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What is a BCD to 7-Segment Display Decoder

Written by Harpreet

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Seven segment displays can be seen everywhere today in devices such as digital clocks, elevators, weight measuring machines, microcontroller kits etc. These consist of seven LED's stocked together in an arrangement of the number eight's and are used for the display of decimal numbers, alphabets, special characters etc.


A BCD to seven segment display decoder is a digital combinational circuit that is used in conjunction with the seven segment display. It enables the seven segments to display 4-bit BCD numbers in their decimal form. The design of such a decoder depends first of all upon drawing a truth table that relates the output state of the different segments to the various 4 bit BCD numbers. This can be done by determining which segment of the display glows for which numbers and then assigning the segment a value of logic 1 for those numbers. E.g. All seven segments of the display glow are thus are assigned a logic 1 when the input is 1000 (8 in decimal) while only the two right most segments of the display glow and are assigned a logic 1 when the input is 0001 (1 in decimal).

Once the truth table is determined, the rest of the decoder can easily be designed by drawing from the truth table, a separate Karnaugh or K-Map for each of the seven outputs and then finding the minimum Sum of Product terms that relates each of these seven outputs to the four input bits. The seven logic circuits thus obtained from the maps are combined together to form the BCD to seven segment display decoder.

A better way of implementing this circuit though is not to do the optimum grouping 1's of in the Karnaugh Maps in order reduce the logical function but to do the grouping in a way that there are as many common terms between the 7 Maps as possible. This is because such an implementation reduces the hardware and thus the cost of the decoder as it is possible to use the same logic circuitry for each of these common terms.

In a similar way, if a seven segment display is to be used to display ASCII characters then it requires an ASCII to seven segment display decoder along with it. Such a decoder can be designed in a similar way as a BCD to seven segment decoder though it is much more complex in terms of digital hardware.

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