- BCD to Seven Segment Decoder Definition: A BCD to seven-segment decoder is defined as a circuit that converts BCD input into signals to drive a seven-segment display.
- Types of Seven Segment Displays: There are two types: common cathode and common anode displays, each requiring different driving signals.
- Common Cathode Display: In a common cathode display, all cathodes are tied together and grounded, so segments light up with a high input.
- Common Anode Display: In a common anode display, all anodes are tied together and powered, so segments light up with a low input.
- Truth Table: The truth table for the decoder indicates which segments to drive high or low to display the correct digit on the seven-segment display.
BCD (Binary Coded Decimal) is defined as an encoding scheme that represents each decimal number with a 4-bit binary pattern. A Seven segment displays is made up of seven segments, usually Light Emitting Diodes (LEDs) or Liquid Crystal Displays (LCDs), arranged in a specific pattern.
For the display to work, these segments must be driven by specific logic levels. There are two types of seven-segment displays: common cathode and common anode.
A common cathode display (Figure 2a) has all the cathode terminals of its LED segments tied together (green line). Further, this is grounded and hence is considered to be at logic 0 state. This means that in order to light up an LED, one needs to drive it high. On the other hand, a common anode display shown by Figure 2b has all its anode terminals connected together which is further driven high by connecting it to a positive supply voltage (green line). Hence for this kind of display to work, one has to drive low on the cathode terminals of the individual LED segments.
A BCD to seven-segment decoder is a circuit that converts BCD input into signals for a seven-segment display. It has four input lines (A, B, C, and D) and seven output lines (a, b, c, d, e, f, and g). For a common cathode display, the truth table for the decoder is shown in Table I.
Table I Truth table for common cathode type BCD to seven segment decoder
This table indicates the segments which are to be driven high to obtain certain decimal digit at the output of the seven segment display. However, it is to be noted that in the case of common anode type, the only change will be to interchange ones and zeros on the table. This means that from the truth table so obtained one can get to know where low has to be driven so as to obtain the required digit at the output.
