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VHDL, which stands for VHSIC Hardware Description Language, is a programming language used to model and design digital systems. VHSIC stands for Very High-Speed Integrated Circuit. It was originally developed by the U.S Department of Defense in the 1980s as a way to document the behavior of ASICs (Application Specific Integrated Circuits). VHDL is commonly used in electronic design automation to describe digital and mixed-signal systems such as field-programmable gate arrays and integrated circuits. It allows for the description of systems at multiple levels of abstraction, from device-level logic gates all the way up to system-level architecture.

Ask the right questions secure the right VHDL talent among an increasingly shrinking pool of talent.

First 20 minutes

General VHDL app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in VHDL application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you define VHDL?

VHDL is a hardware description language used to describe digital and mixed-signal systems such as field-programmable gate arrays and integrated circuits.

What are the basic data types used in VHDL?

The basic data types used in VHDL are bit, bit_vector, integer, real, and Boolean.

What is the use of the 'entity' keyword in VHDL?

The 'entity' keyword is used to declare the interface of a design unit. It includes the name, input and output ports.

How would you declare a signal in VHDL?

A signal in VHDL can be declared using the 'signal' keyword followed by the signal name, its type and initial value. For example: signal my_signal: bit := '0';

What are the different types of operators available in VHDL?

VHDL provides a variety of operators. These can be categorized into seven types: logical, relational, shift, arithmetic, concatenation, and miscellaneous operators.

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What you’re looking for early on

Does the candidate have a solid understanding of VHDL language?

Has the candidate demonstrated problem-solving skills?

Is the candidate able to explain complex concepts in a simple way?

Has the candidate shown experience with hardware description languages other than VHDL?

Next 20 minutes

Specific VHDL development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

What is the difference between 'variable' and 'signal' in VHDL?

In VHDL, 'variable' is a storage object whose value can be changed within a process, while 'signal' is a storage object that retains its value until another value is assigned to it in the following process.

Describe the difference between 'component' and 'entity' in VHDL?

In VHDL, an 'entity' is a design unit that includes the name, input and output ports. A 'component', on the other hand, is a declaration within an architecture that allows an entity to be used.

How would you implement a flip-flop in VHDL?

A flip-flop can be implemented in VHDL using a process statement with a sensitivity list, where the flip-flop's state is stored in a signal and updated depending on the input signals.

What are the different types of modeling styles in VHDL?

The different types of modeling styles in VHDL are data flow modeling, behavioral modeling, and structural modeling.

What is the use of 'attribute' in VHDL?

'Attribute' in VHDL is a property or characteristic of an object or entity. It can be used to assign certain characteristics or properties to signals, variables, constants, etc.

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At this point, a skilled VHDL engineer should demonstrate strong problem-solving abilities, proficiency in VHDL programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with VHDL.

What does this simple VHDL code do?
entity HelloWorld is
end HelloWorld;
architecture Behavior of HelloWorld is
begin
process
begin
assert false report "Hello, World!" severity note;
wait;
end process;
end Behavior;
This code is a simple VHDL program to print 'Hello, World!' It's similar to the 'Hello, World!' program in other languages, used to illustrate the basic syntax of the language. It creates an entity 'HelloWorld' and an architecture 'Behavior' for that entity. The process block inside the architecture contains an assertion that always fails, causing it to report the message 'Hello, World!'
What does the following VHDL code do?
architecture Behavioral of mux is
begin
process(a, b, sel)
begin
if (sel = '0') then
z <= a;
else
z <= b;
end if;
end process;
end Behavioral;
This piece of code is implementing a multiplexer (mux). It creates an architecture 'Behavioral' of a previously defined entity 'mux'. Inside the process block, it checks if the 'sel' signal is '0'. If it is, the output 'z' is assigned the value of 'a', otherwise it is assigned the value of 'b'. This effectively allows 'sel' to select which of 'a' or 'b' is output to 'z'.
What does the following VHDL code do?
type array_type is array (0 to 7) of std_logic_vector(7 downto 0);
signal array_signal : array_type;
This VHDL code defines a new type 'array_type' which is an array of 8 elements, each of which is a std_logic_vector of 8 bits. It then declares a signal 'array_signal' of this new type. This could be used to represent a 8x8 bit matrix or a 64-bit memory block, for example.
What does the following VHDL code do?
entity counter is
Port ( clk : in std_logic;
rst : in std_logic;
count : out std_logic_vector(3 downto 0));
end counter;
architecture Behavioral of counter is
begin
process(clk, rst)
begin
if rst = '1' then count <= "0000";
elsif rising_edge(clk) then count <= count + 1;
end if;
end process;
end Behavioral;
This VHDL code implements a 4-bit counter with reset. In the entity declaration, it defines three ports: 'clk' and 'rst' as inputs and 'count' as output. In the architecture, it defines a process that is sensitive to 'clk' and 'rst'. If 'rst' is '1', the count is reset to 0. If there's a rising edge on 'clk', the count is incremented by 1.

Wrap-up questions

Final candidate for VHDL Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle VHDL application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

Explain the difference between 'blocking' and 'non-blocking' assignments in VHDL?

In VHDL, 'blocking' assignments are executed in the order they are written, while 'non-blocking' assignments are executed simultaneously.

How would you implement a state machine in VHDL?

A state machine in VHDL can be implemented using a process statement with a case statement inside it, where the different states are represented by different values of a signal and transitions between states are handled by the case statement.

What are generics in VHDL and how would you use them?

Generics in VHDL are parameters that can be used to customize an entity or architecture. They can be used to set the size of arrays, set the frequency of a clock signal, etc.

VHDL application related

Product Perfect's VHDL development capabilities

Beyond hiring for your VHDL engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in VHDL projects, and can engage in multiple capacities.

VHDL Consultive assessments

VHDL Application Conversion Assessment VHDL Application Integration Assessment VHDL Data & Database Integration Assessment

Custom VHDL development projects

Custom VHDL App Conversion VHDL Custom App Development Process App UI Design for VHDL Facelifts

Hiring guide for VHDL Engineers

VHDL Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

What you’re looking to see on the VHDL engineer at this point.

VHDL Consultive assessments

Custom VHDL development projects