Asynchronous Delay Compensation (ADYCS)
SCANFLEX hardware can be used in a variety of test system configurations, some of which may require rather long cable length for JTAG test bus cables.
Examples are JTAG / boundary-scan system integrations in functional test racks or in environmental test chambers, where the JTAG / boundary-scan tester hardware
is located more than a few feet away from the unit under test (UUT). Extended cable lengths such as these result in additional signal delay.
This added signal delay results in an upper limit for the test clock (TCK) frequency. In order to overcome this limitation, SYSTEM CASCON software and SCANFLEX hardware have
a compensation mechanism called "ADYCS" built in.
The timing diagram on the right shows the time tdelay that expires between the falling TCK edge provided on the JTAG tester hardware TAP interface and the corresponding
change of the TDO signal driven from the UUT TAP interface back to the JTAG tester TAP interface.
tdelay covers both the signal delay of the UUT signal path itself and the test bus cable between JTAG / boundary-scan tester TAP interface and the UUT TAP interface (delay on TCK line plus delay on the TDO line).
∆t is the tolerance of the signal delay. For a high TCK frequency, ∆t has to be determined more precisely.
Given these parameters, the maximum TCK frequency can be calculated.
The diagram on the right shows the signal delay window (defined by +∆t and - ∆t) during which a change on the TDO is expected (if there is indeed a change on the TDO signal). The TDO value must not be captured during this time window.
The TDO value must instead be captured between two subsequent signal delay windows. For delay compensation to be possible the time between two consecutive signal delay windows must be at least as long as the time window itself.
The ADYCS feature designed into GOEPEL Electronics' JTAG / boundary scan tools allows for the compensation of test bus signal delay caused by long distances between tester and UUT and/or by signal propagation delays on the UUT itself,
therefor allowing the highest possible TCK frequency supported by the slowest device in the scan chain.