Introduction

Description
Speed up arithmetic operations using a Carry Look-Ahead Adder (CLA) in Verilog. Learn the concept, code, and simulation, ideal for VLSI learners and FPGA enthusiasts.

Introduction

When speed is crucial in digital design, Ripple Carry Adders fall short due to sequential carry delays. The Carry Look-Ahead Adder (CLA) offers a smarter solution by calculating carry bits in parallel. This blog guides you through the CLA concept, Verilog implementation, and simulation steps—ideal for learners working with the MAX10 FLK FPGA board.

What is a Carry Look-Ahead Adder?

A CLA improves speed by using Generate (G) and Propagate (P) logic to calculate all carry signals in parallel. Unlike ripple carry adders, where each bit must wait for the previous carry, CLA handles carry prediction ahead of time—minimising delay and enhancing performance.

Generate (G): A carry is generated at this bit position.
Propagate (P): A carry input is passed to the next bit.

Verilog Code: CLA Using Dataflow Modeling

Design Code

// Pantech e-learning

// Carry Look Ahead Adder – Dataflow Modeling

module cla_4bit(

  input [3:0] a, b,

  input cin,

  output [3:0] sum,

  output cout

);

  wire [3:0] p, g;

  wire c1, c2, c3;

  assign p = a ^ b;

  assign g = a & b;

  assign sum[0] = p[0] ^ cin;

  assign c1 = g[0] | (p[0] & cin);

  assign sum[1] = p[1] ^ c1;

  assign c2 = g[1] | (p[1] & g[0]) | (p[1] & p[0] & cin);

  assign sum[2] = p[2] ^ c2;

  assign c3 = g[2] | (p[2] & g[1]) | (p[2] & p[1] & g[0]) | (p[2] & p[1] & p[0] & cin);

  assign sum[3] = p[3] ^ c3;

  assign cout = g[3] | (p[3] & g[2]) | (p[3] & p[2] & g[1]) |

                (p[3] & p[2] & p[1] & g[0]) |

                (p[3] & p[2] & p[1] & p[0] & cin);

endmodule

Testbench

// Pantech e-learning

// Testbench for CLA

module cla_4bit_tb;

  reg [3:0] a, b;

  reg cin;

  wire [3:0] sum;

  wire cout;

  cla_4bit uut(

    .a(a), .b(b), .cin(cin),

    .sum(sum), .cout(cout)

  );

  initial begin

    $dumpfile(“dump.vcd”);

    $dumpvars(0, cla_4bit_tb);

    a = 4’b0000; b = 4’b0000; cin = 0; #10;

    a = 4’b0011; b = 4’b0001; cin = 0; #10;

    a = 4’b0101; b = 4’b0101; cin = 0; #10;

    a = 4’b1111; b = 4’b0001; cin = 0; #10;

    a = 4’b1111; b = 4’b1111; cin = 1; #10;

    $finish;

  end

endmodule

Simulation Output

Observe the waveform using GTKWave (VCD file) and verify correct sum and carry outputs for each input combination. This confirms the CLA’s parallel carry generation.

Figure: Carry Look Ahead Adder simulation output

FAQs

Q1: Why is a CLA faster than a ripple carry adder?
Because it computes all carry outputs in parallel, removing dependency on previous stages.

Q2: What is the role of ‘Generate’ and ‘Propagate’?
Generate means a carry is created at that bit. Propagate means the input carry is passed forward.

Q3: Where is CLA used?
CLA is used in high-performance ALUs, CPUs, and processors requiring fast arithmetic.

Q4: What is the limitation of CLA?
As bit-width increases, logic becomes more complex and harder to scale.

Q5: How many full adders are in a CLA?
It doesn’t use full adders directly; it uses logic gates based on G and P terms.

Conclusion

The Carry Look-Ahead Adder offers a brilliant trade-off between speed and complexity. It is ideal for high-speed VLSI design and digital systems where speed is paramount. Whether you’re designing arithmetic units for an ALU or experimenting on your MAX10 FLK FPGA, CLA is a must-know concept in digital logic.

Call to Action

Try implementing this 4-bit Carry Look-Ahead Adder on a MAX10 FLK FPGA board and experience real-time high-speed addition without ripple delay.
Want more logic circuit simulations? Explore our full Verilog series covering all combinational and sequential circuits—perfect for beginners and aspiring FPGA developers!