The hardware
The hardware is a circuit board sized 1.05" x 6.3" (2.7 cm x 16.0 cm) that can be populated in two different ways depending if it is the first in the chain or not.
Chaining
The displays can be chained together to form a string with up to 16 displays. Each board have two 4-pole connectors in the left upper and lower corners. This connector carries the power and the communication channels between the displays. Just connect the top connector of display #1 to the lower connector of display #2 and so on in a chain. Only the first display needs the USB connector and the USB-to-Serial chip, the rest of the displays should have a crystal in place of the USB-chip.
Power
The display is USB powered and draws about 100 mA from the USB bus so it's possible to connect five units in a chain without exceeding the limit of 500 mA per USB socket. Most PC's can deliver much higher currents though, many 2.5" external hard disks requires up to 700 mA and they work just fine.
There are two ways of overcoming the limitation of five displays. Install another USB-connector on the sixth display and don't jumper the plus-connector between display five and six. The new cable will then feed power to display five and higher. The other options to break the plus-jumper between the first and second display and then connect an external 5 volt power supply with enough power to supply the rest of the up to fifteen displays - like 1.5 Amperes.
The circuit
The circuit is basically separated into three parts.
- The USB-to-serial converter
- The microcontroller
- The displays with anode drivers
Almost all of the parts are very common and hole-mounted for easy assembly, there's really no rocket science here. The only exception is the FT232R chip. It's both surface mounted and is most probably not available from your local mom&pop electronics store. Fortunately they are both easy and rather cheap to get hold of by mail order.
The full schematics, in both PDF and Eagle-formats, as well as pre made Gerbers are available in the download section.
USB-to-serial
This is handled by a FTDI FT232R chip housed which is surface mounted (SOIC28) device. These chips can be a bit hard to solder correctly, but it's not impossible if you've done any soldering before. I plan to sell PCB's with the chip pre-soldered as a service for those who are hesitant to try it themselves.
The FT232R handles both the programming of the microcontroller using its synchronous bitbang mode as well as supplying a stable 6 MHz system clock for the microcontroller removing the need for an external crystal or tuning of the microcontroller internal RC oscillator.
When the USB cable is plugged into a PC it will automatically be detected as a new serial port that can be communicated with in an easy way, I've used Hyperterm to test the hardware with.
Microcontroller
The Atmel ATtiny2313 microcontroller is the heart of the circuit. It listens for commands on its usart and checks the destination of the incoming packets to see if it should act on it or just pass it on to the next display in the chain. It continuously updates the multiplexed signals going to the displays in order to display the requested digits and characters on it in a flicker free way,
Since the display is Charlieplexed only nine signals is going to it, with normal multiplexing sixteen (or with extra hardware - eleven) signals would have been required.
The boards that are not first in the chain doesn't have the FT232R chip installed which means that the system clock source on those boards is not available so a crystal and two capacitors must be installed on them. Also a 6-pole ISP header should be installed in order to allow (re)programming of the firmware.
Displays & Drivers
The displays are a dual-digit CA (Common Anode) model from Kingbright. This is a quite common model of display and it should be easy to find replacements for it if necessary.
As mentioned earlier they are Charlieplexed to reduce the number of required io-lines. The anodes are driven by standard NPN small signal transistors like BC547 or 2N2222. The cathodes are current limited by nine 82 ohm resistors..