Nuts and Volts - September 2013

Everywhere you look, there are projects, tutorials, books, and accessories for every conceivable microprocessor — Arduino (and its clones); PICAXE; BASIC Stamp; Propeller — but almost nothing for the TI MSP430 Launchpad. Since it’s priced at under $5 for the development board, two uP ICs, and a USB cable, that is just plain sad. After a crash-course in C language programming, I was ready to dive in to my first project with the Launchpad.

The Bits and Pieces

The Keypad

The first thing you see is the keypad. I used a Datavision 12075 keypad (an Electronic Goldmine bargain) that features a 4 x 4 matrix keypad, six indicator LEDS (of which only two are needed) with built-in current limiting resistors, and a cutout for a piezo speaker element. The keypads being shipped by Goldmine now come with a mating female connector for the 16-pin thin film cable, but for mine I had to improvise. I found that if you glue a piece of single-sided printed circuit board (PCB) copper blank to the back of the film with contact cement, the cable is a perfect fit for an old 5-1/4" floppy drive connector.

As an added bonus, by cutting tracks across the copper blank, you're able to isolate pads to which you could solder the piezo buzzer's wires (to be added later). Refer to Figure 1.

One final modification has to be made to the keypad.

The indicator LEDs share a common ground with the keyboard matrix. For our purposes, this creates havoc in the decoder IC. To solve this, simply cut the ground traces going to the LEDs by cutting a small square out of the plastic film with an Xacto™ knife. Next, wire a new ground connection from one of the LED cathodes to one of the pads that you created on the copper blank (see Figure 2).

Now, hot-glue the piezo element into the cutout provided in the keypad. The negative terminal of the piezo connects to the same pad on the copper blank as the LED’s ground wire. The other terminal is soldered to another one of the empty pads.

Keypad Decoder IC

The next component is the keyboard decoder IC. Since I had a few of the 74C922 decoder ICs in my junk box, it was an easy choice. It is, of course, an obsolete component, but they can still be found on eBay for less than $3 a piece. Yes, the microprocessor does have enough I/O lines to implement a software keypad decoder, but since the 74C922 features full key debounce and multiple key press elimination, using a separate decoder IC lightens the amount of software code necessary.

When a key is pressed on the keypad, this IC sets the DATA_AV (Data Available) line high, alerting the microprocessor that data is available. In response, the processor brings the OE (Output Enable) line low whereby (as per the truth table found in the datasheet) a four-bit BCD is placed on the Data Output lines ABCD.