The purpose of this project is to display a phrase of expandable length, as a scrolling massage across a 24x6 LED matrix. To make this possible there are numerous hardware and software requirements.
Hardware Requirement Description Edit
The hardware system must be capable of multiplexing a 24-bit output serial signal form the HCS12 microprocessor. In this application multiplexing is the combination of six, 24-bit serial signals, which are patterns for each row of the matrix. To receive the serial signal down one line, a 24-bit shift register must be assembled with a clock controlled by the HCS12. Since a 24-bit shift register is not available, three 8-bit 74HC595 shift registers can be tied together to create a 24-bit register output for an active row of LEDs. The active row needs to be selected by the HCS12 to decode the bit patterns. To control a row, a 74LS138 3 to 8 decoder is used to decode a 3-bit output pattern from the HCS12. The aim of the 24-bit shift register and 3-bit decoder is to give the HCS12 a communication system to interface with a 24x6 LED matrix. To interface this communication system with the LED matrix a few more hardware components are needed for functional use. To start the LED matrix configuration is 24-anode by 6-cathode, meaning 24 resistors and 6 transistors. Each of the resistors must be capable of drawing an appropriate amount of current for 6 active LEDs. Each of the transistors must be capable of passing all the current from up to 24 LEDs for a row. The transistors base pins will be controlled by the 74LS138 decoder which has eight controllable outputs. One issue is that the 74LS138 has active low outputs which will not activate the transistors to allow current, becoming like an open switch. To fix this issue a 74LS04 inverter can be used to invert the LOW 0 volt signal to a 5 volt HIGH signal. With this extra hardware the HCS12 has full communication with the 24x6 LED matrix.
The required hardware components needed for this project are as listed below:
- 3x 74HC595 8-bit shift registers
- 1x 74LS138 3 to 8 decoder
- 1x 74LS04 Hex Inverter
- 6x ZTX458 Transistors
- 1x Axman with HCS12 processing system
- 24x 82ohm-1/4w-1%
- 6x 1Kohm-1/4-1%
- 1x 24x6 LED Matrix Display