HEIRARCHICAL DESIGN FOR A FOUR-BIT ADDER USING SCHEAMTIC AND ABEL

Here we would give an example of how to create a hierarchical design using just the schematic. Before going through this example you should familiarize yourself with the section on how to create the block symbols in the schematic editor.

PROBLEM STATEMENT

Our goal here is to create a four-bit adder using full adder block as a new symbol. The full adder block is basically a schematic for a full adder with inputs A, B, Cin and outputs S and Cout.

PROBLEM DESIGN

To implement the four-bit adder we will follow these steps :

• Create the schematic for the full adder
• Create a block symbol for the full adder we have created above
• Create the top-level ABEL source that refrences the lower-level schematic block

CREATING THE SCHEMATIC FOR THE FULL ADDER

Following the steps indicated in Tutorial 1 : Creating a Schematic we create the following schematic for the full adder with inputs A, B, Cin and outputs S and Cout :

CREATING A BLOCK SYMBOL FOR THE FULL ADDER WE HAVE CREATED ABOVE

Following the steps indicated in Tutorial 1 : Creating a block symbol  we create the following full adder block symbol:

CREATING THE TOP-LEVEL ABEL SOURCE THAT REFRENCES THE LOWER-LEVEL SCHEMATIC

Following the steps indicated in Tutorial 2 : Creating an ABEL source we create the following ABEL top-level source that implements the 4-bit adder by refrencing the fulladder schematic block we have created above:

MODULE bitadd

        interface(A0,B0,A1,B1,A2,B2,A3,B3,Cin -> S0,S1,S2,S3,Cout);
TITLE 'four bit adder'

"inputs
A0,B0,A1,B1,A2,B2,A3,B3,Cin     pin;

"outputs
S0,S1,S2,S3,Cout                pin istype 'com';

fuladder interface (A,B,Cin -> Cout, Sum);
ful0..ful3 functional_block fuladder;

EQUATIONS
ful0.A = A0;
ful0.B = B0;
ful0.Cin = Cin;
S0 = ful0.Sum;
ful1.A = A1;
ful1.B = B1;
ful1.Cin = ful0.Cout;
S1 = ful1.Sum;
ful2.A = A2;
ful2.B = B2;
ful2.Cin = ful1.Cout;
S2 = ful2.Sum;
ful3.A = A3;
ful3.B = B3;
ful3.Cin = ful2.Cout;
Cout = ful3.Cout;



END

In the above code, the functional_block statement creates four instances of the fuladder module namely ful0..ful3. The inputs for these instances are assigned values in the top-level module by statements like ful0.A = A0, ful0.B = B0..etc. Finally the ouputs of the top-level ABEL module are obtained from the lower-level schematic by using statements like S0 = ful0.sum, S1 = ful1.sum..etc.