Step 41Build the controller: RS-232

To get the truly amazing animations, we need to connect the LED cube to a PC. The PC can do floating point calculations that would have the AVR working in slow motion.

The ATmega has a built in serial interface called USART (Universal Synchronous and Asynchronous serial Receiver and Transmitter).

The USART communicates using TTL levels (0/5 volts). The computer talks serial using RS232. The signal levels for RS232 are anywhere from +/- 5 volts to +/- 15 volts.

To convert the serial signals from the micro controller to something the RS232 port on a PC can understand, and vice versa, we use the Maxim MAX232 IC. Actually, the chip we are using isn't from Maxim, but it is a pin-compatible clone.

There are some 100nF ceramic capacitors surrounding the MAX232. The MAX232 uses internal charge-pumps and the external capacitors to step up the voltage to appropriate RS232 levels. One of the 100nF capacitors is a filter capacitor.

The RS232 connector is at a 90 degree angle for easy access when the latch array board is mounted on top of the AVR board. We used a 4 pin connector and cut one of the pins out to make a polarized connector. This removes any confusion as to which way to plug in the RS232 cable.

In the second picture you can see two yellow wires running from the ATmega to the MAX232. These are the TTL level TX and RX lines.

1) Connect the GND and VCC pins using solder trace or wire. Place a 100nF capacitor close to the GND and VCC pins.

2) Solder in place the rest of the 100nF capacitors. You can solder these with solder traces, so its best to do this before you connect the tx/rx wires.

3) Solder in place a 4 pin 0.1" header with one pin removed. Connect the pin next to the one that was removed to GND.

4) Connect the tx/rx input lines to the micro controller, and the tx/rx output lines to the 4 pin header.

The wires going to the 4 pin header are crossed because the first serial cable we used had this pinout.