Interview

20 JTAG Interview Questions and Answers

Prepare for the types of questions you are likely to be asked when interviewing for a position where JTAG will be used.

JTAG, or Joint Test Action Group, is a hardware interface specification that is commonly used in electronic equipment and boards. If you are interviewing for a position that involves working with JTAG, it is important to be prepared to answer questions about your experience and knowledge. This article reviews some common JTAG interview questions and provides tips on how to answer them.

JTAG Interview Questions and Answers

Here are 20 commonly asked JTAG interview questions and answers to prepare you for your interview:

1. What are the main components of a JTAG interface?

The main components of a JTAG interface are the TAP controller, the TAP state machine, and the JTAG instruction register. The TAP controller is responsible for controlling the TAP state machine, which in turn controls the movement of data through the JTAG instruction register. The JTAG instruction register is a special register that is used to store the instructions that are used by the JTAG state machine.

2. Can you explain what boundary scan is in context with JTAG?

Boundary scan is a feature of JTAG that allows for the testing of interconnects between devices on a board. By using boundary scan, you can test for shorts, opens, and other defects in the connections between devices without having to physically access the devices themselves.

3. Why do we use JTAG?

JTAG is a standard for connecting debugging and programming interfaces to a CPU or other chip. JTAG can be used for boundary scan testing, programming flash memory, and other debug functions.

4. How does JTAG work?

JTAG is a standard for connecting debuggers and other testing equipment to processors and other chips. It uses a 4-pin connector to provide a connection between the chip and the debugger. JTAG is typically used for boundary scan testing, which can test for shorts and open circuits on a board.

5. What makes up the JTAG chain?

The JTAG chain is made up of four main components: the Test Access Port (TAP), the Test Instruction Register (TIR), the Test Data Register (TDR), and the Test Output Register (TOR). The TAP is used to control the flow of information between the other three registers, while the TIR contains the instructions that tell the TDR what data to output. The TDR itself contains the data that is to be output, and the TOR outputs that data to the device under test.

6. Is it possible to add more devices to an existing JTAG chain? If yes, then how?

Yes, it is possible to add more devices to an existing JTAG chain. This can be done by adding more devices to the TDI or TMS lines of the JTAG interface.

7. Can you give me some examples where JTAG has been used?

JTAG is often used in the development and debugging of electronic hardware. It can be used to test individual components on a board, to test interconnections between components, or to test the board as a whole. JTAG can also be used to program flash memory or other types of non-volatile memory.

8. What’s the difference between JTAG and IEEE P1149.1?

JTAG is a standard for testing and debugging electronic hardware, while IEEE P1149.1 is a standard for testing and debugging printed circuit boards. JTAG can be used to test and debug both digital and analog hardware, while IEEE P1149.1 is limited to digital hardware.

9. What is its relationship with BGA packages?

JTAG is a type of boundary-scan testing that is often used for BGA packages. JTAG can be used to test the connection between the BGA balls and the pads on the PCB. JTAG can also be used to test the connection between the BGA balls and the solder joints.

10. In your opinion, why isn’t there much standardization for JTAG interfaces?

JTAG is a very complex technology, and as such there is a lot of room for variation in how it is implemented. This lack of standardization can be a problem for companies who want to use JTAG to test their products, as it can be difficult to find compatible equipment. However, there are some efforts underway to try to standardize JTAG, which should help to address this issue in the future.

11. What happens if the TCK clock stops working?

If the TCK clock stops working, then the JTAG controller will be unable to communicate with the JTAG devices. This can cause a number of problems, including the inability to program or debug the devices.

12. What is the purpose of the TMS pin on the JTAG header?

The TMS pin is the Test Mode Select pin, and it is used to select the operating mode of the JTAG controller. There are four different modes that can be selected: reset, idle, instruction fetch, and data register access. The TMS pin is used to cycle through these different modes.

13. What are some common issues that can occur when using JTAG?

Some common issues that can occur when using JTAG include:

– JTAG pins can become damaged or broken if they are not handled carefully.
– If the JTAG cable is not properly shielded, it can pick up electromagnetic interference (EMI) that can corrupt the data being transmitted.
– JTAG can be slow, especially when trying to access large amounts of data.

14. Can you explain the importance of the TDI pin on the JTAG header?

The TDI pin is the Test Data In pin, and it is used to send test data into the JTAG device. This data is then used by the device to perform various tests, such as boundary scan tests. Without the TDI pin, it would not be possible to perform these tests.

15. Can you explain how JTAG is used to debug hardware problems?

JTAG is a technology that is used to debug hardware problems. It is typically used in conjunction with a debugger, which is a piece of software that allows you to step through code and examine the state of the system at each step. JTAG can be used to debug problems with hardware that is not working correctly, or to test new hardware designs.

16. What happens if the JTAG device IDCODE doesn’t match the expected value?

If the JTAG device IDCODE doesn’t match the expected value, it could be indicative of a hardware issue. In this case, it would be best to check the JTAG connection and make sure that everything is properly connected. If the issue persists, then it is possible that the JTAG device itself is defective and will need to be replaced.

17. What are some limitations of JTAG?

JTAG is not well suited for testing digital circuits with a large number of inputs and outputs, or for testing circuits with a high clock speed. JTAG is also not well suited for testing circuits with a large number of memories or embedded processors.

18. Do you think there will come a time when JTAG becomes obsolete? If so, what would be a better alternative?

JTAG is a very powerful tool, but it is not without its limitations. There are some newer technologies that are starting to emerge that could potentially replace JTAG in the future. These include FPGA configuration over Ethernet (FCOE) and JTAG over IP (JoIP). While JTAG is still the standard for debugging and testing today, it is possible that one of these newer technologies could eventually take its place.

19. What is multi-core debugging?

Multi-core debugging is the process of debugging code that is running on multiple cores simultaneously. This can be a challenge because each core will be running its own code and may be accessing shared resources. In order to debug this type of code, you need to be able to see what each core is doing and how it is interacting with the other cores.

20. Can you explain the difference between active and passive probes?

Active probes contain their own power source, which means they can draw power from the target device to operate. Passive probes do not have their own power source, so they must be powered externally. This can make passive probes less expensive and easier to use, but they may not be as reliable as active probes.

Previous

20 Data Center Interview Questions and Answers

Back to Interview
Next

20 Service Catalog Interview Questions and Answers