Use Mode - Continuous Time Rx Linear Equalizer Only; Use Mode - Fixed Tap Mode - Xilinx Virtex-6 FPGA User Manual

Use Mode – Continuous Time RX Linear Equalizer Only
Modes with greater gain at high frequencies are intended for lossy (usually longer)
channels. Following is a simple way to determine how to use the RX equalizer:
1.
2.
3.
Based on these results, the appropriate setting of RXEQMIX can be picked.
These settings must always be verified in hardware because there are several
considerations, such as discontinuities in the system, jitter on the TX clocks, which cannot
be compensated for by the linear equalizer.
Example
Data Rate = 6.25 Gb/s, which means the fundamental frequency is 3.125 GHz.
Channel Loss in dB at 3.125 GHz = 12 dB
RXEQMIX[2:0] = 000 or 110 is sufficient.
This setting allows compensation of 12 dB at 3.125 GHz hence providing the needed gain.
Use Mode – Fixed Tap Mode
This mode requires that DFETAPOVRD = 1 (DFETAPx value override). It is recommended
that DFEDLYOVRD = 0 (optimized DFE clock delay calibration). DFE_CFG bits should be
at their default values. The values to be written to the DFE taps are applied to DFETAPx.
DFETAPxMONITOR reflects the value entered.
The chip-to-chip and backplane application examples in this section provide starting
points for setting DFETAP1 and RXEQMIX based on the trace length and the associated
loss of the channel. DFETAP2, DFETAP3, and DFETAP4 are set to 0 in these examples. For
channels with different characteristics or lengths than given in the examples, the designer
should use the example with the closest match as a starting point. Either the Virtex-6 FPGA
GTX Wizard example design or IBERT can be used to fine tune the settings to the user's
particular channel.
Example – RX Linear Equalizer and DFE Settings for Chip-to-Chip Applications
Table 4-13
chip-to-chip applications.
www.BDTIC.com/XILINX
Virtex-6 FPGA GTX Transceivers User Guide
UG366 (v2.5) January 17, 2011
Determine the operating data rate.
Determine the channel loss (board) in dB at data rate/2. This is the differential
insertion loss from measured or extracted S-parameter data commonly referred to as
Sdd21.
Pick the appropriate RXEQMIX setting from the relative gain plot.
Always make sure that the transmit amplitude is sufficient when picking modes with
a higher gain because there is DC attenuation of the signal. Reference the absolute gain
plot.
provides DFETAP1 and RXEQMIX settings for 3.125 Gb/s operation for
www.xilinx.com
RX Equalizer
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