking of LCD Character Display Modules.  There are other types of LCD displays, both more and less elaborate, but for 90% of hobbyist projects we use LCD Character Display Modules.

My first exposure to these was an interesting board from B. G. Micro that contained an Optrex 20434 20 character by 4 line display, and a PIC that served as a serial to parallel converter.  This was also part of my introduction to PICs, and may well have served to get me interested.

This display from a standard parts house would have been quite expensive, probably in the $50 neighborhood, but B.G. Micro deals in surplus, pulls and other goodies, so although it was a little expensive with the board, it wasn't horrible.  If I recall, it was in the $25 neighborhood, about what a 16 character LCD would have cost at the time from the typical parts house.

The Optrex had some interesting quirks which probably turned out to be a good thing as it prevented me from being blindsided by later, less obvious, quirks.  You see, most LCDs at the time had 128 bytes of memory for the display.   Line 1 started at address 0, and line 2 at address 64 in that memory.  The Optrex was a little odd in that line 3 started at address 20, and line 4 at address 84.   If you went past the line length, line 1 wrapped to line 3, and line 2 to line 4.  Perhaps more curious, line 4 wrapped back to line 1.

For a long time I pretty much only bought LCDs from B.G. Micro.  They had a beautiful, backlit, 40x1 which made a perfect display for a Morse code decoder.  This was one of my first major PIC projects.  The original code was from IK3OIL, but he used a 16 character display, and he left out a number of Morse characters.  For a long time, improving that code provided many hours of PIC programming experience.

This display had another annoying feature that provided a memorable lesson in LCDs.  It was what was called a "low temperature" LCD.  What that really means was something of a nasty surprise.  You see, LCDs have a contrast pin on which you provide a low voltage, usually through a pot, to control the contrast.  On these "low temperature" LCDs, that voltage has to be negative.  Normally you don't have negative voltages laying around on PIC projects, so this required another supply.  Fortunately, the current demads aren't severe, so a simple charge pump can often do the trick.  Not insurmountable, but certainly annoying.

Back then it was critical that a display had a Hitachi HD44780 controller.  Any other controller and the code was likely to be a problem.  The controller was almost always visible on the back of the LCD, so at a hamfest you could usually tell what you were getting.  More recently, the controllers are potted, so you can't see what they are, but HD44780 clones have gotten a lot better so it is no longer an issue.

B. G. Micro had a cheap 16-character LCD they called a "Medical LCD" as it was pulled from some piece of medical equipment.  They had a plastic bezel in an odd shape that could not be removed without destroying the display (although it could be filed down to something sensible), and the connection was a very obnoxious cable with 0.05" spacing, making it very hard to work with.  But they were cheap.  At the time they were $3.  It is a lot easier to experiment with a three buck part than a fifteen buck part, so they became the display to use for quick experiments.  Although the physical aspects of this display were annoying, it turns out the display itself was quite nice.  B.G. still occasionally has this display, and the most recent price was fifty cents!

When the original PIC-EL came out, it had an 8 character display, quite a nice display if somewhat limited in size.  The display was not backlit - the green color is due to a green reflective background behind the display.

The original PIC Elmer lessons worked within the confines of this 8 character display.  Although a lot of applications require more characters, the limited 8 character display wasn't too much of a problem for lessons, and it did provide an excuse to demonstrate techniques like scrolling.

Quite soon after the introduction of the PIC-EL, the 8 character display was replaced with a 16 character display.  While a lot more flexible, this turned out to be something of a problem.

You see, the Hitachi HD44780 controller could not display a 16 character line.  To go beyond 8 characters required the addition of an HD44100.  This was fairly expensive, although the "cheap" medical LCD contained this addition.  To get around this, many early 16 character displays were actually two line 8 character displays, with the two lines side by side.  This required some programming gymnastics, and of course, code that worked on these displays would not work with a "proper" 16 character display.

Later PIC-ELs had a backlit, 2 line by 16 character display which AA0ZZ was able to get at very attractive quantity prices.  These are quite a bit nicer, and in fact, almost all current LCDs avoid a lot of the quirks of the older displays. Prices have dropped quite a bit as well.  Backlit LCDs, which used to start in the $20 neighorhood, can now be found for around $10, even from the major suppliers.  Cheaper houses like B.G., Sparkfun, etc. often have some very interesting displays for half that.  White or blue backlights, which can look a lot nicer than the traditional yellow-green are often inexpensive and take quite a bit less current than the older displays as well.

No comments:

Post a Comment