Introduction to ASIC flow and Verilog HDL

What is Verilog ?

IEEE industry standard Hardware Descriptive Language (HDL) – used to describe a digital system

Used in both hardware simulation and synthesis

HDL : A text programing language used for model a piece of hardware

More Terminology:

Register Transfer Level : A type of behavioral modeling, for the purpose of synthesis.

Synthesis : Translating HDL to a circuit and then optimizing the represented circuit.

RTL Synthesis : Translating the RTL model of hardware into an optimized technology specific gate level implementation.

What is Synthesis?

Synthesis Implementation (Basic)

Basic VLSI Design Flow:

Synthesis vs Simulation

Synthesis Flow

Basics of Combinational Digital Design

Using a NAND gate, how many ways can you come up with an Inverter?

Implementing an Inverter using 2:1 MUX

Equation of the Mux :

Output = S * A + S (bar) * B

If we replace A with zero and B with 1 we get the functionality of an Inverter

Output = S * 0 + S (bar) * 1

Output = S (bar)

You can verify using truth tables, the following circuit describes an inverter using 2:1 MUX

And Gate using 2:1 MUX Equation of the Mux :

Output = S * A + S (bar) * B

Tie Input B to zero we get

Output = S*A (AND gate)

2:1 MUX using only NAND Gates

Verilog : The module

Behavioral Description (Continuous /Dataflow Assignment)

Continuous assignment use the assign keyword.

A simple natural way to express the circuit.

Specify the logic expression instead of describing the gate level.

HDL more useful if used as a higher level of abstraction.

The RHS is continuously evaluated as a function of arbitrarily changing

inputs

The target / output is a net driven by combinational logic

Implementation of a Dataflow Assignment

Behavioral Description (Procedural Assignment) An alternative, often higher level of abstraction in behavioral class

Two structured procedural statements : always and initial.

Rather than specifying a circuit by Boolean expression, we use if – else (case, while, for statements)

Supports richer control structures and provides greater flexibility.

Mux Implementation of Procedural statement with always

Gate Level : Structural Description

Provides gate level details of the design.

Net represents connections between hardware elements. Nets are declared with the keyword wire

This coding style is error prone, it’s used only when we are sure about the exact circuit implementation

Verilog supports basic logic gates as primitives

: and, or, nor, NAND, XOR, XNOR, NOT, buf

Structural Implementation of MUX

Verilog Registers In digital design registers represents memory elements.

Digital registers need a clock to operate and update their state at a particular edge.

Registers in Verilog are different from digital design, please don’t confuse

In Verilog (reg) simple means a variable that can hold value, which can be changed anytime by assigning a new one

Combining Procedural and Continuous

The case statement: Case and if can be used interchangeably to implement conditional

execution within always blocks.

Case statements can be more easily read

N - bit signals in verilog

2:1 MUX with 8 – bit operands.

Multi bit arithmetic the easy way:

Port Connection Implementation

Connecting module instantiation Ports

module Full_Adder(Cin, x, y, Cout, s);

input Cin;

input x, y;

output Cout;

output s;

wire c1, c2, s1;

Half_Add HA1(x, y, c1, s1);

Half_Add HA2(s1, Cin, c2, s);

assign Cout = c1 | c2;

endmodule

Basic Verilog Construct

Verilog Combinational Logic

Alternative coding style for CL

Combined Verilog code for Flip Flop

Incorrect specification in Verilog:

Use always block sensitivity list to wait for clock to change

Sequential vs Combinational in always block

Synchronous vs Asynchronous clear

Implementing Asynchronous/Synchronous Reset

Verilog Implementation for Asynch/Synch Reset

Synchronous vs Asynchronous -III

Synchronous vs Asynchronous Reset

Getting the best of both worlds: Synchronize the asynchronous external reset signal.

Use this synchronous signal as input to all asynchronous flip flops.

Asynch reset FF takes less logic to implement, consumes less power.

The Asynchronous Metastable Problem

Blocking vs Non-Blocking

Swap function using blocking/non blocking

Assignment style for Sequential design

Use Non-Blocking for Sequential Logic

Use Blocking for Combinational Logic

Structural Representation (Revisited)

Structural Representation of a 4-bit Adder

Modeling Finite State Machines

Moore Machine example

Code for Traffic light controller

Traffic light controller code(contd.)

Further thoughts:

Non-latched Traffic light controller code:

Non-latched code (continued):

Moore Machine : Example 2

Code for parity detector:

Parity Detector code (continued):

Reasoning it’s a Moore

Mealy Machine : Example

Sequence Detector : Code

Sequence Detector code :(continued)

Example with Multiple Modules

The Compliment module

The Adder Module

The parity checker module

Top Level Module: Interconnecting the lower level modules