Using a PC parallel port to control external devices is a popular approach these days but I certainly couldn't afford to tie up a PC for the few weeks leading up to Christmas, just to flash a few LEDs! Not to mention the power bill! So, why not use a small microcontroller? They are cheap and easy to use and if the design doesn't work first time (when does it?), you simply re-program it.

Also, you can easily create something using a micro that is the equivalent of many discrete logic chips. In this case, the circuit is simple enough to lash up on VeroBoard" though it is much easier to use a printed circuit board. To do the star in discrete logic would be a nightmare!

Given a few inexpensive software tools, a microcontroller such as the Atmel AT89C2051 should be just as easy to use as a handful of 4000-series CMOS chips. In my experience, the micro is usually easier!

Another reason for using a micro is that micros are the future of electronics. While it is useful to know how to design with 4000-series logic, most new products require more than can be easily done in discrete logic. I believe there is actually a commercially available toaster that uses a micro! While some would say that this is an extreme example, it does indicate how far micros have entered our lives.

So what's in a 2051?

The Atmel AT89C2051 is a relatively recent derivative of the venerable 8051. It comes in a diminutive 20 pin skinny-DIP plastic package and contains 2k bytes of program memory, 128 bytes of RAM, 15 programmable I/O lines, on-chip oscillator, two 16 bit counter/timers, six interrupt sources and a full duplex serial port (UART).  This all sounds very much like a small 8051 until we add that the program memory is re-programmable Flash with 1000 erase/write cycles, the oscillator runs to 24MHz (double that of the original 8051), the I/O pins can sink 20mA for directly driving LEDs and two I/O pins are connected to an on-chip analogue comparator!

The appropriate combination of LEDs in a column is switched on for a short time (about 2ms in this case). This process is repeated for each column in turn taking 10ms for a full cycle.

Provided the multiplexing is done quickly enough, the persistence of the human eye "fills in the gaps" and we see any combination of LEDs on without any flicker. The minimum practical multiplexing frequency is about 100Hz which is the frequency used by the star.

The power supply uses the ubiquitous 7805 three terminal regulator with bypass capacitors C4 and C5. Diode D1 provides reverse polarity protection.

The maximum current drawn by the star is about 150mA with all LEDs on but less than about 50mA for most patterns. The maximum temperature rise of the 7805 when the star is run from a typical 9Vdc unregulated plug-pack is about 30 degrees which is quite acceptable. It gets warmer when run from a 12Vdc unregulated plug-pack and should be provided with a small heatsink.

In the spirit of Christmas, we are making the basic source code available for free! An extended version that uses the EEPROM for storage is available at minimal cost. The software was written in the C language using the low cost Dunfield Development Systems Micro/C compiler. There is nothing particularly "smart" or "tricky" about the software it was written to be easy to understand and to encourage use of small micros.  Consequently, there are no interrupt routines and no use of the counter/timers, the UART or the comparator though Micro/C can make use of these resources. The software is table driven. This means that the display patterns and sequences are determined by data stored in a table (an array of bytes). There is a simple interpreter that scans through the table to perform the specified operations. The defined byte values are listed in the following table.