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AR# 11814: Virtex-II/Pro DCI - What is the error tolerance of DCI? Does it change if I use the FreezeDCI patch?
Virtex-II/Pro DCI - What is the error tolerance of DCI? Does it change if I use the FreezeDCI patch?
What is the error tolerance of DCI termination?
How closely does it match the VRN and VRP reference resistors?
Is this error tolerance adequate for maintaining good signal integrity?
Will the tolerance change if I use the FreezeDCI patch?
DCI error tolerance (assuming 1% reference resistors) is +/-5% under nominal temperature and voltage conditions, and +/-10% over the entire range of recommended temperature and voltage conditions (note that the worst-case of +/-10% is extremely rare).
These tolerance values also assume that the "FreezeDCI" BitGen option is set. If the FreezeDCI option is not set, the tolerance range will be even smaller.
For Virtex-II Pro devices, please see (Xilinx Answer 15619) for information on the "DCIUpdateMode" BitGen option. DCIUpdateMode is equivalent to FreezeDCI for Virtex-II Pro devices.
If the termination is based on 2R, such as the LVDS_25_DCI standard, the worst-case error will be +/-20% (two times +/-10%).
In most cases, a 2:1 impedance match is acceptable (for example, 25 to 100 ohms buffer impedance with a 50 ohm transmission line). In some critical situations, a better match is required, but seldom is the match required to be better than +/-25% (38 to 62 ohms buffer impedance with a 50 ohm transmission line).
Using the FreezeDCI option (i.e., disabling fine adjustment) does not prevent DCI from performing its job. Compensating for process (a coarse adjustment that occurs during configuration) is more important than compensating for temperature and voltage, and this DCI feature is very effective.
Internally, DCI is implemented with transistors (not resistors). As a result, the I/V curve is not linear. This is acceptable since it is designed to be 50 ohms (i.e., match the reference resistors) at the most critical points.
For Virtex-II Pro, the LVDS_25_DT I/O standard is available to provide true internal 100 Ohm differential termination. In most applications, LVDS_25_DT is a much better solution than LVDS_25_DCI. Please see (Xilinx Answer 17244) for more information.