Description
PART 1 Calculator utilizing Ripple Carry Adder (RCA)
PART 2 Calculator utilizing Carry Lookahead Adder (CLA)
PART 3 Speed and Area Comparison of RCA v/s CLA
Objectives
This FPGA application development assignment builds on your knowledge of datapath
component design to implement a calculator that adds two 4-bit. The numbers are to be
entered using the Basys3 board switches and the result of addition will be stored on a 5-
bit register controlled by a button. The number stored on the register is displayed on the
LEDs.
The design of the calculator will be done using two different adder designs, namely,
ripple carry adder (RCA) and carry look-ahead adder (CLA). The designs will be
followed by a small analysis exercise to understand the differences in complexity and
delay of the two architectures.
Figure 1. Adder, Register controlled by Store Button, and Output LEDs
Equipment
● PC or compatible
● Digilent’s Basys3 FPGA Evaluation Board
Software
● Xilinx Vivado Design Software Suite
Part 1. Ripple Carry Adder
The basic building block of a great variety of adders is the full adder. The full adder adds
two input bits (a and b) and a carry-in bit (c), and outputs a sum bit (s) and a carry-out
bit (co). The adder is defined by the following equations –
𝑐𝑜 = 𝑏𝑐 + 𝑎𝑐 + 𝑎𝑏
𝑠 = 𝑎 ⊕ 𝑏 ⊕ 𝑐
Multiple full adders can be cascaded to enable the addition of two N-bit numbers. A
ripple carry adder is an adder in which the carry-out of one adder is the carry-in of the
next adder. Figure 2 shows an adder design that adds two 4-bit numbers [a3…a0] &
[b3…b0] and carry-in, and returns a sum and carry-out.
Figure 2. Ripple Carry Adder – Block Diagram
Procedure
Step 1 (Design) – Design a 4-bit ripple carry adder using the data-flow style of
Verilog. The design should consist of three modules –
– Full-Adder
– 4-bit Adder
– Register Logic
As shown in Figure 1, it is required to load the result of the adder (Cout and Sum bits)
into a 5-bit register (the MSB bit must be loaded with Cout and the remaining four bits
with the Sum bits). Consider a 5-bit register as a set of 5 flip-flops, which loads the value
on its input to the output at the positive edge of the clock if it is enabled. The I/O
definitions for the modules is given in Figure 3, and this information should be used to
build the code.
Note that the full_adder and register_logic blocks needs to be instantiated inside the
RCA_4bits module, and hence, RCA_4bits module will be the top-level block for this
design. The loaded value on the register (“Q”) will be a concatenation of the output Sum
bits and the Cout bit. The value won’t be reflected on the LEDs unless the enable button
is pressed.
Figure 3. Module I/O definitions for RCA, full adder and register logic modules
Step 2 (Verification) – To verify the design, create a testbench that tests the input
combinations mentioned as part of Table 1.
A[3:0] B[3:0] Cin Sum[3:0] Cout
0001 0101 0 ? ?
0111 0111 0 ? ?
1000 0111 1 ? ?
1100 0100 0 ? ?
1000 1000 1 ? ?
1001 1010 1 ? ?
1111 1111 0 ? ?
Table 1. Testcases for Ripple Carry Adder Verification
Step 3 (Synthesis and Implementation) – After behavioral simulation, the
constraints file needs to be created according to the following mapping specifications –
– “clk” input port to internal clock W5
– “enable” input port to button U18
– “A[3:0]” input ports to switches SW[3:0]
– “B[3:0]” input ports to switches SW[7:4]
– “Cin” input port to switch SW8
– “Q[4:0]” output ports to LEDs LD[4:0]
Part 2. Carry Lookahead Adder
In this part of the lab, a faster adder called the Carry Look-Ahead Adder will be
designed. In the case of the ripple carry adder, the sum and the output carry cannot be
produced until the input carry of the previous stage is available. This leads to delay
which grows linearly with the number of bits. The carry look-ahead adder is a faster
adder: it reduces the propagation delay at the cost of using more gates. (In Part 3 of this
lab, the comparison between the delay and the complexity of the two adders needs to be
done!)
Here, the basic information about the CLA design is summarized. The lecture notes and
the textbook contain a more extensive description of the CLA.
Per-bit Propagate and Generate Variables
In addition to the sum and carry signals, the CLA introduces two additional values
which are defined per bit in the following manner –
– Propagate 𝑃𝑖 = 𝑎𝑖 ⊕ 𝑏𝑖
– Generate 𝐺𝑖 = 𝑎𝑖𝑏𝑖
The Sum and Carry outputs can now be defined in terms of the propagate and generate
values in the following manner –
– Sum 𝑆𝑖 = 𝑃𝑖 ⊕ 𝐶𝑖
– Carry 𝐶𝑖+1 = 𝐺𝑖 + 𝑃𝑖𝐶𝑖
Procedure
Step 1 (Design) – To start the design of the CLA adder, the first step is to derive the
equations for Sum bits (S3, S2, S1, S0) and C4, C3, C2, C1. It should be noted that C0 will be
the input carry and C4 will be the output carry for the adder.
For example, 𝐶2 = 𝑃1𝐶1 + 𝐺1 = 𝑃1
(𝑃0𝐶0 + 𝐺0
) + 𝐺1 = 𝑃1𝑃0𝐶0 + 𝑃1𝐺0 + 𝐺1
Once the equations are obtained, the adder needs be designed in Verilog using either
Dataflow modeling or Structural modeling or a combination of both. Similar to part 1, it
is required to load the result of the adder (Cout and Sum bits) into a 5-bit register so
that the value is no reflected on the LEDs unless the enable button is pressed. The
module I/O definitions is given in Figure 4, and this information should be used to build
the code.
Note → Only the expanded equations (equations depending only on C0) must be
implemented in Verilog, as that ensures that carry look ahead adder delay is minimized.
Figure 4. Module I/O definitions for CLA and register modules
Step 2 (Verification) – To verify the design, create a testbench that tests the input
combinations mentioned as part of Table 2.
A[3:0] B[3:0] Cin Sum[3:0] Cout
0000 0101 0 ? ?
0101 0111 0 ? ?
1000 0111 1 ? ?
1001 0100 0 ? ?
1000 1000 1 ? ?
1101 1010 1 ? ?
1110 1111 0 ? ?
Table 2. Testcases for Carry Lookahead Adder Verification
Step 3 (Synthesis and Implementation) – After behavioral simulation, the
constraints file needs to be created according to the following mapping specifications –
– “clk” input port to internal clock W5
– “enable” input port to button U18
– “A[3:0]” input ports to switches SW[3:0]
– “B[3:0]” input ports to switches SW[7:4]
– “Cin” input port to switch SW8
– “Q[4:0]” output ports to LEDs LD[4:0]
Part 3. Comparison
In parts 1 and 2, two different techniques to add binary numbers, namely the ripple
carry adder and the carry lookahead adder have been designed. This part is meant for a
comprehensive understanding of the two designs, and pros & cons of each technique.
The two techniques will be evaluated in terms of their speed (lesser delay for faster
addition) and area. The sample values given for delay (time taken by a gate to give a
stable output) and area (proportional to the number of transistors a gate uses) are given
in Table 3, and need to be used for the calculation of delay and area of the two
architectures.
Gate Delay
(ns) Area
XOR 3 6
AND 3 4
OR 2 4
Table 3. Sample Delay and Area Values for Various Gates
Procedure
Step 1 (View Gate Level Schematic) – The first step is to generate the gate-level
schematic for the design. Vivado’s capability to give the schematic from the design
description will be used for the same. After the simulation of the two designs is
complete, click on “Open Elaborated Design” under RTL Analysis in the left pane, as
shown in Figure 5. The gate-level schematic for the design will show up. For the Ripple
Carry Adder, the schematic will come up as four blocks for the four full adders. Doubleclicking on each block will expand the design.
Figure 5. Opening Elaborated Design in Vivado
Step 2 (Calculating Delay and Area) – For each of the adder designs, find the
delay of the critical path, i.e., the path with the maximum delay and the area using the
schematic obtained and the information provided in Table 3. All the calculations need to
be shown and it is required to draw a conclusion regarding the pros and cons of each of
the techniques.
