Verilog
IEEE | |
---|---|
First appeared | 1984 |
Stable release | IEEE 1800-2023
/ 6 December 2023 |
.vh | |
Website | https://ieeexplore.ieee.org/document/10458102 |
Dialects | |
Verilog-AMS | |
Influenced by | |
Pascal, Ada, C, Fortran | |
Influenced | |
SystemVerilog | |
|
Verilog,
Overview
Hardware description languages such as Verilog are similar to
The designers of Verilog wanted a language with syntax similar to the
A Verilog design consists of a
Verilog's concept of 'wire' consists of both signal values (4-state: "1, 0, floating, undefined") and signal strengths (strong, weak, etc.). This system allows abstract modeling of shared signal lines, where multiple sources drive a common net. When a wire has multiple drivers, the wire's (readable) value is resolved by a function of the source drivers and their strengths.
A subset of statements in the Verilog language are
History
Beginning
Verilog was created by
Verilog is a portmanteau of the words "verification" and "logic".[5]
Verilog-95
With the increasing success of
In the same time frame Cadence initiated the creation of Verilog-A to put standards support behind its analog simulator Spectre. Verilog-A was never intended to be a standalone language and is a subset of Verilog-AMS which encompassed Verilog-95.
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
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 (e.g. always, @*, named parameter override, C-style function/task/module header declaration).
Verilog-2001 is the version of Verilog supported by the majority of commercial EDA software packages.
Verilog 2005
Not to be confused with
A separate part of the Verilog standard, Verilog-AMS, attempts to integrate analog and mixed signal modeling with traditional Verilog.
SystemVerilog
The advent of hardware verification languages such as OpenVera, and Verisity's e language encouraged the development of Superlog by Co-Design Automation Inc (acquired by Synopsys). The foundations of Superlog and Vera were donated to Accellera, which later became the IEEE standard P1800-2005: SystemVerilog.
SystemVerilog is a
Updates since 2009
The SystemVerilog standard was subsequently updated in 2012,[6] 2017,[7] and most recently in December 2023.[2]
Example
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. This is known as a "non-blocking" assignment. Its action does not register until after the always block has executed. This means that the order of the assignments is irrelevant and will produce the same result: flop1 and flop2 will swap values every clock.
The other assignment operator =
is referred to as a blocking assignment. When =
assignment is used, for the purposes of logic, the target variable is updated immediately. In the above example, had the statements used the =
blocking operator instead of <=
, flop1 and flop2 would not have been swapped. Instead, as in traditional programming, the compiler would understand to simply set flop1 equal to flop2 (and subsequently ignore the redundant logic to set flop2 equal to flop1).
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.
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 <= {size{1'b0}};
else
if (cet && cep) // Enables both true
begin
if (count == length-1)
count <= {size{1'b0}};
else
count <= count + 1'b1;
end
// 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. it executes whenever any of the entities in the list (the b or e) changes. When one of these changes, a is immediately assigned a new value, and due to the blocking assignment, b is assigned a new value afterward (taking into account the new value of a). 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:
- 12'h123 – Hexadecimal 123 (using 12 bits)
- 20'd44 – Decimal 44 (using 20 bits – 0 extension is automatic)
- 4'b1010 – Binary 1010 (using 4 bits)
- 6'o77 – Octal 77 (using 6 bits)
Synthesizable constructs
There are several statements in Verilog that have no analog in real hardware, such as the $display command. However, the examples presented here are the classic (and limited) subset of the language that has a direct mapping to real gates.
// 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 example
reg latch_out;
always @(gate or din)
if(gate)
latch_out = din; // Pass through state
// Note that the else isn't required here. The variable
// latch_out will follow the value of din while gate is
// high. When gate goes low, latch_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;
Note: If this model is used to model a Set/Reset flip flop then simulation errors can result. Consider the following test sequence of events. 1) reset goes high 2) clk goes high 3) set goes high 4) clk goes high again 5) reset goes low followed by 6) set going low. Assume no setup and hold violations.
In this example the always @ statement would first execute when the rising edge of reset occurs which would place q to a value of 0. The next time the always block executes would be the rising edge of clk which again would keep q at a value of 0. The always block then executes when set goes high which because reset is high forces q to remain at 0. This condition may or may not be correct depending on the actual flip flop. However, this is not the main problem with this model. Notice that when reset goes low, that set is still high. In a real flip flop this will cause the output to go to a 1. However, in this model it will not occur because the always block is triggered by rising edges of set and reset – not levels. A different approach may be necessary for set/reset flip flops.
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"
Note that there are no "initial" blocks mentioned in this description. There is a split between FPGA and ASIC synthesis tools on this structure. FPGA tools allow initial blocks where reg values are established instead of using a "reset" signal. ASIC synthesis tools don't support such a statement. The reason is that an FPGA's initial state is something that is downloaded into the memory tables of the FPGA. An ASIC is an actual hardware implementation.
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. 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) // Any time 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 language 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, and may purposefully be randomized by the simulator. This allows the simulation to contain both accidental race conditions as well as intentional non-deterministic behavior.
Notice that VHDL cannot dynamically spawn multiple processes like Verilog.[8]
Race conditions
The order of execution isn't always guaranteed within Verilog.[9] 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=%d Value of b=%d",a,b);
end
What will be printed out for the values of a and b? Depending on the order of execution of the initial blocks, it could be zero and zero, or alternately zero and some other arbitrary uninitialized value. The $display statement will always execute after both assignment blocks have completed, due to the #1 delay.
Operators
Note: These operators are not shown in order of precedence.
Operator type | Operator symbols | Operation performed |
---|---|---|
Bitwise | ~ | Bitwise NOT (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 | |
** | Exponentiation (*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 | {n{m}} | Replicate value m for n times |
Conditional | ? : | Conditional |
Four-valued logic
The IEEE 1364 standard defines a four-valued logic with four states: 0, 1, Z (high impedance), and X (unknown logic value). For the competing VHDL, a dedicated standard for multi-valued logic exists as IEEE 1164 with nine levels.[10]
System tasks
System tasks are available to handle simple I/O and various design measurement functions during simulation. All system tasks are prefixed with $ to distinguish them from user tasks and functions. This section presents a short list of the most frequently used tasks. It is by no means a comprehensive list.
- $display – Print to screen a line followed by an automatic newline.
- $write – Print to screen a line without the newline.
- $swrite – Print to variable a line without the newline.
- $sscanf – Read from variable a format-specified string. (*Verilog-2001)
- $fopen – Open a handle to a file (read or write)
- $fdisplay – Print a line from a file followed by an automatic newline.
- $fwrite – Print to file a line without the newline.
- $fscanf – Read from file a format-specified string. (*Verilog-2001)
- $fclose – Close and release an open file handle.
- $readmemh – Read hex file content into a memory array.
- $readmemb – Read binary file content into a memory array.
- $monitor – Print out all the listed variables when any change value.
- $time – Value of current simulation time.
- $dumpfile – Declare the VCD (Value Change Dump) format output file name.
- $dumpvars – Turn on and dump the variables.
- $dumpports – Turn on and dump the variables in Extended-VCD format.
- $random – Return a random value.
Program Language Interface (PLI)
The PLI provides a programmer with a mechanism to transfer control from Verilog to a program function written in C language. It is officially
The PLI (now VPI) enables Verilog to cooperate with other programs written in the C language such as test harnesses, instruction set simulators of a microcontroller, debuggers, and so on. For example, it provides the C functions tf_putlongp()
[11]and tf_getlongp()
[12] which are used to write and read the 64-bit integer argument of the current Verilog task or function, respectively. For 32-bit integers, tf_putp()
[13] and tf_getp()
[14] are used.
Simulation software
For information on Verilog simulators, see the
See also
Additional material
- List of HDL simulators
- Waveform viewer
- SystemVerilog Direct Programming Interface (DPI)
- Verilog Procedural Interface (VPI)
Similar languages
- VHDL, the main competitor to Verilog and SystemVerilog.
- Verilog-A and Verilog-AMS: Verilog with analog extensions.
- SystemC — C++ library providing HDL event-driven semantics
- SystemVerilog
- OpenVera
- e (verification language)
- Property Specification Language
- Chisel, an open-source language built on top of Scala
References
- PMID 27034378.
- ^ IEEE, 2023
- ^ "Verilog's inventor nabs EDA's Kaufman award". EE Times. 7 November 2005.
- OCLC 696254754.
- ^ "Oral History of Philip Raymond "Phil" Moorby" (PDF). Computer History Museum. 22 April 2013. pp. 23–25.
- IEEE, 2012
- IEEE, 2017
- ^ Cummings, Clifford E. (2003). "SystemVerilog – Is This The Merging of Verilog & VHDL?" (PDF). SNUG Boston 2003.
- ISBN 978-1-4613-5691-2.
The initial procedure in Verilog is not executed prior to simulation. It begins execution at time zero, when simulation starts running, and is executed in parallel with other initial or always procedures. When there are multiple initial procedures, there is no defined order to the activation of the procedures.
- ISBN 978-1-59829-190-2.
- ^ "IEEE Standard Verilog® Hardware Description Language" (PDF). p. 652(25.38). Retrieved 12 November 2023.
- ^ "IEEE Standard Verilog® Hardware Description Language" (PDF). p. 628(25.28). Retrieved 12 November 2023.
- ^ "IEEE Standard Verilog® Hardware Description Language" (PDF). p. 653(25.39). Retrieved 25 November 2023.
- ^ "IEEE Standard Verilog® Hardware Description Language" (PDF). p. 631(25.21). Retrieved 25 November 2023.
- Notes
- IEEE Standard for Verilog Hardware Description Language. 2006. ISBN 0-7381-4850-4.
- IEEE Standard Verilog Hardware Description Language. 2001. ISBN 0-7381-2826-0.
- IEC 61691-4 Ed.1 (IEEE STD 1364(TM)-2001): Behavioural Languages - Part 4: Verilog(C) Hardware Description Language. 2004. ISBN 2-8318-7675-3.
- IEEE Standard Hardware Description Language Based on the Verilog(R) Hardware Description Language. 1996. ISBN 978-0-7381-3065-1.
- Thomas, Donald E.; Moorby, Phillip R. (2013). The Verilog® Hardware Description Language (3rd ed.). Springer. ISBN 978-1475724646.
- Cornell ECE576 Course illustrating synthesis constructs
- Bergeron, Janick (2012). Writing Testbenches: Functional Verification of HDL Models (2nd ed.). Springer. ISBN 978-1-4615-0302-6. (The HDL Testbench Bible)
External links
Standards development
- "IEEE Standard for Verilog Hardware Description Language". IEEE STD 1364-2005 (Revision of IEEE STD 1364-2001): 1–590. April 2006. ISBN 978-0-7381-4851-9. – The official standard for Verilog 2005 (not free).
- IEEE P1364 – Working group for Verilog (inactive).
- IEEE P1800 – Working group for SystemVerilog (replaces above).
- Verilog syntax – A 1995 description of the syntax in Backus-Naur form. This predates the IEEE-1364 standard.
Language extensions
- Verilog AUTOs — An open-source meta-comment used by industry IP to simplify maintaining Verilog code.