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 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
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
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.
September 2013 41
■ FIGURE 1. Homemade ribbon cable connector.