Find the most up-to-date version of IEEE at Engineering DRAFT. Summary of Key Verilog Features (IEEE ) ∗. Module. Encapsulates functionality; may be nested to any depth module module name (list of ports);. What this means, however, is that two different Verilog standardization efforts will be ongoing. One is IEEE , an upcoming revision of the IEEE.
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It is most commonly used in the design and verification of digital circuits at the register-transfer level of abstraction. It is also used in the verification of analog circuits and mixed-signal circuitsas well as in the design of genetic circuits. Since then, Verilog is officially part of the SystemVerilog language. The current version is IEEE standard Hardware description languages such as Verilog are similar to software programming languages because they include ways of describing the propagation time and signal strengths sensitivity.
The non-blocking assignment allows designers to describe a state-machine update without needing to declare and use temporary storage variables. Since these concepts are part of Verilog’s language semantics, designers could quickly write descriptions of large circuits in a relatively compact and concise form. At the time of Verilog’s introductionVerilog represented a tremendous productivity improvement for circuit designers who were already using graphical schematic capture software and specially written software programs to document and simulate electronic circuits.
The designers of Verilog wanted a language with syntax similar to the C programming languagewhich was already widely used in engineering software development. Verilog requires that variables be given a definite size. In C these sizes are assumed from the ‘type’ of the variable for instance an integer type may be 8 bits.
A Verilog design consists of a hierarchy of modules. Modules encapsulate design hierarchyand communicate with other modules through a set of declared input, output, and bidirectional ports.
Internally, a module can contain any combination of the following: However, the blocks themselves are executed concurrently, making Verilog a dataflow language. Verilog’s concept of ‘wire’ consists of both signal values 4-state: 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 synthesizable. Verilog modules that conform to a synthesizable coding style, known as RTL register-transfer levelcan be physically realized by synthesis software. Synthesis software algorithmically transforms the abstract Verilog source into a netlista logically equivalent description consisting only of elementary logic primitives AND, OR, NOT, flip-flops, etc.
Further manipulations to the netlist ultimately lead to a circuit fabrication blueprint such as a photo mask set for an ASIC or a bitstream file for an FPGA.
Verilog was one of the first popular [ clarification needed ] hardware description languages to be invented. Su, for his PhD work. Cadence now has full proprietary rights to Gateway’s Verilog and the Verilog-XL, the HDL-simulator that would become the de facto standard of Verilog logic simulators for the next decade.
Originally, Verilog was only intended to describe and allow simulation, the automated synthesis of subsets of the language to physically realizable structures gates etc. Verilog is a portmanteau of the words “verification” and “logic”. With the increasing success of VHDL at the time, Cadence decided to make the language available for open standardization.
In the same time frame Cadence initiated the creation of Verilog-A to put standards support behind its analog simulator Spectre. Extensions to Verilog were submitted back to IEEE to cover the deficiencies that users had found in the original Verilog standard. Verilog is a significant upgrade from Verilog 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 can be more succinctly described by one of the built-in operators: And finally, a few syntax additions were introduced to improve code readability e. Verilog is the version of Verilog supported by the majority of commercial EDA software packages. Not to be confused with SystemVerilogVerilog IEEE Standard consists of minor corrections, spec clarifications, and a few new language features such as the uwire keyword.
A separate part of the Verilog standard, Verilog-AMSattempts to integrate analog and mixed signal modeling with traditional Verilog. The advent of hardware verification languages such as OpenVeraand Verisity’s e language encouraged the development of Superlog by Co-Design Automation Inc acquired by Synopsys. SystemVerilog is a superset of Verilog, with many new features and capabilities to aid design verification and design modeling.
A simple example of two flip-flops follows:. 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: 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:. The always clause above illustrates the other type of method of use, i. 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.
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 13644 not necessarily imply a hardware veripog. The definition of constants in Verilog supports the addition of a width parameter. The basic syntax is:. There are several statements in Verilog that have no analog in real hardware, e.
Consequently, much of the language can not be used to describe hardware. The examples presented here are the classic subset of the language that has a direct 13664 to real gates. 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 stores it upon transition of the gate signal to “hold”.
The output will remain stable regardless leee the input signal while the gate is set to “hold”. Idee the example below the “pass-through” level of the gate would be when the value of the if clause is true, i.
The flip-flop is the next significant template; in Verilog, the D-flop is the simplest, and it can be modeled as:. The significant thing to notice in the example is the use of the non-blocking assignment.
A variant of the D-flop is one with an asynchronous reset; there is a convention that the reset state will be iieee first if clause within the statement. The next variant is including both an asynchronous reset and asynchronous set condition; again the convention comes into play, i.
Verilog – Wikipedia
Consider the following test sequence of events. 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 verioog 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. The final basic variant is one that implements a D-flop veeilog a mux feeding its input. The mux has a d-input and feedback from the flop iese.
This allows a gated load function. Note that there are no “initial” blocks mentioned in this description. 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. An ASIC is an actual hardware implementation. 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.
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 It is possible to use always as shown below:.
The other interesting exception is the use of the initial keyword with the addition of the forever keyword. Execution continues after the join upon completion of the longest running statement or block between the fork and join. This allows the simulation to contain both accidental race conditions as well as intentional non-deterministic behavior.
The order of execution isn’t always guaranteed within Verilog. This can best be illustrated by a classic example. Consider the code snippet below:. What will be printed out for the values of a and b?