What makes the text scrolling on the silicon chips? The heart of scrolling display is dot matrix led display unit. One or more dot matrix display is multiplexed to form a display panel. A set of hex values define a character which are send to the dot matrix display. To minimize the pin usage led matrix is formed having certain number of rows and columns. The concept is to refresh each display unit with a frequency at which human eye cannot perceive changes.
Dot matrix displays are available with common cathodes as the columns and common anodes as the columns. The leds may be single color, bi-color or RGB.
Here we are interfacing 5x7 dot matrix display with common cathode columns and having bright red leds. The cathodes are shorted along the column and anodes shorted along the row.5x7 dot matrix display needs 7 Row drivers and 5 Column drivers.
Block Diagram
Due to the limitation of source and sink current of AT89C51 we have used SK100 pnp transistors along with 200-ohm current limiting resistors as row drivers. We have used ULN2003A to increase the sinking capacity of the AT89C51 as column drivers.
Ciruit Diagram
The transistors are turned on by the TTL voltages applied by the Port 2 of AT89C51 to their bases through 1-kilo ohm resistors. To make any led segment glow just make the respective row and column bits on.We send the hex value of character to be displayed in first column to Port2 and send columns selection byte to Port1.Then we send another hex value to row and select the respective column. This process is done very fast and it seems that the whole character is displayed at the same time.
