1149.6 testing is coming of age, and though XJTAG has supported IEEE Std. 1149.6 since software version 2.1 (released in 2008) it seems to us that 1149.6 devices are becoming much more common in the last couple of years.
1149.6 is a means of testing higher-speed links via JTAG, and particularly for supporting signals on capacitively coupled lines.
As more customers start using 1149.6-enabled devices, we are seeing similar questions about our support for it repeated, and so here is an overview of how XJTAG tools support 1149.6.
To test using 1149.6, results will be best if the devices at both ends of the signal support the 1149.6 standard.
If any device in a circuit supports 1149.6 XJTAG will by default enable 1149.6 testing – this can be disabled if required in the Connection Test Options on the XJRunner setup screen.
When 1149.6 testing is enabled, XJTAG operates with 2 views of the circuit – the “normal” 1149.1 view where capacitors are viewed as open circuit devices, and the AC 1149.6 view where coupling capacitors are regarded as connections between two nets. Using the two views XJTAG’s connection test automatically contructs a set of 1149.6 interconnection tests which are performed after the 1149.1 tests are completed. These tests can detect faults with the coupling capacitors or termination resistors on the high-speed lines, and detect shorts between the high-speed lines and other nets on the board.
Apart from categorising the coupling capacitors and termination resistors, no additional work is needed to set up a project for 1149.6. You will see during the connection test that it prints out a “Performing AC tests” message at the start of the 1149.6 testing.
XJEase always operates using the 1149.1 view of the circuit, because it tests non-JTAG devices which by definition therefore do not support 1149.6.
There are various options described by the 1149.6 standard which are intended to work around potential problems in AC interconnect testing. In particular, the Connection Test tab in the XJRunner Setup screen allows timings to be adjusted, and the setup for each JTAG device on the JTAG Chain screen allows for further adjustments to timing as well as selection of which AC test instruction to use. The default (and recommendation) is to use EXTEST_PULSE, but EXTEST_TRAIN is available for more troublesome devices.
1149.6 test coverage is reflected in the DFT screen in XJDeveloper. To allow the additional effect of 1149.6 to be seen and separated from the test coverage given by 1149.1 testing, the additional coverage is shown as a separate “functional test” in the DFT analysis results.
In summary, 1149.6 support is integrated into XJTAG, and by default is enabled. In terms of setting up XJTAG, the user needs to do nothing beyond categorising the coupling capacitors and termination resistors in their high-speed lines, and XJTAG will automatically use the 1149.6 functionality where it is present. In terms of board design, the user should try to select devices which support 1149.6 at both ends of high-speed lines, especially those with coupling capacitors in them.
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