Articles

14.05.2013

SAU510-USB Iso Plus - one of the most reliable XDS510 class emulators

For 5 years (2005-2008) we have been producing SAU510-USB emulators (including “Plus” version) without galvanic isolation. Long-term operating experience of this emulator in various conditions proved the emulators to be highly reliable although there were some shortages revealed in that old version of emulators. Such shortages primarily include a quite low hot JTAG connection stability which despite of being explicitly prohibited in the documentation is commonly practiced by engineers, and an insufficient stability in case of reverse JTAG connection which, despite to the guide pin present in the connector is, unfortunately also a common practice. In view of these shortages there was SAU510-USB Iso Plus emulator developed in 2008. This version incorporated unprecedented measures on protection of the emulator electronics against external influences.

The analysis of failures revealed during maintenance of emulators of the previous version (without galvanic isolation, SAU510-USB Plus) showed the following statistics:

Cause

Failed assembly

% of all failures

Exposure of JTAG circuits to high voltage

JTAG buffer circuits

47%

Exposure of JTAG circuits to reverse voltage

JTAG buffer circuits

39%

Exposure to current generated due to a difference of potentials between the ground terminals of the device being debugged and those of PC

Complete circuit breakdown

8%

Exposure to voltage in excess of the permissible USB power supply voltage limit

Voltage regulators, other components in the worst case

5%

Other causes (mechanical impact, failure of parts for no apparent reasons, misc)

1%

In view of these statistics we have upgraded and/or integrated the following units in SAU510-USB Iso Plus emulators:

Galvanic isolation unit

There is a variety of projects where it is impossible to provide a proper ground connection between the device being debugged and the debugger host PC common wire. This causes a high difference of potentials between the same: each engineer knows what a spark jumps when interconnecting ground terminals of two different switched-on devices equipped with switch-mode power supply units and unconnected to the common grounding bus. A difference of potentials in this event constitutes several hundreds of volts and even more in some cases, and this is not a direct but pulse voltage that not direct but DC voltage but pulse voltage that seriously interferes with a propagation of signals between the PC and the device being debugged. In the best case the operation of the emulator that does not have a galvanic isolation will be unstable in such conditions and in the worst case, especially in case of an accident hot connection, the emulator may fail completely (those 8% of the statistics).

In order to ensure a comfortable and safe operation in such conditions we have a galvanic isolation unit implemented in SAU510-USB Iso Plus emulator to enable isolation of high speed JTAG signals and power supply of JTAG buffer amplifiers:

galvanic_isolation.JPG

This unit enables operation with a difference of potentials between the PC and the device being debugged reaching 2500V with a low input-to-output capacitance, particularly below 70 pF, of which 60 pF is applied to buffer power supply isolation.

JTAG interface protection.

The failure statistics showed how frequent are failures of emulators (old versions of SAU510 as well as emulators of other manufacturers) caused both by the reverse polarity of signals transmitted to JTAG connector and the excessive voltage supplied to JTAG. Following the studies of failure causes and the activity of the above signals causing failures there was a highly reliable two-level JTAG interface buffer protection circuit developed and implemented.

Integral_ESD.JPG

Such structure of JTAG circuit signals protection enables the reliable protection of emulator circuits against failures, momentary static discharges or pulse interference as well as against a long term signal injection beyond the permissible limits. The first protection level (integrated elements and R resistors) absorbs short-term effects while the second level (VD zener diode and the same R resistors) being more powerful than the integrated protection element protects the circuit against long-term effects.

USB interface protection.

The practice showed that the standard protection of USB signal circuits appeared to be quite efficient as there were no failures of emulator USB signal circuits revealed during the operation period.

However it was found that there is a certain number of failures caused due to an emulator being supplied from USB bus with voltage far beyond the USB rated limits. We cannot guess the real causes of such events which look quite weird and unexpected however they happen often enough and share 5% of all old emulator failures. Based on this fact it was decided to enhance the USB power protection. There was a more powerful power protection element installed in SAU510 Iso Plus emulators and a thermal resistor (resettable fuse) added to SAU510-USB Iso v.2 and SAU510-USB Iso Dot.7 emulators of the next generation.

Due to the general-circuit design measures implemented at present there have been no failures of emulators of this model which could be caused by exposure to any electrical signals recorded during the whole SAU510 Iso Plus emulator operation period of five years from the year 2008 through the year 2012. Thus, today SAU510-USB Iso Plus JTAG emulator can be recognized as one of the most reliable models among the entire range of JTAG emulators available in the market!


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