Basics of Digital Design and Verilog

Digital Design & Verilog
PRITHIVIRAJ. R, RITHIK NAMBIAR
SRMIST, KTR.

Digital Systems
u The foundation of all digital systems is binary numbers.
u The binary representation has only two digits - 0 & 1.
u Binary representation simplifies the design process and the circuitry itself.
u It also provides a higher efficiency and smaller area/footprint.
u The mathematics/algebra of binary numbers is called Boolean algebra.
u Basic Boolean operations are implemented as a circuit/hardware in the
form of logic gates.
u Boolean algebra is used to design, simplify and analyze digital circuits.
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Binary to Decimal Conversion
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Logic Gates
u It is the building block of all digital systems.
u They implement the basic Boolean operations like AND, OR, NOT, XOR, etc.
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Combinational Circuits
u The output depends only on a combination of the present inputs.
u Because of its dependence only on the present inputs, there is no
feedback loop involved.
u Examples include adders, multipliers, multiplexers, encoders, PLA, PLDs, etc.
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Classification of Combinational Circuits
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Sequential Circuits
u Sequential Circuits the inputs depend on present as well as past inputs.
u Sequential Circuits involve combinational blocks of logic and memory
elements.
u Examples include flip flops, FSM, RAM, etc.
u Sequential Circuits are of two types:
u Synchronous: They output changes are synchronized to edges of the clock
(positive/rising edge or negative/falling edge)
u Asynchronous: The output changes are triggered by the value of the clock(high
or low), i.e., it is level sensitive.
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Asynchronous vs Synchronous Circuits – Timing Diagram

Finite State Machines (FSMs)
u Its is generally used to control the behavior of systems and dataflow paths.
u It is typically implemented in control units in the digital system.
u It consists of multiple states, with each state being associated with a particular
function or condition.
u Eg. The system may go into low power mode when it is in ‘IDLE’ state and so
on.
u Depending on the state, different events/functions may be triggered.
u Eg. When registers are full, the FSM can enter the ‘OVERLOAD’ state. Upon
entering the OVERLOAD state, we can force the processor to put a hold on
any future instructions to be executed until any registers are freed up.
u There are two types of FSMs, Moore and Mealey.
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An FSM used to
control the Data
Link Layer of a
PCIe Gen3 Link

Modern Digital Design
u Modern Digital Design uses the concept of Datapath and Control Units.
u The Datapath include functional blocks like ALUs.
u The Control Units are typically implemented as FSMs.
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An Example of Datapath and Control Unit of a Processor

Verilog
u During the early days of IC Design, engineers drew the circuits by hand.
u This was a very difficult, complex, time consuming and error prone process.
u VHDL was introduced in order to simplify this process.
u The idea behind this is that the designer will “describe” the circuit using the
HDL and the tools would infer a circuit that matches the designers intent.
u Verilog was later designed in order to make the Hardware Description
Language (HDL) more robust.
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Verilog vs Conventional Programming
Languages
u Verilog is used to describe hardware, while programming languages are
instructions for the hardware to do certain tasks.
u Verilog is a concurrent(parallel) language.
u The above code would infer two separate “AND” logic gates, while a
similar statement in C++ would merely “and” the values of the variables.
u In C++, the value of ‘A’ will be calculated first and then the value of ‘D’
will be calculated
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Types of Modelling:
u There are three types of modelling in Verilog:
u Gate Level Modelling
u Behavioral Modelling
u Dataflow Modelling
u Switch Level Modelling
u RTL or Register transfer level is a mixture of behavioral and dataflow
modelling that is used for designing synthesizable circuits.
u RTL models the dataflow between registers and logical operations
performed on the data.
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Gate Level Modelling
u It is practically the lowest level of abstraction as switch level is rarely used.
u It is used to implement modules in the lower levels of hierarchy like adders,
multipliers, etc.
u Here the connection between the pins of the logic gates are manually
specified.
u Netlist obtained from synthesis tools are generally gate level models in
Verilog.
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A NAND gate designed using AND & NOT
gates using gate level modelling in Verilog.
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Behavioral Level Modelling
u It is one of the highest level of abstraction provided by Verilog.
u Procedural blocks and ‘reg’ variables are used the describe the behavior
of the intended hardware.
u Other statements like case, if, loops are used to model the behavior of the
hardware.
u It is not necessarily synthesizable.
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Behavioral Level Model o 4x1 Multiplexer
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Dataflow Modelling
u This involves the use of operators to model the flow of data in the digital
system, mainly using assign statements.
u It is mainly used for designing combinational circuits.
u It is always synthesizable.
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2x1 Multiplexer using Dataflow Modelling
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Switch Level Modelling
u This is the lowest level of modelling in Verilog.
u It is similar to gate level modelling except that connections between the
pins of the transistor is specified(like gate, drain ,etc.)
u It involves the use of transistor primitives.
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NOT gate using Switch Level Modelling
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References
u Morris Mano Digital Design 5th edition
u www.chipverify.com
u www.asic-world.com
u PCI Express Technology 3.0 - Mindshare
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Basics of Digital Design and Verilog

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