From HDLBits


You are provided with a BCD (binary-coded decimal) one-digit adder named bcd_fadd that adds two BCD digits and carry-in, and produces a sum and carry-out.

module bcd_fadd {
    input [3:0] a,
    input [3:0] b,
    input     cin,
    output   cout,
    output [3:0] sum );

Instantiate 4 copies of bcd_fadd to create a 4-digit BCD ripple-carry adder. Your adder should add two 4-digit BCD numbers (packed into 16-bit vectors) and a carry-in to produce a 4-digit sum and carry out.

Module Declaration

module top_module( 
    input [15:0] a, b,
    input cin,
    output cout,
    output [15:0] sum );

  • The BCD representation for the 5-digit decimal number 12345 is 20'h12345. This is not the same as 14'd12345 (which is 14'h3039).
  • The circuit is structured just like a binary ripple-carry adder, except the adders are base-10 rather than base-2.

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