What is this type of digits-only LCD called?

Question

I've seen this type of LCD many times in weigh scales, calculators, gauges, micrometers, etc. I know it's a fairly traditional display but I really like its compactness, simplicity, and maybe it even costs less, versus TFTs or smartphone-type touch displays.

[What is it called? Answered by @kevlar1818 and @stevenvh: "Seven-segment display"]

I would like to work with this type of LCD, for example, I just found this 8-digit one called VIM-878 from the Digikey catalog; here is its datasheet.

How do I interface with it? I would like to know what would be a good/common way of interfacing with it from a simple AVR microcontroller like an Atmega8 -- preferably with interfacing circuitry/parts that aren't too physically large.

• I presume some sort of driver or multiplexer would be necessary? I guess I'm looking for some beginning perspective from others who may have better experience interfacing with this type of LCD.

Answer 1

It's a 7-segments display. Unlike the dot-matrix character displays kevlar refers to these are most often not intelligent module. Most dot matrix displays have an HD44780-compatible controller which you simply can write ASCII codes to, but a 7-segment LCD will often be just the glass, with connections for segments and a number of backplanes (often up to 4).

Driving LCDs can be awkward since they don't use just two levels, so you can't drive them with common digital logic.

The best thing you can do is select a microcontroller with integrated LCD controller, which you can connect the display directly to, like the TI MSP430x4xx. Like most controllers this one also knows just segments; it isn't aware of digits or anything. (Great, first we had a dumb display, now we have a dumb driver as well!) There's reason for this. These LCD drivers are often used to drive custom LCDs which may be a mix of a numeric part, bar graphs and custom symbols. Such a symbol is also a single segment, so it makes no sense to talk about digits.

This display has symbols like "battery" and "alarm clock", but also all text fields, like "AM", "PM" and "SNOOZE" are symbols consisting of a single segment (i.e. controlled by a single bit).

Further reading
MSP430x4xx Family User's Guide. LCD controller is covered on p.709 ff.

Answer 2

What you are thinking of is called a 7 segment LCD. If you know how to light up a regular LED then your 1/8th of the way there.

Here is the Wiki to it. It has some theory on how you would create numbers by lighting up different arrays of the LED's: http://en.wikipedia.org/wiki/Seven-segment_display

(de)Multiplexing is a good method for lighting up a large 7-Segment panel, although not necessary. If your I/O ports are limited then it's the way to go. The basic's of demuxing work like this (assuming you know binary here). Look at this picture:

With a 3 bit binary number like 000 we can represent up to the number 7 (I.E. to select port 5 we'd have ABC be 101). What would happen in the demux in your situation is one input is something like a high signal, and AB&C are I/O ports. The 0 to 7 are connect to each segment of the LCD and by writing to ABC from your controller you can light up a segment. You may be saying wait but if I want to light up a number I need many segments lit, not just the 5th segment. Well after that you can look into two roads to get the LCD to display a number. The first is add more demux's for additional needed segments. The second is a software approach which involves very very quickly flashing each of the segment required to create the illusion that the panel is completely lit. The AVR chip in your mega is operating in the Megahertz range if you write a loop to display ABGED (from the wiki pic) you will end up seeing something that looks like a number 2. Hope this gets you started.

Answer 3

A seven-segment display or dot matrix display are what you're referring to I believe.

Here's an example on Sparkfun with the relevant datasheet. Found this AVR example/tutorial in the "Documents" section of the product page. Isn't Sparkfun great?

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Given this 14-segment display, and its datasheet, let's figure out how to use it.

Let's take the example of showing a 7 in the first (leftmost) segment cluster.

To write a 7, we need to assert segments 1A, 1B, and 1C, as seen in the datasheet. These segments all map to pin 35 of the device, but on different COM lines. With such a cryptic datasheet, my best guess would be that these COM lines map to pins 17 through 20, given the table in the datasheet.

Thus, to assert each segment above, you would hold pin 35 high while quickly cycling between asserting COM1, COM2, and COM3 via pins 18, 19, and 20 respectively. The three segments would all appear lit simultaneously, creating a 7.

This being said, well-written code simply will use some sort of map for any given character to be displayed. The challenge is to make this map work for ANY of the segment clusters.

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I still recommend a dot-matrix display like the first one I linked to; there are certainly more lightweight designs out there. The benefit to using a dot-matrix display is that most have a data register built-in, so you can just pass it 8-bit ASCII characters, no funky implementation issues like in the 14-seg example above.