LED & Multiplexed Display Interfacing with Microprocessor

LED and Multiplexed Display Interfacing with Microprocessor – Display devices are used by the microprocessor-based systems to provide the information from the microprocessor to the external world. Most of the microprocessor-based applications display the letters of the alphabet and numbers to the users. This information can be displayed using different display devices like CRT, LED, LCD, etc. A CRT display is used to display a large amount of information. For a small amount of data display, LED and LCD can be used.

LED Display Interfacing

LED (Light Emitting Diode) displays are commonly available in 7-segment format. A seven-segment display, as its name indicates, is composed of seven elements. This type of display is generally used as a numerical indicator and consists of seven LEDs, as shown in the figure below.

The seven segments of the display are labelled from a to g, and the dot is labelled as h. By lighting up the particular segments, a number or alphabet is displayed. The seven-segment displays are of two types:

1. Common Anode Type
2. Common Cathode Type

In a common anode, all anodes of LEDs are connected together, whereas in a common cathode, all cathodes are connected together. An interfacing circuit to drive a single, seven-segment, common anode LED display is shown in the figure below.

To interface the LED display with the microprocessor, an additional IC (7447-BCD-to-7-segment decoder) is required. The BCD to 7-segment decoder IC 7447 provides the signals on the segments of the LED display. The kind of arrangement to display works properly for just one or two digits. When there are more digits, the power requirements may increase, resulting in a decrease in the LED glow. The another problem which will be there in that each display digit requires a separate BCD to 7-segment decode IC 7447.

Multiplexed Display Interfacing

To solve the problems that occur with LED displays, multiplexed displays are used. Multiplexed displays are electronic displays where the entire display is not driven at one time. The figure below shows the four 7-segment displays connected in a multiplexed method.

The above figure shows that the four 7-segment displays with one BCD to 7-segment decoder. The microprocessor can select one 7-segment display through the transistor circuit. IT means, in a multiplexed display, the segment information is sent for all digits on the common lines, but only one display digit is turned on at a time. In a multiplexed display, the segment current is held between 40mA and 60mA so that they will appear as bright as they would if not multiplexed.