Arduino Teaches Old Coder New Tricks
Now that I had invested a lot of time into learning the Arduino system and the Atmel microcontrollers, I wanted to take the next logical step: move a design from the breadboard to a printed circuit board. Some interesting projects exist in this area, such as Fritzing, which is designed to facilitate doing exactly that. It's a clever project and you should check it out, but I took a different path—using the gEDA open-source Linux software suite for printed circuit development.
I looked at my inventory of parts and started thinking of what I could create that wasn't already readily available. I settled upon the LCD display. The displays being used in Arduino projects were interfaced with a lot of I/O pins and code space, neither of which are in great supply on the Atmel chips. I felt that if I could create a same-size daughter board that I could attach onto the back of the display and put the smarts into the board that would communicate with the LCD display via an ASCII serial interface, I would have something useful that didn't exist in the marketplace in an affordable form. This is commonly called a serial LCD.
Being somewhat of an old-timer, I spent a lot of time using and coding for the DEC VT100 display terminals upon which the ECMA-48/ANSI X3.64 standards are based. I felt that if I coded the daughter board to turn an LCD display into a tiny, affordable DEC-VT100, I would have something reasonably unique and useful. Serial-driven LCD displays do exist, but they typically have proprietary protocols, and some are rather expensive. As far as I have been able to determine, there exists no open-source (software and hardware) serial LCD display with VT100 protocol. I found my project!
Gathering the Parts
I selected parts for the VT100-LCD project, such that the parts would be as affordable as possible. In fact, I purchased all the parts from two sources, eBay and Digi-Key, based on cost. Table 1 shows the required materials to build one vt100lcd. Costs are shown on a per-item basis; however, I purchased most of these items in quantities of five or more.
|1602 HD44780 LCD||1||eBay seller (China)||$2.95|
|Atmel ATtiny84||1||Digi-Key ATTINY84-20PU-ND||$3.01|
|Switch, tactile FSM4JH||1||Digi-Key 450-1650-ND||$0.80|
|Socket, IC, 14-pin||1||eBay seller (USA)||$0.15|
|Header, 1X20, Female, 2.54mm||1||eBay seller (China)||$0.39|
|Header, 1X40, Male, 2.54mm||1||eBay seller (China)||$0.20|
|Resistor, 330 ohm, 1/4W||1||eBay seller (Thailand)||$0.02|
|Resistor, 10k ohm, 1/4W||1||eBay seller (Hong Kong)||$0.02|
|Pot, trim, 5k, RM-065||1||eBay seller (USA)||$0.30|
|Capacitor, .1uf, ceramic disc, 50V||1||eBay seller (Hong Kong)||$0.05|
|Transistor, 2N3906||1||eBay seller (Thailand)||$.01|
|Diode, 1N4148||1||eBay seller (Thailand)||$.01|
|Commercial PCB||1||Panel Aggregator||$7.43|
|Capacitor, 22pf, ceramic disc, 50V||2||eBay seller (USA)||$0.40|
|Crystal, 20MHz, ATS200-E||1||Digi-Key CTX1105-ND||$0.64|
To design the circuitry for the VT100-LCD, I chose gschem of the gEDA Project by http://geda-project.org. This suite includes not only the schematic design program but also a PCB layout program, as well as various helper programs. A number of schematic/PCB design software programs exist, but I'm focusing on the open-source software of the gEDA Project by geda-project.org here. Other open-source projects that run on Linux, include KiCad, as well as several commercial products, the most popular of which is Eagle PCB by CadSoft, which runs pretty well under WINE.
gschem is fairly straightforward, and many functions are intuitive, but a few, useful but arcane commands necessitate printing out a cheat sheet (hey, I'm getting older and I can't memorize all of those keystrokes). Yes, although gschem is a GUI program, useful keyboard shortcuts appear nowhere in the GUI's menus. This is especially true of the PCB layout program that I discuss later.
The process consists of inserting electronic component symbols into the schematic drawing either from the built-in library or from your private library and then connecting the pins by drawing traces. I highly recommend reviewing the gEDA Project's on-line documentation before starting your own schematic. There are a few tutorials on the Web about using the gEDA suite, and Stuart Brorson wrote a tutorial article in the November 2005 issue of Linux Journal (see Resources).
I created two versions of my VT100-LCD project: one using the eight-pin ATtiny85 microcontroller and another using the 14-pin ATtiny84 microcontroller. The schematic for the ATtiny84 microcontroller version is shown in Figure 2.
Figure 2. Schematic for VT100LCD e/w ATtiny84
Edward Comer is retired from the telecommunications industry, having worked for the real AT&T, BellSouth and Numerex Corp during a 30-year career.
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