1-bit BCD Adder

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Decimal adders are used often to add two 4-bit representations of a decimal number. These decimal numbers are in their BCD or 8421 code. These adders add two coded numbers and deliver the sum in the same coded form. Recollect that BCD code has only 10 valid combinations out of the sixteen possibilities of a four-bit code. Numbers from 0 to 9 have the same representation in binary as well as BCD. Beyond 9, the representation changes BCD 10 (ten) is represented as 0001 0000, BCD 15 is represented as 0001 0101, BCD 79 is represented as 0111 1001 and so on. As seen, there are two 4-bit binary adders. The first adder adds the two BCD digits. If the result is more than 1001 than is ‘9’, then 0110 is added to the binary sum to come at the BCD value. This is done by checking if the second and third bits of the binary is high and if so 0110 id added to the sum as shown in the circuit.

Circuit info


Created on: 25 Feb 2013
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