System Test using JTAG

Chintan Panchal, Parth Rao (Einfochips)

Abstract:

Boundary Scan technique is most often thought of as a board-level test method, but certain techniques makes system level test with JTAG quite effective. Many types of faults can arise when systems are assembled. JTAG testing techniques are well suited to finding and diagnosing many of these problems. For example, a connector may not be making good electrical connect or a connector’s pins might be damaged during assembly, some parts of device might be missing or not functioning properly. This paper will show the certain techniques like “Ring Architecture”, “Star Architecture” and “Multi-drop Architecture” for System level test using JTAG. Though the JTAG IEEE 1149.1 standard is widely used for board level test, this paper would show the applications of the same standard at the system level test. The system level test using JTAG techniques can be very helpful to support and maintenance personnel long after a system has been assembled in a manufacturing environment and installed at a customer location.

I. INTRODUCTION

What is system test and why would you want to do it?

To begin the discussion of System level testing using JTAG standard, certain basic aspects of system test must be cleared like what is system test and how it is distinguished from board level test. A system can be defined as “a group of interrelated, interacting, or interdependent elements forming a complex whole” [1]. System can be simply defined as the connection or combination of more than one board, to accomplish certain task. This could be an assembly as simple as a mother/daughterboard combination or an extremely complex backplane-based computer with hundreds of boards. So, with the help of this definition, it can be said that an electronic system is a group of interconnected devices and boards that function together to accomplish a task. The basic goal of system test is to verify the structural integrity and/or functionality at the level of the complex whole, though the components that comprise an electronic system may be tested individually without regard to the task of the entire system. At a fundamental level, testing a system’s structural integrity means to verify that each component is properly connected to all other components that require such connections.

II. DFT TECHNIQUES

Why systems test?

Most electronic products can be seen as a system. Even if all of the system’s components are fully tested individually, the system still may not function properly. Many problems can arise when systems are assembled. Boundary scan techniques can provide the possible solution to find out and diagnose these problems. For example, a connector may not be making good electrical contact or a connector’s pins might be damaged during assembly. Boards on a backplane may be missing, out of place or simply not functioning properly. Functional tests can identify that the system is not functioning as per the expectation, but isolating and diagnosing the fault with functional tests would be very time consuming and often would require high level system expertise to troubleshoot the cause.

In addition, boundary scan techniques can also perform various operations apart from the test functions. Besides structural integrity tests, boundary-scan can also perform on-board programming of flash memory, in-system configuration of PLDs, on-board programming with I2C data and emulation-based functional testing through a processor’s JTAG debug port. Beginning with system-level debug and production test, boundary scan provides many other system benefits when it is extended to field support. For example, boundary scan can be used for quick and easy updates of programs stored in flash memory or updates to system functionality in the field through the re-configuration of on-board PLDs [2].

III. ARCHITECTURES

How is system test implemented with boundary scan?

Three primary methods are available for implementing boundary-scan-based system tests. All of these methods involve connecting an external boundary-scan test tool to the system.

Ring Architecture:-

The first external method connects all of the boundary scan paths on all of the boards in the system to a single scan path with one point of access for the boundary scan test system (Figure 1). This method requires that the configuration of the system must remain exactly the same in every assembled product. Also, any break in the single scan path will disable all boundary-scan test access. One very long scan path can also result in slow access times.

Fig. 1: Ring Architecture [3]

Star Architecture:-

The second method of testing system is the star architecture. In a backplane star configuration as shown in figure 2, the backplane drives TCK and TDI directly to all boards and each board outputs a TDO signal to the backplane. Each board requires a unique TMS signal from the backplane and a separate circuit trace for each signal. During testing, only one board is enabled at a time and only the TMS signal for that board is active. With this architecture, virtually all chips can be tested independently. STAR configuration is used to test the connectivity between IC and PCB and its separate functionality at a time.

The advantage of the ring and star architectures is that they do not require any additional components or new protocols beyond what is required by the boundary-scan specification. This makes them straightforward to implement, but in a large multiple-board system they are often too cumbersome to use [3].

Fig. 2: Star Architecture [3]

Multi-drop Architecture:

The third method is to design a method for controlling access to the scan paths on the individual boards from one external JTAG port (Figure 3). A multi-drop gateway device must be inserted on the board which controls the board for the purpose of testing system. A multi-drop architecture uses only one set of 1149.1 data and control signals which is wired in parallel to each board in the system. An addressable gateway device must be implemented on each board, to ensure that boundary-scan operations are applied to only one board at a time. A special selection protocol is applied to the boundary-scan bus by the system-level boundary-scan controller to connect the chosen board’s scan chain to the bus. This solves the problems mentioned above (in Ring and Star configuration) at the expense of extra selection logic in each board.

Fig. 3: Multi-drop Architecture [3]

There are commercially available multi-drop gateway devices, including the following:

Several methods are available for controlling multi-drop devices and the secondary scan paths associated with them. In some cases, a certain scan path configuration may be needed for several test operations. Or, the scan path may be dynamically reconfigured before an action can be applied. In either case, a particular scan path configuration would be maintained until it is explicitly changed by the Scan Works or until a JTAG Test Logic/Reset operation occurs.

The table below provides a summary of the advantages and disadvantages of the three system-level test methods [2].

IV. CONCLUSION

Boundary Scan testing has established a strong position in the board level testing. As more and more board-level boundary scan tests are developed, the feasibility of implementing system level test using JTAG (Boundary Scan) techniques increases because of the requirement of testing at the system level. It is important that the board and system designers are aware of the benefits and effectiveness of extending the IEEE1149.1 standard in a system test environment. The architectures discussed in this paper have been readily adopted by the leading silicon vendors of the system test access devices so that the system architects have far more flexibility in implementing the system level test strategy. So this would be considered as solutions to the system level testing.

REFERENCES

[1] Wordnet Dictionary

[2] ASSET Whitepaper : system Test with Boundary Scan (JTAG)

[3] VLSI Test Principles and Architectures Design For Testability, Laung-Treng Wang, Cheng-Wen Wu, Xiaoqing Wen

[4] Design Considerations In Using 1149.1 As a Backplane Test Bus, Pete Collins, petec at jtag.co.uk, JTAG TECHNOLOGIES

Coronavirus: Test and trace system will start on Thursday

By James GallagherHealth and science correspondent

A massive system to find people who come into close contact with those infected with coronavirus will start in England on Thursday, Boris Johnson has said.

The prime minister said it "will change people's lives".

The aim of the test and trace system is to move from lockdown for all towards more targeted measures.

However, scientists have warned it is not a "magic bullet" and may prevent between 5% and 15% of infections.

Northern Ireland has its own version up and running, Scotland has announced its own system will start on Thursday and Wales' system is due to start in early June.

As is currently the case, anyone who develops symptoms of coronavirus - a persistent cough, fever or a sudden loss of taste or sense of smell - will have to isolate for seven days and the rest of their household for 14 days.

The difference is that from Thursday, everyone with symptoms should ask for a test online or call to arrange a test by calling 119.

If the test comes back negative, everyone in your household can go back to normal.

But if the test comes back positive, the NHS Test and Trace team will get in touch - via text, email or phone call - to discuss whom you have come into close contact with.

Any of those contacts deemed at risk of catching the virus will be instructed by the NHS to go into isolation for 14 days, whether they are sick or not.

They will be tested only if they develop symptoms. The rest of their household does not have to isolate, unless someone becomes ill.

Those isolating will be eligible for statutory sick pay.

Health Secretary Matt Hancock told the daily Downing Street briefing this "must become a new way of life" and will require a "national effort" - otherwise lockdown would have to continue.

"It is your civic duty, so you avoid unknowingly spreading the virus and you help to break the chain of transmission," he said.

He also confirmed that, from Thursday, anyone with symptoms - including the under-5s - can be tested for coronavirus.

Earlier, Mr Johnson told MPs on the Commons Liaison Committee that the government "will consider bringing in financial sanctions" if people do not self-isolate.

Only people who have been "close" to an infected person will be contacted by NHS Test and Trace.

It will not be the case that one teacher testing positive would lead to the whole school going into isolation, for example.

Close contacts are:

The contact must have taken place between two days before and up to seven days after symptoms appeared.

At the moment coronavirus is being suppressed by lockdown restrictions which are applied across the whole of society.

The ambition is to pivot from lockdown for the many to isolation for the few. How far that can go will depend on the effectiveness of the scheme.

Baroness Dido Harding, the chairwoman of NHS Test and Trace, said: "[It] is designed to enable the vast majority to get on with their lives in a much more normal way... trading national lockdown for individual isolation."

Test and trace will also give far more detail on how and where the virus is spreading. Mr Hancock said this information could lead to local lockdowns to tackle flare-ups in towns, schools or workplaces.

A report published earlier by the Royal Society said the success of the scheme would depend on how quickly contacts can be found and whether the public buys in.

It concluded that between 5% and 15% of infections could be prevented. But the 15% figure depends on finding contacts within three days and 80% of people reporting their symptoms or going into isolation when asked.

Prof Anne Johnson, one of the report's authors, said: "It is an important part in bringing the pandemic under control, but we're very clear it is not a magic bullet."

This is a big ask - there will be a lot for people to get used to.

First, if you test positive for the virus you will be expected to pass on contact details of people you have recently met. Some may be concerned about that though NHS leaders emphasise that confidentiality and data security will be rigorously observed.

On the other hand you may get a call from a contact tracer and advised you are at risk because of a recent encounter with someone with the virus. And even if you are fit and well you will still be told to self-isolate for two weeks.

Health Secretary Matt Hancock has called on people to do their civic duty. The big question is whether sufficient numbers will respond positively to that.

Sanctions have not been ruled out if they don't. There's a lot riding on policies like this to be adopted in every part of the UK, not least as a vehicle to help each nation out of lockdown.

Instead, social distancing, hand hygiene and isolating everyone who gets ill will have a far greater effect, the report said.

By comparison, isolating cases and quarantining members of their household is thought to cut infections by 50%.

And scientists, including those advising the government, have warned that the row over the PM's chief adviser Dominic Cummings will make it harder to get public support such measures.

The government says it has 25,000 contact tracers ready to go and that it has plenty of testing capacity.

The contacts of 2,000 people who have just tested positive for coronavirus will be chased on Thursday.

However, the real pressure will be on the speed of testing.

Prof John Newton, the national coordinator of Test and Trace said: "In order to be successful it requires rapid testing."

Mr Johnson said on Wednesday there was now a target to get test results in less than 24 hours.

There is a particular problem with home testing kits, which have to be posted out. Prof Newton said it would be "very difficult" to get those results in less than 48 hours.

The NHS contact tracing app is not ready to roll out and was described by Mr Hancock as a "complement". He denied technical issues were delaying its roll out.

In other developments:

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