Interviewing for an entry-level embedded systems engineering position

In my current position, I am often tasked to interview recent college graduates for our internships and also for our entry-level embedded systems positions. Since we are a relatively small company, we are looking for well rounded engineers with some general understanding of embedded software as well as circuit design. We need our new-hires to help designing new circuits at the schematic level, develop software for various micro-controllers and debug faulty circuit boards returned from the field.
Over the last couple of months I have interviewed dozens of candidates and I was shocked to see that the vast majority of these engineers, many coming out of prestigious universities, are unable to answer the most basic questions. I understand that job interviews can be a stressful event, but some of the most basic concepts should be second nature to anyone with an electrical/ computer engineering degree. In this post I will outline our technical interview routine, which does not differ much from other embedded systems engineering entry-level interviews. While the human-resources department is mainly interested in personality fit, the role of the technical portion of the interview, is to assess the candidate skills.

Unsolicited advice from the other side
For the young engineers out there interviewing, I have 4 general suggestions:
1- During the interview, do not drop industry-buzzwords that you don’t fully understand what they mean. For example, don’t say that you love chips manufactured by ARM. ARM does not make chips.
2- Come prepared with a portfolio with some of your work (e.g. source code, circuits, layouts, relevant blog posts) and leave a copy with the interviewers.This makes you look professional and organized.
3- In a technical engineering position no one is interested how cool the candidate was in college or how smart the candidate is for wanting to go to graduate school to learn more. In most companies there is a need for dedicated engineers to design, implement and maintain a project in its entire life-cycle. We simply have no time to train someone for a couple of months, so that they can jump-ship in six months to go to graduate school.
4- Go to interviews dressed business casual. I have noticed that the strongest candidates are usually the ones who don’t over-or-underdress.

Resume skill check
When we start the interview process, the first thing we do is re-read the skills section. If the candidate lists a particular skill, we will inevitably ask some very basic questions about it. For example, if C/C++ is listed as a skill, we generally ask the candidate to tell us the difference between C and C++. Unfortunately, only about 25% of the candidates know this.The simplest answer, as defined by the creator of C++ himself (Bjarne Stroustrup), is that C++ is C with classes.This would be the perfect time to talk about how C++ is an object oriented language and that objects allow the programer to secure (hide) data inside classes.The diligent engineer could also talk about how in C++, two functions can have the same name but different arguments (function overloading) and how variables in standard versions of C need to be declared at the start of each function. Similarly, if you do not understand the difference between Linux and Unix, do not put both on your resume.

Here are some of the commonly listed skills and some possible simple questions about them:
Matlab or Mathematica – Can you put 10 numbers in a 1D Matrix (orTable) and compute their average?
Linux – What is your Linux distribution? What command do you use to create and change a directory?
ASM – What does the command JMP do? What about MOV?
Python or Perl – Can you create a for loop that prints “hello world” 10 times?
HTML What does the and tag to?
Spice – Can you write a spice file with a circuit with a power supply and a single resistor?
Labview – Who makes Labview?

Finally, my recommendation is to avoid listing skills that you don’t fully understand what they mean. One of our candidates listed Xilinx as a software skill.This particular candidate not only did not know that Xilinx was a company, but he also could not remember the purpose of that particular piece of software (“I think it was to program some sort of chip.”).

Assessing the candidate interest in embedded systems
We are always interested in candidates with a passion for anything related to embedded systems, as these tend to be the strongest engineers. We often ask them where they buy their components, what electronics magazines they read and which websites they frequent. We also like to know what is their favorite prototyping system (e.g. arduino, raspi, beagleboard) and favorite programming language.We also like candidates to tell us about how they normally go about testing and implementing their prototypes. For example, how would they go about etching a PCB in their basement? While the answers to these questions are far from being deal-breakers, they do give us an insight about how passionate the candidate is about the field.
We also like our entry-level candidates to know a bit of whats out there. For example, what are the names of some companies that manufacture micro-controllers and the names of their products (e.g. Atmel manufactures ATmega328P which is used in the Arduino prototyping system).

The deal breakers
We have, what we like to call the “five deal breaker questions”. If candidate misses one of them, we cannot in good conscience hire them.
Question #1 – The most fundamental question regarding electrical circuit theory is how to measure current and voltage in a simple circuit. In the interview room we have a very simple circuit with a single resistor in a prototype board. After we power the circuit we give the candidate a digital multimeter and ask them to measure the voltage across, and the current through, the resistor; Voltage levels are measured by connecting the two probes across the resistor, while current is measured by opening the circuit and completing it with the multi-meter connected in series. Surprisingly about 10% of the candidates miss this question.

Question #2 – We need to know how confident the candidate is with basic circuit analysis. We draw on the board a schematic very similar to the one shown on Figure 1 and ask the candidate to determine the measured voltage across R2.


Figure 1 – Interview question: Determine the voltage across R2.

A confident engineer does not have to make any calculations.Two equal resistors in series with 10V across them, means that the voltage across just one of these resistors is 5V. The candidate can also determine this voltage by “hand”:

Ohms law implies that,
V1 = i (R1 + R2)
10 V = i (100 + 100)
So the current through R1 and R2 is 10 / 200 Amps or 20 mAmps.
VM = i (R2) = 10 / 200 * 100 = 10 / 2 = 5 Volts

One quarter of our candidates, most with degrees in electrical engineering had trouble with this question.

Question #3 – We also ask very simple practical design questions. For example, when should you use a transistor instead of a relay in a design? There are many possible answers here, and we are just looking for the basic ones. For example, you can mention that the relay, since it is an electromechanical switch, is much slower than a transistor, but it can usually carry a lot more current. Relays in general are also much bigger than transistors.

Question #4 – It is fundamental that our new hires know the purpose of basic circuit concepts. Here are the three short-answer questions we always ask:

Q:What is a servo motor?

It is a motor in which you can control the precise position of its shaft through a well-defined pulse.

Q:What is a potentiometer?

It is a resistor whose resistance can be changed by adjusting a knob, usually with a small screwdriver.

Q:What is does it mean for a signal to have 30% duty cycle?

It means the signal is in its high-state 30% of the time.

Question #5 – Finally, we always ask the candidate to explain what are the defining characteristics of an OP-AMP. Most candidates are able to tell us that the V+ and V- inputs have high impedance and they are often able to write out the (usually memorized) equations for OP- AMP circuits with a feedback loop. Unfortunately they can’t do much more than this, and the concept of the OP-AMP rail is often not understood.

We then ask the candidate to determine the output voltage of an ideal OP-AMP with a “comparator” configuration, exemplified in Figure 2.

The voltage across R1 is 15V. The two inputs V1 and V2 are compared and the output voltage will be set by the appropriate rail supply, which in this example is the one associated with the negative input (V3).


Figure 2 – Interview question: Determine the voltage across R1.

Alternatively we also ask about the purpose of the voltage follower configuration, shown in Figure 3.


Figure 3 – Voltage follower op-amp configuration: If the voltage V1 is both at the V- and VM1, what is the purpose of this configuration?

The voltage follower with an ideal OP-AMP gives simply V1 = VM1. However, because the high input impedance, the OP-AMP output (VM1) is isolated from the OP-AMP input (V1). In essence the current drawn from the circuitry connected to the output is not drawn from the signal source at the OP-AMP input, but from the OP-AMP rails.

Basic Embedded-C
We need our candidate to know how to program in C.There is no work around this one; the embedded systems engineer must know how to handle the most basic programming tasks.
Question #6 – The first programming question is always about distinguishing between the ‘=’ and ‘==’ operator (‘=’ is assignment, while ‘==’ is a comparator).
Question #7 – We then ask the candidate to write out the C-code that solves the commonly used fizzbuzz problem.The assignment is as follows:
“Write a program that prints the numbers from 1 to 100. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”.”

While this is not a particularly hard, most of our candidates were unable to solve this assignment. Here is a possible solution:

We always ask something related to C-pointers. We understand that pointers can be a bit confusing to engineers, but the candidate must have at least a very general concept about what a pointer is.

Question #8 – Consider an integer k. How do you change its value through a pointer?

If the candidate shows confidence with pointers we normally ask slightly more advanced questions. For example,

Question #9 – What is the benefit of passing elements to a function by reference instead of by value? Can you write a small piece of code that demonstrates passing by reference?

Every time something is passed into a function by value, it is pushed down into the stack. If we are sending a structure with 10,000 members, all those members will be pushed down, needlessly using a lot of memory space.

Intermediate Embedded Systems topics
At this point we have a pretty good idea about the candidate and his abilities. If time allows we ask slightly more complex questions, allowing us to differentiate between the strongest candidates.

Question #10 – Consider the generic schematic on Figure 4. In this circuit, the solenoid L1 has been energized for a long time and suddenly we open the switch SW1. Can you foresee any problem? What can be done to fix it?


Figure 4 – Interview question: What might happens when we suddenly open SW1?

When the solenoid L1 has been energized for a long time, it behaves as if it were a short with current flowing from the positive terminal of the voltage source to its negative terminal. When the switch is opened, the inductor will attempt to resist the sudden drop of current. A large negative potential is created where there once was positive potential, and a positive potential is created where there was once negative potential. This large potential difference can cause the electrons to “arc” across the switch.
A flyback diode solves this arc problem by allowing the inductor to draw current from itself in a continuous loop until the energy is dissipated through losses in the wire and across the diode. This is shown in Figure 5.


Figure 5 – Correct location and orientation of a flyback diode.

Question #11 – What is the purpose of an H-Bridge? Can you draw a generic H-bridge schematic?

An H-Bridge is a circuit that allows us to control the direction of current flowing through a solenoid (e.g. a DC motor). Figure 6 shows how to control the direction of the current through L1 with the state of the switch SW1.


Figure 6 – Generic implementation of an H-Bridge using npn-transistors.

Question #12 – Consider the very generic circuit on Figure 7. What happens when SW1 changes states?

Whenever SW1 is open, no current will flow into the base of the Q1 npn transistor, so the bjt will be OFF. This means both inputs of the NAND gate will be pulled to ground (which is a zero binary sate). Two logic-zeros at the input of a NAND gate imply a logic-one at the output. Since the output of the NAND gate is at the same potential as V1, no current will flow through R2 and D1, which means the LED will be off.
If SW1 is closed, the Q1 bjt will be ON, and the inputs of the NAND gate will be set at logic-ones. This implies a logic-zero at the output of the NAND gate, which in turn enables current to flow through R2 and D1.


Figure 7 – Interview question: What happens to D1 whenever SW1 is pressed?

Question #13 – What is an interrupt?

An interrupt is the result of an event that tells the microprocessor to stop executing the current task, when appropriate, and execute another piece of code, called the interrupt service routine (ISR).

Question #14 – What is RS232?

RS-232 is a communication standard for a series of standards for serial binary signals. This standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pin out of connectors.

  • In RS-232, user data is sent as a time-series of bits.
  • Both synchronous and asynchronous transmissions are supported.
  • The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the signal: valid signals are plus or minus 3 to 15 volts; the ±3 V range near zero volts is not a valid RS-232 level.
  • The devices at both ends of a RS-232 compatible cable agree on the bit transmission rate (e.g. 9600 baud, which corresponds to 9600 bits transferred per second).
  • Conclusion
    Even with a relatively straight forward interview process, with a strong emphasis on the basics, we have had a difficult time recruiting skilled recent graduates. For those looking for a entry-level position, I strongly recommend to come prepared to the interviews. Spend a couple of days solving technical exercises straight from inexpensive books such as:

  • Schaum’s Outline of Electronic Devices and Circuits, by Cathey
  • Schaum’s Outline of Basic Electricity, by Gussow
  • Practical Electronics for Inventors, by Scherz and Monk
  • In addition keep reading and subscribing to magazines such as Elektor, Make, Circuit Cellar and, of course, Nuts And Volts.