XJTAG Blog » XJLink2

XJLink2 Diagnostics

Bob Storey — Mon, 08 Mar 2010 08:00:47 +0000

XJlink2 comes with a detailed self test program. Accessed via the “Test…” button in XJLinkManager this enables you to run a diagnostic report on your XJlink2.

When you first test a new circuit board that doesn’t work this diagnostics test can give you confidence that your XJlink2 hardware is working correctly. The test takes a few minutes to run but tests all of the internal features of the XJlink2 in detail. The test includes checks on:

all of the connector signal driver transistors
all of the signal input buffers
the connector power supplies
the internal signal communication buses
the internal clock sources
and the integrity of the internal program.
the push-button
and the LEDs.

You can optionally use a loop-back connector ( pin1 to pin11, pin2 to pin 12 etc) to provide enhanced testing of the connector itself.

If you think you might have damaged your XJlink2 then just run the self test. If a test fails please copy the entire report along with your serial number to support@xjtag.com so that we can help you.

The diagnostics feature is an excellent way to confirm that the XJLink2 is working correctly.

TCK Termination

Dominic Plunkett — Mon, 22 Feb 2010 08:00:54 +0000

We all pay close attention to the termination of high speed signals, especially clock signals. However in many of the designs I see this doesn’t seem to have happened for the JTAG clock signal TCK. But TCK is a clock signal just like any other clock signal.

Some people believe that because JTAG is only for test it doesn’t matter. Other people think that you can just reduce the clock frequency during testing to solve problems. But terminating the TCK signal correctly really does matter! High speed JTAG interfaces have a need to have fast rise and fall times to ensure good signal quality. It is these fast edges that cause problems with poorly terminated boards. The usual problem is a reflection causing an extra clock pulse. If you want to try and have a look at this on a scope you will need a fast scope (500MHz or greater bandwidth), a good probe and a very short ground connection (<1 inch). Turn up the timebase and look at the rising edge of the clock. In a good system the edge will rise continuously and have no dips on the way up to the logic 1 level. A poorly terminated system will have a rising edge and then a dip and continue rising again. This dip can cause an extra clock edge. JTAG interfaces with slow rise and fall times mask poorly terminated boards. This is why sometimes cheap low speed JTAG interfaces work and high speed ones don’t.

Correct TCK Termination

To correctly terminate a board, place a 100R resistor to ground at the end of the TCK net on the circuit board. Track the TCK net as a 100R track to each device in turn with out any stubs etc.
If you buffer the TCK signal, remember to terminate the TCK signal at the input to the buffer. Also each output of the buffer will need termination as above.

Dealing with poorly terminated boards

XJLink2 helps you to run tests on poorly terminated boards in two ways:

1) XJLink2 enables variable source termination impedances to match your board.
2) Variable Slew rates.

Variable source termination impedance match enable the XJLink2 to better match the termination on you board so if a 220R resistor is fitted for instance then you can select 220R as the source impedance. If you have no termination fitted then there is a “none” option as well.

The variable Slew rates enable the rise and fall times to be adjusted to improve the signal quality of the TCK. The slower the Slew rate the better the signal quality will be but the lower the maximum TCK frequency will be as well.

By adjusting the Source termination impedance and the Slew rate, XJLink2 can provide high quality signals across long cables into poorly terminated systems. For example, we recommend only using 15cm of JTAG cable; we have however used XJLink2 with over 4metres of cable at 20MHz, which was greater than the devices on the board were rated at.

Voltage Measurements with XJLink2

Dominic Plunkett — Mon, 08 Feb 2010 08:00:05 +0000

One of the exciting new features of XJLink2 is the ability to measure a number of voltages. Via the 20-way XJlink2 connector you can measure up to 18 different voltages. This means supply rail voltages can be measured before (and during) JTAG testing, and analogue measurements are now simple to make during your tests.

Many modern boards have a number of supply rails. Some of these need to be quite accurate to ensure stable operation. The XJLink2 is +/-( 0.2% + 10 mV) accurate, which is easily good enough to test supplies that have to be within 1% tolerance – we find most supplies have to be within 5%.

The input voltage range of the XJlink2 is 0 to 5 V. If you have a higher voltage rail then, then a potential divider will be required.

To use the voltage measurement you simply have to make sure the voltages / supply rails you would like to measure are brought out to spare pins on the XJLink2 connector.

You can take this a step further, If you what to check the current though an LED this becomes easy with XJLink2. Most LEDs have a series resistor to limit the current. If this resistor is between ground and the LED, you just need to measure the voltage across the resistor and do a little bit of maths with the resistor value to calculate the current in the LED ( V=IR ). If the LED takes the correct current then it is very likely to be working. LEDs not fitted / fitted the wrong way round / open circuit or short circuit will almost always give very different reading from the correct value. This might enable you to test the LED without asking the test operator if the LED lights.

This an example of how XJTAG can save you time and reduce the amount of test equipment you need during production testing and board bring up.

XJLink2 Frequency Measurement

Dominic Plunkett — Mon, 25 Jan 2010 08:00:48 +0000

XJLink2 is the only JTAG interface that can accurately measure frequency.

Why Measure Frequencies?

Often in products oscillators are used, for instance in USB you require an oscillator to be within 100 ppm. With crystals this is seen as easy, but it can be a little harder than you expect. A standard crystal may be accurate to +/- 30 ppm with a temperature stability of +/- 50 ppm and an ageing of +/-5 ppm per year. So in the worst case we are at 85 ppm after the first year. Now add on the effects from tolerances of the load capacitors (often 22 pF or 33 pF) and you might be over the limit!

Low power devices often use crystals which specify lower load capacitances but which are as a result more sensitive to errors in load capacitance. These errors can come from the capacitors themselves (tolerance), track layout and IC capacitance. At manufacture you might want to check the crystal is operating at the correct frequency or close to it. XJLink2 is able to measure up to 18 frequencies with an accuracy of 10 ppm. In fact, typically at room temperature this is 2 ppm.

For the above USB case we might in the factory be happy with a product that is +/-40 ppm. This still gives a little margin for temperature and ageing of the crystal. Frequency measurement will find errors such as wrong load capacitors or crystal used. If a wrong crystal is fitted it could be the wrong frequency part or more subtly could be a 12 pF load capacitance crystal used instead of a 33 pF load capacitance crystal.

If you have the wrong crystal (not frequency but load capacitance) the oscillator may not even start. More likely is the situation in which it will oscillate but at the wrong frequency. If it does this in the USB case the product may not enumerate correctly on all PCs to which the product is connected to, which might cause a customer support issue.

Measuring Frequencies

To measure a frequency all you need to do is connect up the signal you would like to measure to a spare IO pin on the XJLink2 connector. Don’t connect the crystal directly to the connector because the capacitive loading of the cable will change the frequency. Fortunately most devices that use crystals can output a frequency. It doesn’t have to be the same frequency as the crystal to measure the ppm error, so if for instance a processor has a crystal oscillating at 24 MHz but can output 48 MHz this will be fine for measuring the ppm error.

For EMC reasons you might like to turn off the output when not testing the product. You might use another spare output from the XJLink2 if required to enable the frequency only during testing.

XJLink2 button and LEDs

Bob Storey — Tue, 05 Jan 2010 08:00:31 +0000

The new XJlink2 has a button and some LEDs, which aren’t found on most JTAG interface devices. These are simply there to make life a little simpler on a crowded workbench. Once you have a project open in XJRunner you can start testing by just pressing the button – this can be quicker than finding your mouse which has at some point almost certainly moved off the on-screen Run button.
You get visual indication on the XJLink2 if the test has passed or failed with the red and green lights.

Test jig integration

Some of our customers built the original XJLink into custom test rigs. The XJLink2 goes further in supporting this kind of customisation: the button signal and the LED signals can be replicated on the 20-way connector. In the Pinmap settings for your project just enable the advanced option and you are able to assign these signals to spare I/O pins. You are then able to wire this up however you like. For example, you may put a button on the front panel of a test jig and use that to start the tests. In addition, the LED signals can be replicated to the front panel of the test jig or to an alarm if required. If you have an externally controllable PSU you may connect the Yellow ( running ) light so that only when a test has started will the PSU be turned on. Once the test is stopped the PSU will automatically be turned off.

The new XJLink2

John Barton — Mon, 14 Dec 2009 08:00:31 +0000

The new XJLink2 USB-to-JTAG interface is beginning to make its way out to our distributors and depending on where you are is, or shortly will be, available for trial and purchase.

The XJLink is the interface between your board and the XJTAG software, and this second-generation version has many improvements over the original XJLink – many of which will be covered in detail in future blog entries. The original XJLink is not being deprecated – it is still in production and ships with our standard development systems, whilst the XJLink2 is an addition to our product portfolio, and ships when you buy one of our systems with “XTR” at the end of the name.

In addition to the original XJLink’s features, the XJLink2 supports:

4 JTAG TAP connections can be made from the XJLink2 to your board
Run Tests button – with XJRunner or XJDeveloper open, hit the button on the XJLink2 to start tests.
Status LEDs – to indicate tests running/passed/failed
Programmable I/O – pins on the XJLink2 which are not part of the JTAG signals to the board can be controlled during XJEase testing as well as during Test Reset sequences
Two voltage banks – variable between 1.1 V and 3.3 V.
Built-in Voltmeter capability on all I/O pins, accessible during XJEase tests
Frequency measurement capability on all I/O pins
Higher TCK frequencies – up to 166 MHz if your board supports it
Adjustable termination on JTAG TAP signal

We will be issuing a full press release for the XJTAG XTR systems in the near future; in the meantime your distributor or local sales rep should be able to give pricing information or get any other information you need.

Highlights of the 2.3 Release

John Barton — Mon, 31 Aug 2009 07:00:29 +0000

This post outlines what is new in the 2.3 release. Some aspects will be explored in more detail in future entries.

Development of 2.3 has mainly been driven by the design of the new XJLink2:

The JTAG Chain Debugger supports configuring the new pinmap used by XJLink2. The pin mapping is much more configurable for an XJLink2. This will be covered by future blog entries.

XJDeveloper is also capable of configuring the new XJLink2 pin mapping. As the XJLink2 supports up to four JTAG chains the JTAG Chain Screen has been redesigned to support this.

XJRunner supports running tests on an XJLink2 by pressing the button on the unit. If you have several XJLinks you can run through a batch of boards continuously. When one board has finished being tested unplug it and plug a new one in and press the button on the unit (or screen) to start testing the new board whilst tests are still running on your other XJLinks.

The XJEase language now has two new keywords to support the new features of the XJLink2. PIN_FREQUENCY will return the frequency in Hz of an XJLink2 PIO pin, and PIN_VOLTAGE will return the voltage in mV.

XJDeveloper has a new BSDL File Editor to view and edit your BSDL and package files. Like the XJEase editor it has syntax highlighting and supports code folding.

XJRunner has a new Analysis feature which gives statistics about the percentage of boards that have failed as well as Fault Trend Analysis which highlights which tests are more prone to fail. There is also a log file viewer to show the complete current log file as well as allowing older log files to be viewed.

On top of all this there have been literally hundreds of minor tweaks and usability enhancements. We are constantly striving to provide the best possible user interface to enable you to work more efficiently. As always we welcome any feedback about where you feel we could improve.