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Verilog is a hardware description language (HDL) used to model electronic systems. In Electronics, a hardware description language or HDL is any language from a class of Computer languages and/or Programming languages for formal Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical The language (sometimes called Verilog HDL) supports the design, verification, and implementation of analog, digital, and mixed-signal circuits at various levels of abstraction. Analogue electronics (or analog in American English) are those electronic systems with a continuously Variable signal Digital electronics are Electronics systems that use Digital signals Digital electronics are representations of Boolean algebra also see A mixed-signal Integrated circuit combines Analog circuits, used for analog signal processing with Digital signal processing (DSP circuits on a single In Computer science, abstraction is a mechanism and practice to reduce and factor out details so that one can focus on a few concepts at a time

The designers of Verilog wanted a language with syntax similar to the C programming language so that it would be familiar to engineers and readily accepted. tags please moot on the talk page first! --> In Computing, C is a general-purpose cross-platform block structured The language is case-sensitive, has a preprocessor like C, and the major control flow keywords, such as "if" and "while", are similar. Text sometimes exhibits case sensitivity; that is words can differ in meaning based on differing use of uppercase and lowercase letters In Computer science, a preprocessor is a program that processes its input data to produce output that is used as input to another program In Computer science control flow (or alternatively flow of control refers to the order in which the individual statements, instructions or Function In Computer programming, a keyword is a Word or Identifier that has a particular meaning to the Programming language. The formatting mechanism in the printing routines and language operators and their precedence are also similar. Programming languages generally support a set of operators that are similar to operators in mathematics. In Programming languages, common operator notation is just one way of notating mathematical expressions as alinear sequence of tokens or Operators but this is not

The language differs in some fundamental ways. Verilog uses Begin/End instead of curly braces to define a block of code. The definition of constants in Verilog require a bit width along with their base, consequently these differ. Verilog 95 and 2001 don't have structures, pointers, or recursive subroutines, however SystemVerilog now includes these capabilities. In the Semiconductor and electronic design industry SystemVerilog is a combined Hardware Description Language and Hardware Verification Language based Finally, the concept of time —so important to a HDL— won't be found in C.

The language differs from a conventional programming language in that the execution of statements is not strictly sequential. A programming language is an Artificial language that can be used to write programs which control the behavior of a machine particularly a Computer. In Computer programming a statement can be thought of as the smallest standalone element of an imperative Programming language. A Verilog design consists of a hierarchy of modules. Modules are defined with a set of input, output, and bidirectional ports. Internally, a module contains a list of wires and registers. Concurrent and sequential statements define the behaviour of the module by defining the relationships between the ports, wires, and registers. Sequential statements are placed inside a begin/end block and executed in sequential order within the block. But all concurrent statements and all begin/end blocks in the design are executed in parallel, qualifying Verilog as a Dataflow language. In Computer programming, dataflow programming implements Dataflow principles and architecture and models a program conceptually if not physically as a Directed A module can also contain one or more instances of another module to define sub-behavior.

A subset of statements in the language is synthesizable. Logic synthesis is a process by which an abstract form of desired circuit behavior (typically Register transfer level (RTL or behavioral is turned into a design implementation If the modules in a design contain only synthesizable statements, software can be used to transform or synthesize the design into a netlist that describes the basic components and connections to be implemented in hardware. The word netlist can be used in several different contexts but perhaps the most popular is in the field of Electronic design. The netlist may then be transformed into, for example, a form describing the standard cells of an integrated circuit (e. For the battery used as a voltage reference see Weston cell. In semiconductor design standard cell methodology is a method of designing Application Specific Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside g. an ASIC) or a bitstream for a programmable logic device (e. A bitstream or bit stream is a Time series of Bits A Bytestream is a series of Bytes typically of 8 bits each and can be A programmable logic device or PLD is an electronic component used to build reconfigurable Digital circuits Unlike a Logic gate, which has a g. a FPGA). FPGAs should not be confused with the Flip-chip pin grid array, a form of integrated circuit packaging

Verilog stands for verify logic.

Contents

History

Beginning

Verilog was invented by Phil Moorby and Prabhu Goel during the winter of 1983/1984 at Automated Integrated Design Systems (later renamed to Gateway Design Automation) in 1985 as a hardware modeling language. Phil Moorby is an Engineer and Computer scientist. Moorby was born and brought up in Birmingham England and studied Mathematics at Southampton University "Verilog HDL originated at Automated Integrated Design Systems (later renamed as Gateway Design Automation) in 1985 Year 1985 ( MCMLXXXV) was a Common year starting on Tuesday (link displays 1985 Gregorian calendar) Gateway Design Automation was later purchased by Cadence Design Systems in 1990. Cadence Design Systems Inc ( is an Electronic design automation (EDA Software and engineering services company founded in 1988 by the merger of SDA Systems and Year 1990 ( MCMXC) was a Common year starting on Monday (link displays the 1990 Gregorian calendar) Cadence now has full proprietary rights to Gateway's Verilog and the Verilog-XL simulator logic simulators. Logic simulation is the use of a computer program to Simulate the operation of a Digital circuit.

Verilog-95

With the increasing success of VHDL at the time, Cadence decided to make the language available for open standardization. Standardization (or standardisation) is the process of developing and agreeing upon technical standards. Cadence transferred Verilog into the public domain under the Open Verilog International (OVI) (now known as Accellera) organization. Accellera was founded in 2000 from the merger of Open Verilog International and VHDL International. Verilog was later submitted to IEEE and became IEEE Standard 1364-1995, commonly referred to as Verilog-95. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization

Verilog 2001

Extensions to Verilog-95 were submitted back to IEEE to cover the deficiencies that users had found in the original Verilog standard. These extensions became IEEE Standard 1364-2001 known as Verilog-2001. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization

Verilog-2001 is a significant upgrade from Verilog-95. First, it adds explicit support for (2's complement) signed nets and variables. Previously, code authors had to perform signed-operations using awkward bit-level manipulations (for example, the carry-out bit of a simple 8-bit addition required an explicit description of the boolean-algebra to determine its correct value. ) The same function under Verilog-2001 can be more succinctly described by one of the built-in operators: +, -, /, *, >>>. A generate/endgenerate construct (similar to VHDL's generate/endgenerate) allows Verilog-2001 to control instance and statement instantiation through normal decision-operators (case/if/else). Using generate/endgenerate, Verilog-2001 can instantiate an array of instances, with control over the connectivity of the individual instances. File I/O has been improved by several new system-tasks. And finally, a few syntax additions were introduced to improve code-readability (eg. always @*, named-parameter override, C-style function/task/module header declaration. )

Verilog-2001 is the dominant flavor of Verilog supported by the majority of commercial EDA software packages.

Verilog 2005

Not to be confused with SystemVerilog, Verilog 2005 (IEEE Standard 1364-2005) consists of minor corrections, spec clarifications, and a few new language features (such as the uwire keyword. In the Semiconductor and electronic design industry SystemVerilog is a combined Hardware Description Language and Hardware Verification Language based The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization )

A separate part of the Verilog standard , Verilog-AMS, attempts to integrate analog and mixed signal modelling with traditional Verilog. Verilog-AMS is a derivative of the Hardware description language (HDL Verilog (IEEE 1364-1995 Verilog HDL

SystemVerilog

Main article: SystemVerilog

Systemverilog is a superset of Verilog-2005, with many new features and capabilities to aid design-verification and design-modeling. In the Semiconductor and electronic design industry SystemVerilog is a combined Hardware Description Language and Hardware Verification Language based

The advent of High Level Verification languages such as OpenVera, and Verisity's E language encouraged the development of Superlog by Co-Design Automation Inc. OpenVera is a Hardware Verification Language developed by Synopsys. In Mathematics, the super-logarithm is one of the two inverse functions of Tetration. Co-Design Automation Inc was later purchased by Synopsys. Not to be confused with Synopsis Synopsys Inc is one of the largest companies in the Electronic design automation industry The foundations of Superlog and Vera were donated to Accellera, which later became the IEEE standard P1800-2005: SystemVerilog. Accellera was founded in 2000 from the merger of Open Verilog International and VHDL International. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization

Example

A hello world program looks like this:

module main;
  initial 
    begin
      $display("Hello world!");
      $finish;
    end
endmodule

A simple example of two flip-flops follows:

module toplevel(clock,reset);
 input clock;
 input reset;

 reg flop1;
 reg flop2;

 always @ (posedge reset or posedge clock)
 if (reset)
   begin
     flop1 <= 0;
     flop2 <= 1;
   end
 else
   begin
     flop1 <= flop2;
     flop2 <= flop1;
   end
endmodule

The "<=" operator in verilog is another aspect of its being a hardware description language as opposed to a normal procedural language. A "Hello World" program is a Computer program that prints out "Hello world!" on a Display device. In Digital circuits a flip-flop is a term referring to an Electronic circuit (a Bistable Multivibrator) that has two stable states and thereby This is known as a "non-blocking" assignment. When the simulation runs, all of the signals assigned with a "<=" operator have their assignment scheduled to occur after all statements occurring during the same point in time have executed. After all the statements have been executed for one event, the scheduled assignments are performed. This makes it easier to code behaviours that happen simultaneously.

In the above example, flop1 is assigned flop2, and flop2 is assigned flop1. These statements are executed during the same time event. Since the assignments are coded with the "<=" non-blocking operator, the assignments are scheduled to occur at the end of the event. Until then, all reads to flop1 and flop2 will use the values they had at the beginning of the time event.

This means that the order of the assignments are irrelevant and will produce the same result. flop1 and flop2 will swap values every clock.

The other choice for assignment is an "=" operator and this is known as a blocking assignment. When the "=" operator is used, things occur in the sequence they occur much like a procedural language.

In the above example, if the statements had used the "=" blocking operator instead of "<=", the order of the statements would affect the behaviour: the reset would set flop2 to a 1, and flop1 to a 0. A clock event would then set flop1 to flop2, which is a 1 after the reset. The next statement would be executed subsequently and would set flop2 to flop1, which is now a 1. Rather than swap values every clock, flop1 and flop2 would both become 1 and remain that way.

An example counter circuit follows:

module Div20x (rst, clk, cet, cep, count,tc);
// TITLE 'Divide-by-20 Counter with enables'
// enable CEP is a clock enable only
// enable CET is a clock enable and
// enables the TC output
// a counter using the Verilog language

parameter size = 5;
parameter length = 20;

input rst; // These inputs/outputs represent 
input clk; // connections to the module. In Digital logic and Computing, a counter is a device which stores (and sometimes displays the number of times a particular event or process 
input cet;
input cep;

output [size-1:0] count;
output tc;

reg [size-1:0] count; // Signals assigned 
                      // within an always 
                      // (or initial)block 
                      // must be of type reg
                      
wire tc; // Other signals are of type wire

// The always statement below is a parallel
// execution statement that
// executes any time the signals 
// rst or clk transition from low to high

always @ (posedge clk or posedge rst)
  if (rst) // This causes reset of the cntr
    count <= 5'b0;
  else
  if (cet && cep) // Enables both  true
    begin
      if (count == length-1)
        count <= 5'b0;
      else
        count <= count + 5'b1; // 5'b1 is 5 bits 
    end                        // wide and equal 
                               // to the value 1. 

// the value of tc is continuously assigned 
// the value of the expression
assign tc = (cet && (count == length-1));

endmodule

An example of delays:

. . . 
reg a, b, c, d;
wire e;
. . . 
always @(b or e)
 begin
   a = b & e;
   b = a | b;
   #5 c = b;
   d = #6 c ^ e;
 end

The always clause above illustrates the other type of method of use, i. e. the always clause executes any time any of the entities in the list change, i. e. the b or e change. When one of these changes, immediately a and b are assigned new values. After a delay of 5 time units, c is assigned the value of b and the value of c ^ e is tucked away in an invisible store. Then after 6 more time units, d is assigned the value that was tucked away.

Signals that are driven from within a process (an initial or always block) must be of type reg. Signals that are driven from outside a process must be of type wire. The keyword reg does not necessarily imply a hardware register.

Definition of Constants

The definition of constants in Verilog supports the addition of a width parameter. The basic syntax is:

<Width in bits>'<base letter><number>

Examples:

Synthesizeable constructs

As mentioned previously, there are several basic templates that can be used to represent hardware. 

// Mux examples - Three ways to do the same thing. 

// The first example uses continuous assignment
wire out ;
assign out = sel ? a : b;

// the second example uses a procedure 
// to accomplish the same thing. 

reg out;
always @(a or b or sel)
 begin
  case(sel)
   1'b0: out = b;
   1'b1: out = a;
  endcase
 end
 
// Finally - you can use if/else in a 
// procedural structure. 
reg out;
always @(a or b or sel)
  if (sel)
    out = a;
  else
    out = b;

The next interesting structure is a transparent latch; it will pass the input to the output when the gate signal is set for "pass-through", and captures the input and store it upon transition of the gate signal to "hold". The output will remain stable regardless of the input signal while the gate is set to "hold". In the example below the "pass-through" level of the gate would be when the value of the if clause is true, i. e. gate = 1. This is read "if gate is true, the din is fed to latch_out continuously. " Once the if clause is false, the last value at latch_out will remain and is independent of the value of din. 

// Transparent latch example

reg out;
always @(gate or din)
 if(gate)
   out = din; // Pass through state
   // Note that the else isn't required here.  The variable
   // out will follow the value of din while gate is high. 
   // When gate goes low, out will remain constant.

The flip-flop is the next significant template; in verilog, the D-flop is the simplest, and it can be modeled as:

reg q;
always @(posedge clk)
  q <= d;

The significant thing to notice in the example is the use of the non-blocking assignment. A basic rule of thumb is to use <= when there is a posedge or negedge statement within the always clause.

A variant of the D-flop is one with an asynchronous reset; there is a convention that the reset state will be the first if clause within the statement. 

reg q;
always @(posedge clk or posedge reset)
  if(reset)
    q <= 0;
  else
    q <= d;

The next variant is including both an asynchronous reset and asynchronous set condition; again the convention comes into play, i. e. the reset term is followed by the set term. 

reg q;
always @(posedge clk or posedge reset or posedge set)
 if(reset)
   q <= 0;
 else if(set)
   q <= 1;
 else
   q <= d;

The final basic variant is one that implements a D-flop with a mux feeding its input. The mux has a d-input and feedback from the flop itself. This allows a gated load function. 

// Basic structure with an EXPLICIT feedback path
always @(posedge clk)
  if(gate)
    q <= d;
  else
    q <= q; // explicit feedback path

// The more common structure ASSUMES the feedback is present
// This is a safe assumption since this is how the
// hardware compiler will interpret it.  This structure
// looks much like a Latch.  The differences are the
// '''@(posedge clk)''' and the non-blocking '''<='''
//
always @(posedge clk)
  if(gate)
    q <= d; // the "else" mux is "implied"

Looking at the original counter example you can see a combination of the basic asynchronous reset flop and Gated input flop used. The register variable count is set to zero on the rising edge or rst. When rst is 0, the variable count will load new data when cet && cep is true.

Initial and Always

There are two separate ways of declaring a verilog process. These are the always and the initial keywords. The always keyword indicates a free-running process that triggers on the accompanying event-control (@) clause. The initial keyword indicates a process executes exactly once. Both constructs begin execution at simulator time 0, and both execute until the end of the block. Once an always block has reached its end, it is rescheduled (again). It is a common misconception to believe that an initial block will execute before an always block. In fact, it is better to think of the initial-block as a special-case of the always-block, one which terminates after it completes for the first time. 

//Examples:
initial
  begin
    a = 1; // Assign a value to reg a at time 0
    #1; // Wait 1 time unit
    b = a; // Assign the value of reg a to reg b
  end

always @(a or b) // Anytime a or b CHANGE, run the process
begin
  if (a)
    c = b;
  else
    d = ~b;
end // Done with this block, now return to the top (i. e.  the @ event-control)

always @(posedge a)// Run whenever reg a has a low to high change
  a <= b;

These are the classic uses for these two keywords, but there are two significant additional uses. The most common of these is an always keyword without the @() sensitivity list. It is possible to use always as shown below:

always
 begin // Always begins executing at time 0 and NEVER stops
   clk = 0; // Set clk to 0
   #1; // Wait for 1 time unit
   clk = 1; // Set clk to 1
   #1; // Wait 1 time unit
 end // Keeps executing - so continue back at the top of the begin

The always keyword acts similar to the "C" construct while(1) {. . } in the sense that it will execute forever.

The other interesting exception is the use of the initial keyword with the addition of the forever keyword.

The example below is functionally identical to the always example above. 

initial forever // Start at time 0 and repeat the begin/end forever
 begin
   clk = 0; // Set clk to 0
   #1; // Wait for 1 time unit
   clk = 1; // Set clk to 1
   #1; // Wait 1 time unit
 end

Fork/Join

The fork/join pair are used by Verilog to create parallel processes. All statements (or blocks) between a fork/join pair begin execution simultaneously upon execution flow hitting the fork. Execution continues after the join upon completion of the longest running statement or block between the fork and join. 

initial
 fork
   $write("A"); // Print Char A
   $write("B"); // Print Char B
   begin
     #1; // Wait 1 time unit
     $write("C");// Print Char C
   end
 join

The way the above is written, it is possible to have either the sequences "ABC" or "BAC" print out. The order of simulation between the first $write and the second $write depends on the simulator implementation. This illustrates one of the biggest issues with Verilog. You can have race conditions where the language execution order doesn't guarantee the results.

Race Conditions

The order of execution isn't always guaranteed within verilog. This can best be illustrated by a classic example. Consider the code snippet below:

initial
  a = 0;


initial
  b = a;
 
initial
  begin
    #1;
    $display("Value a=%b Value of b=%b",a,b);
  end

What will be printed out for the values of a and b? Well - it could be 0 and 0, or perhaps 0 and X! This all depends on the order of execution of the initial blocks. If the simulators scheduler works from the top of the file to the bottom, then you would get 0 and 0. If it begins from the bottom of the module and works up, then b will receive the initial value of a at the beginning of the simulation before it has been initialized to 0 (the value of any variable not set explicitily is set to X. ) This is the way you can experience a race condition in a simulation. So be careful! Note that the 3rd initial block will execute as you expect because of the #1 there. That is a different point on the time wheel beyond time 0, consequently both of the earlier initial blocks have completed execution.

Operators

Operator type Operator symbols Operation performed
Bitwise ~ 1's complement
& Bitwise AND
| Bitwise OR
^ Bitwise XOR
~^ or ^~ Bitwise XNOR
Logical  ! NOT
&& AND
|| OR
Reduction & Reduction AND
~& Reduction NAND
| Reduction OR
~| Reduction NOR
^ Reduction XOR
~^ or ^~ Reduction XNOR
Arithmetic + Addition
- Subtraction
- 2's complement
* Multiplication
/ Division
** exponent (*Verilog-2001)
Relational > Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to
== logical equality (bit-value 1'bX is removed from comparison)
 != Logical inequality (bit-value 1'bX is removed from comparison)
=== 4-state logical equality (bit-value 1'bX is taken as literal)
 !== 4-state Logical inequality (bit-value 1'bX is taken as literal)
Shift >> Logical Right shift
<< Logical Left shift
>>> Arithmetic Right shift (*Verilog-2001)
<<< Arithmetic Left shift (*Verilog-2001)
Concatenation { , } Concatenation
Replication {{ }} Replication
Conditional  ? : Conditional

System tasks

System tasks are available to handle simple I/O, and various design measurement functions. All system tasks are prefixed with $ to distinguish them from user tasks and functions. This section presents a short list of the most often used tasks. It is by no means a comprehensive list.

Program Language Interface (PLI)

Program Language Interface provides a programmer with transferring control from Verilog to a program function written in C language. It is officially deprecated by IEEE Std 1364-2005 in favor of the newer Verilog Procedural Interface, which completely replaces the PLI. In Computer software standards and documentation the term deprecation is applied to Software features that are superseded and should be avoided The Verilog Procedural Interface (VPI is an interface primarily intended for the C programming language.

The PLI enables Verilog to cooperate with other programs written in the C language such as test harness, Instruction Set Simulator of microcontroller, debugger, and so on. In Software testing, a test harness or automated test framework is a collection of Software and test data configured to test a program unit by running it An instruction set simulator (ISS is a Simulation model, usually coded in a High-level programming language, which mimics the behavior of a mainframe or A microcontroller (also MCU or µC is a functional Computer system-on-a- chip. A debugger is a Computer program that is used to test and Debug other programs For example, it provides C functions named tf_putlongp() and tf_getlongp() which are used to write and read the argument of the current Verilog task or function, respectively.

Simulation software

For information on Verilog simulators, see List of Verilog Simulators. Verilog simulators are software packages that emulate the Verilog Hardware description language.


See also

Additional material

Related languages

External links

Verilog Resources

Standards Development

Verilog Tools

Open Source Verilog Tools

References

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