Xilinx Virtex-4 RocketIO User Manual page 142

Chapter 4: PMA Analog Design Considerations
Table 4-2: TXDAT_TAP_DAC and TXPOST_TAP_DAC Settings (Continued)
The TXPRE_TAP_DAC value can be set to some fraction of the TXPOST_TAP_DAC value.
However, for most channels, TXPRE_TAP_DAC can be set to off, unless it is required to
improve the performance of the linear receiver equalizer and decision feedback equalizer.
It is recommended that all the attributes listed in this section are made accessible through
the Dynamic Reconfiguration Port (see
This allows transmitter parameters to be fine-tuned in the lab to match the characteristics
of a particular channel.
With MGT emphasis, the initial differential voltage swing is boosted to create a stronger
rising or falling waveform. This method compensates for high-frequency loss in the
transmission media that would otherwise limit the magnitude of this waveform. The
effects of emphasis are shown in four scope screen captures,
(Also, see
Effects.)
Note:
actual MGT data.
Figure 4-4
corresponds to the signal after 36 inches of FR4.
link with pre-emphasis over driving the rising and falling edges. This produces a less than
optimal eye at the transmit side, but after the attenuation of the rising and falling edges of
36 inches of FR4
characteristic of MGT emphasis is that the overdriving level is reduced after some time to
the normal driving level, thereby minimizing the voltage swing necessary to switch the
differential pair into the opposite state.
Lossy transmission lines cause the dissipation of electrical energy. This emphasis
technique extends the distance to which signals can be driven down lossy line media and
increases the signal-to-noise ratio at the receiver.
Emphasis can be described from two perspectives, additive to the smaller voltage (V
(pre-emphasis) or subtractive from the larger voltage (V
benefits in compensating for channel loss are identical. It is simply a relative way of
specifying the effect at the transmitter.
The equations for calculating pre-emphasis as a percentage and dB are as follows:
The equations for calculating de-emphasis as a percentage and dB are as follows:
142
Line Loss Differential Swing
(dB) (mV)
14
17
"Appendix C, "Dynamic Reconfiguration Port"
Figure 4-4 through Figure 4-7
shows the transmit portion of a link with minimal pre-emphasis and
(Figure 4-7), the optimal eye is restored for the receiver. A second
Pre-Emphasis% = ((V -V
LG
Pre-Emphasis dB = 20 log(V
De-Emphasis% = (V - V
LG
De-Emphasized dB = 20 log(V
www.xilinx.com
TXDAT_TAP_DAC
400
00110
600
01101
800
10100
400
00110
600
01100
800
10010
Appendix C, "Dynamic Reconfiguration
are for illustration purposes only and do not correspond to
Figure 4-6
)/V ) x 100
SM SM
/V )
LG SM
)/V ) x 100
SM LG
/V )
SM LG
TXPOST_TAP_DAC
00110
01101
10100
01001
10001
11000
Port").
Figure 4-4 through Figure
for the signal or attribute
Figure 4-5
shows the transmit portion of a
) (de-emphasis). The resulting
LG
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
R
4-7.
)
SM