Step 37: Build the controller: Address selector The 74HC138 is responsible for toggling the clock pin on the 74HC574 latch ICs. We call this an address selector because it selects which one of the 8 bytes in the latch array we want to write data to. The three blue wires running from the connector to the 74HC138 is the 3 bit binary input used to select which of the 8 outputs is pulled low. From each of the outputs on the 74HC138, there is a wire (white) running to the clock pin on the corresponding 74HC574 latch IC. Start by soldering the GND and VCC connections. If you use the solder trace method to run GND/VCC lines you want to do this before you solder any other wires in place. A 100nF ceramic filtering capacitor is placed close to the VCC and GND pins of the 74HC138. Then connect the address lines and the 8 clock lines. If you look carefully at the connector, you can see two pins that are not used. These will be used for a button and debug LED later.
Image Notes 1. 3 bit address select bus
Step 38: Build the controller: AVR board Braaaaainzz!!! This board is the brain of the LED cube. The main component is an Atmel AVR ATmega32. This is an 8 bit microcontroller with 32 KB of program memory and 2 KB RAM. The ATmega32 has 32 GPIO (General Purpose IO) pins. Two of these will be used for serial communication (TX+RX). Three IO pins are used for ISP (In-circuit Serial Programming). This leaves us with 27 GPIO to drive the LED cube, buttons and status LEDs. A group of 8 GPIO (8 bits, one byte) is called a port. The ATmega32 has 4 ports. PORTA, PORTB, PORTC and PORTD. On PORTC and PORTD some of the pins are used for TX/RX and ISP. On PORTA and PORTB, all the pins are available. We use these ports to drive the data bus of the latch array and layer select transistor array. PORTA is connected to the data bus on the latch array. Each pin on PORTC is connected to a pair of transistors that drive a ground layer. The address selector on the latch array (74HC138) is connected to bit 0-2 on PORTB. Output enable (OE) is connected to PORTB bit 3. In the first image, you see the AVR board right-side-up. The large 40 pin PDIP (Plastic Dual Inline Package) chip in the center of the board is the ATmega32, the brainz! Just to the left of the ATmega, you see the crystal
http://www.instructables.com/id/20-Unbelievable-Arduino-Projects/