From the datasheet, the HDSP-253x looks like the LED dot-matrix equivalent of an HD44780-controlled LCD display:
- data is sent on a 8-bit bus;
- ability to decode 128 ASCII characters, which are permanently stored in ROM;
- allows definition of 16 user-programmable symbols that are stored in the on-board RAM.
I wired the display to an ATmega328 (internal oscillator at 8MHz) through a 595 shift register, using the schematic in this post from nycresistor blog.
They use the HDSP-2111 display, which is very similar to the HDSP-2534 I have. They also provide demo source code for writing data to the display. Needless to say everything works as described. My contribution to the code is a function that sets the display brightness at one of the predefined 7 levels.
As shown in the photo above, all components fit on a 5cm x 7cm prototype board. The RTC is DS1307 with backup battery. The hours and the minutes can be set from the two buttons.
The easiest solution to "finish" the clock was to mount it on a phone dock charger I had around (which can be also found on ebay for $3 or so). Plugging the clock board into the dock's USB connector requires a "breakout" miniB (like this one sold by adafruit). I improvised one by cannibalizing a Geiger PCB, as shown in the photo below.
The "breakout" miniB connector is attached to the back of the clock board by soldering a few header pins in matching holes of the 2 boards. The assembly is mechanically solid.
The current draw at the lowest brightness (which is still very visible, as shown in the photo above) is between 20 and 40mA. At the highest brightness, the clock draws about 200mA.
Future improvements may include a Bluetooth module to display messages sent from a smart phone. And of course, designing a proper PCB :)
Update (Jul 20, 2014): A kit for this clock is now available.