Verilog vs. VHDL: What are the differences between them

What is Verilog?

In reality, Hilo, an antiquated hardware description language, and the C programming languages are the roots of Verilog. All the predefined data types are present in it, which makes it an extremely constrained and weakly typed language. The datatypes are displayed on a bit-level basis. Strings and Verilog can be combined with other data types. In comparison to VHDL, Verilog's simulation semantics are more vague. Because of this, developers find Verilog to be incredibly versatile; yet, if coding standards are not followed, race situations may emerge owing to ambiguity. The reusability function of packages must be provided, however, because of the language's restricted scope and inadequate packaging ability, this is exceedingly challenging. Basic simulation control commands are available in Verilog and assist in carrying out system activities. Because of the preset system tasks and straightforward data types in Verilog, command-line or batch simulations are frequently utilized by programmers. Waveforms from the simulation can also be used for debugging.

Figure 1: Verilog example code

What is VHDL?

The programming language Ada, a very strongly typed and densely typed hardware description language, is where VHDL actually derives from. VHDL is very verbose in comparison to Verilog, another HDL, due to the language requirement, which also increases the number of self-documenting designs. Strong typing in VHDL ensures that datatypes are explicitly transformed from one to another, such as when going from a bit-vector to an integer. The semantics of the VHDL language were created in a way that makes it exceedingly clear and unambiguous. The designs of VHDL are easily portable which also adds up the functionality to move from one tool to another very easily. Due to this, there is no necessity to be concerned about race conditions. In order to increase the language's usefulness, the VHDL design heavily relies on IEEE standard 1164 and uses the Math and Numeric packages. VHDLs are totally tool-dependent and do not offer monitoring or simulation control features. Because VHDL types are user-defined and there is no built-in simulation control, debugging design issues is made more difficult and requires the usage of interactive GUIs.

Figure 2: VHDL example code  

Verilog vs. VHDL tutorial

 

How to use Verilog and VHDL working on FPGA

Let's look at a very simple illustration of how to use an FPGA. Assume for the moment that you are creating a 1-bit full adder and have already obtained the adder's logic diagram, which is depicted in the image below.

Figure 3: Logic diagram of the adder As previously noted, the logic gates XOR, AND, and OR are required on FPGA in order to create the aforementioned adder. Either Verilog or VHDL can be used to join those gates together as indicated in the adder's logic diagram in order to demonstrate how the adder works on an FPGA.

Figure 4: Verilog code for the adder

Figure 5: VHDL code for the adder Simply by linking the logic gates on the FPGA together to serve as a full adder, the Verilog/VHDL code for the adder performs the "wiring" task. We can synthesize, run the adder on FPGA, and test the code after simulation verification. There are numerous simulators that are available that we can utilize for simulation.  

Verilog vs. VHDL Syntax Comparison

VHSIC is for Very High-Speed Integrated Circuit, while VHDL stands for VHSIC Hardware Description Language. VHDL stands for Very High-Speed Integrated Circuit Hardware Description Language, therefore in the end. That is a mouthful if ever I heard one. Being a tightly typed language, which means that the language itself has predefined each data type (integer, character, etc.), is one of VHDL's distinguishing characteristics. One of the data types must adequately describe each value or variable specified in this language. Compared to Verilog, VHDL is more verbose and has a different syntax. You have a better possibility of producing more lines of code while using VHDL. Additionally, using VHDL occasionally may feel more intuitive. VHDL can make programming seem to flow more naturally. But perhaps that's just my opinion. The language is more condensed in Verilog since it is more of a language for modeling hardware. A small amount of code must be typed, similar to the C language. Verilog offers a lower level of programming structures but a greater understanding of hardware modeling. Verilog is more compact because it is less verbose than VHDL. All in all, Verilog is pretty different from VHDL. There are some similarities, but their differences overshadow them.  

Verilog vs. VHDL Design

Verilog vs. VHDL: HDL Modeling Capacity

Both Verilog and VHDL are capable of modeling hardware, as the aforementioned graph demonstrates. Verilog is superior to VHDL for low-level hardware modeling, nevertheless. It makes sense because Verilog was designed from the start to model and simulate logic gates. In actuality, designers may instantiate the primitives in Verilog code since it has built-in primitives or low-level logic gates, whereas VHDL does not. Gate primitives in Verilog include and, nand, or, nor, xor, xnor, buf, not, bufif0, notif0, bufif1, pullup, and pulldown. pmos, nmos, rpmos, rnmos, cmos, rcmos, tran, rtran, tranif0, rtranif0, and tranif1, rtranif1 are examples of switch primitives in Verilog. Verilog includes User-Defined Primitives (UDP), which allows designers to create custom cell primitives. For designers of ASICs, this trait is particularly important and well-liked. However, as seen in the graph above, VHDL is superior to Verilog in terms of high-level hardware modeling. Compared to Verilog, VHDL offers additional facilities and constructs for high-level hardware modeling.

Figure 6: HDL Modeling Capacity

Verilog versus VHDL's verbosity

VHDL code must be carefully designed with matching and defined data types because it is a highly strongly typed hardware description language. If you mix data types or mismatch signals when assigning in VHDL, a compiler error will result. Verilog, however, is a loosely typed language. When assigning in Verilog, you can combine data types and mismatch signals. An example VHDL code for mismatched signals is provided below:

Figure 7: VHDL code for ALU When you assign a 4-bit signal to an 8-bit signal in Verilog Compiler, no syntax problems will be introduced. Signals with various bit widths can be assigned to one another in Verilog. The width of the source signal will be adjusted by the Verilog compiler to match the width of the destination signal. During synthesis, unused bits will be optimized. Another VHDL example of combining data types while assigning signals is provided below:  

Verilog simple data types versus VHDL complex data types

Users can define numerous additional complicated data types in addition to the numerous complex data types that come with VHDL. Given that Verilog only has two main data types and does not permit user-defined data types, this also makes VHDL more verbose than Verilog. To represent the same circuit, VHDL code is typically verbose and lengthier than Verilog code since VHDL's strong typing requires us to do conversions between many complex data types. It might be advantageous or detrimental. In fact, it is more likely that the VHDL compiler will add syntax warnings when something is incorrectly assigned in your VHDL code. Compared to Verilog, your VHDL code is more likely to be working when you successfully compile it. Contrarily, Verilog is weakly typed, shorter, and simpler. However, even after successful compilation, it is more likely that your Verilog code still contains errors. Below is another example code that makes VHDL more verbose than Verilog:

Figure 8: Verilog vs. VHDL Code  

Verilog vs. VHDL Comparison

Verilog	VHDL
By utilizing the C API standards, such as tf, acc, and vpi, foreign interfaces are supported.	VhPI is also supported but not yet standardized, as is limited support for external interfaces using foreign attribute standards.
Verilog lacks assertion support.	By utilizing the C API standards, such as tf, acc, and vpi, foreign interfaces are supported.
Utilizing input-output file operations, fork-join, the generation of random numbers, and recursive subprograms, the limited support for verification capabilities is carried out.	Access types, postponed processes, comprehensive file input-output operations, recursive subprograms, and the creation of random numbers using a standard package are used to partially meet the verification targeted capabilities.
If, if-else, and case statements can be used to implement conditional statements, and the for loop can be used to achieve iterative behavior.	The if statement and the for loop can be used to implement the conditional and iterative statements, respectively.
Support for binding and configuration to a limited extent through managing instance-to-module binding.	Complete support for binding and configuration thanks to the ability to manage components and apply instance binding to newly formed entities.
Complete support for binding and configuration thanks to the ability to manage components and apply instance binding to newly formed entities.	No facility for items to be created and deleted on the fly.
Such processes do not exist.	Reactive region processes can be introduced using postponed processes.
Other hierarchies are not supported.	By employing different entities and utilizing the architecture of the interface and implementation, further hierarchies can be established.
The Verilog language supports the bit and integer equivalence.	Partially supporting the bit/vector integer equivalence can be done using standard packages.
It is not possible to employ sparse or associative arrays.	Using associative and sparse arrays, we may somewhat mimic the different access types.
Events with names can be used.	In VHDL, we are unable to use named events.
With datatypes like bit, bit-vector, wire, reg, unsigned, signed, integer, real, and occasionally strings as well, Verilog is a weakly typed language.	Strongly typed language is used.
Verilog does not support user-defined datatypes.	In VHDL, user-defined datatypes are supported.

 

Conclusion of Verilog vs. VHDL

The HDL that is entirely emerging and changing, with new features being introduced on a constant basis, is called Verilog. While Verilog is a weakly typed language and comes with all the predefined datatypes, VHDL is tightly typed and extremely verbose. While Verilog provides the capability of simulation support, which makes it easier to troubleshoot design-related issues with the aid of waveforms that are presented based on the database and may be further investigated, VHDL does not internally offer simulation control as an in-built feature.