Integrated Circuit Design-Lab1Question

March 2011

Integrated Circuit Design-Lab1Question

Swinburne University of Technology

Faculty of Engineering and Industrial Sciences

HET378 Integrated Circuit Design

Mini-Project 1

Hardware modeling

 Design of a Basic ALU

Laboratory

Part 1 : Hardware Modelling

Write a VHDL source code that implements the hardware of the sequencer shown below. Synthesize your source code and compare the results to the circuit.

(Your objective is to reproduce the same circuit . Simulation is not required)

                                   

Fig 1

Part 2

Design and Implementation of a 

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This lab design project which gives you more freedom to come up with your own solutions. The following write-up serves as a guideline to help you design the lab. However, if you find more efficient or more elegant ways to implement parts of the ALU, go ahead. Just make sure you justify your design and explain it clearly in the lab report write-up.

 

Pre-laboratory work:

Do the pre-lab4-bit ALU work for part 2

An Arithmetic and Logic Unit (ALU) is a combinational circuit that performs logic and arithmetic micro-operations on a pair of n-bit operands. The operations performed by an ALU are controlled by a set of function-select inputs. In this lab you will design a 4-bit ALU with 3 function-select inputs: Mode M, Select S1 and S0 inputs. The mode input M selects between a Logic (M=0) and Arithmetic (M=1) operation.

A block diagram of the ALU is given in Figure 2.

 

Figure 2: Block diagram of the 4-bit ALU.

The functions performed by the ALU are specified in Table I.

 

Table 1: Functions of ALU
M = 0 Logic
S1	S0	C0	FUNCTION	OPERATION (bit wise)
0	0	X	AiBi	AND
0	1	X	Ai + Bi	OR
1	0	X	AiÅ Bi	XOR
1	1	X	(AiÅ Bi)’	XNOR
M = 1 Arithmetic
S1	S0	C0	FUNCTION	OPERATION
0	0	0	A	Transfer A
0	0	1	A + 1	Increment A by 1
0	1	0	A + B	Add A and B
0	1	1	A + B + Carry in	sum of A and B and Cin
1	0	0	A + B'	A plus one's complement of B
1	0	1	A - B	Subtract B from A (i.e. B' + A + 1)
1	1	0	A' + B	B plus one's complement of A
1	1	1	B - A	B minus A (or A' + B + 1)

Note that the zero and Carry flag are cleared for logical operation and are affected by the arithmetic operations

This design project gives you freedom to come up with your own solutions. The following write-up serves as a guideline to help you design the ALU. However, if you find more efficient or more elegant ways to implement parts of the ALU, go ahead. Just make sure you justify your design and explain it clearly in the lab report write-up.
When doing arithmetic, you need to decide how to represent negative numbers. As is commonly done in digital systems, negative numbers are represented in  two’s complement. This has a number of advantages over the sign and magnitude representation such as easy addition or subtraction of mixed positive and negative numbers. Also, the number zero has a unique representation in two’s complement.

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Design strategies

There are different ways to design the ALU. One method consists of writing the truth table for the one-bit ALU. This table has 6 inputs (M, S1, S0, C₀, A_i and B_i) and two outputs F_i and C_i+1. This can be done but may be tedious when it has to be done by hand.

An alternative way is to split the ALU into two modules, one Logic and one Arithmetic module. Designing each module separately will be easier than designing a bit-slice as one unit. A possible block diagram of the ALU is shown in Figure 2. It consists of three modules: 2:1 MUX, a Logic unit and an Arithmetic unit.

Fig 2

A third option is design the ALU using modular STD_logic functions 

Laboratory work

(1)  Your task is to design and implement the 4-bit ALU using the Xilinx Foundation and Modelsim tools.  Follow the guidelines of the pre-lab in designing the 4-bit ALU. Develop two models for the ALU, and use Xilinx and Modelsim tools to simulate and synthesise  the design

(2)  Extend the design to a 16-bit ALU

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Hand-in

You have to hand in a lab report that contains the following:

1. Synthesis results for part 1
2. VHDL code (design and test bench), simulation and synthesis results for part 2 
3. Conclusion and discussion

The lab report is an important part of the laboratory. Write it carefully, be clear and well organized. It is the only way to convey that you did a great job in the lab.

Ismat Hijazin

March 2011