In part II we discussed the conversion of binary numbers into decimal digits. It

Question

In part II we discussed the conversion of binary numbers into decimal digits. It is sometimes useful to build circuits that use this method of representing decimal numbers, in which each decimal digit is represented using four bits. This scheme is known as the binary coded decimal (BCD) representation. As an example, the decimal value 59 is encoded in BCD form as 0101 1001. You are to design a circuit that adds two BCD digits. The inputs to the circuit are BCD numbers A and B, plus a carry-in, cin. The output should be a two-digit BCD sum S1S0. Note that the largest sum that needs to be handled by this circuit is S1S0 = 9 + 9 + 1 = 19. Perform the steps given below.

1. Create a new Quartus II project for your BCD adder. You should use the four-bit adder circuit from part III to produce a four-bit sum and carry-out for the operation A + B. A circuit that converts this five-bit result, which has the maximum value 19, into two BCD digits S1S0 can be designed in a very similar way as the binary-to-decimal converter from part II. Write your Verilog code using simple assign statements to specify the required logic functions - do not use other types of Verilog statements such as if-else or case statements for this part of the exercise.

2. Use switches SW7-4 and SW3-0 for the inputs A and B, respectively, and use SW8 for the carry-in. Connect the SW switches to their corresponding red lights LEDR, and connect the four-bit sum and carryout produced by the operation A + B to the green lights LEDG. Display the BCD values of A and B on the 7-segment displays HEX6 and HEX4, and display the result S1S0 on HEX1 and HEX0.

3. Since your circuit handles only BCD digits, check for the cases when the input A or B is greater than nine. If this occurs, indicate an error by turning on the green light LEDG8.

PLEASE EXPLAIN EACH STEP AND USE SIMPLE CODES ONLY. THANKS :)

Explanation / Answer

module bcdadd(a,b,cin,s,cout); //Main module of one digit BCD adder

input [3:0]a;

input [3:0]b;

input cin;

output[3:0]s;

output cout;

wire [3:0]w;

wire y,c0,c1,c2,c3,c4,c5;

fulladd m1(a[0],b[0],cin,w[0],c0);

fulladd m2(a[1],b[1],c0,w[1],c1);

fulladd m3(a[2],b[2],c1,w[2],c2);

fulladd m4(a[3],b[3],c2,w[3],c3);

assign y=c3|(w[3]&(w[2]|w[1]));

halfadd m5(w[1],y,s[1],c4);

fulladd m6(w[2],y,c4,s[2],c5);

halfadd m7(w[3],c5,s[3],cout);

assign s[0]=w[0];

endmodule

module fulladd(a,b,cin,s,cout); //submodule for fulladder

input a,b;

input cin;

output s;

output cout;

wire w1,w2,w3;

halfadd g1(a,b,w1,w2);

halfadd g2(w1,cin,s,w3);

assign cout=w3|w2;

endmodule

module halfadd(a,b,s,cout); //submodule for halfadder

input a,b;

output s,cout;

assign s=a^b;

assign cout=a&b;

endmodule

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