Emphasis - Xilinx Virtex-4 RocketIO User Manual

Chapter 4: PMA Analog Design Considerations

Emphasis

The MGT contains a 3-tap transmitter (shown in
cursor driver, and post-cursor driver to help create a cleaner signal at the receiver.
GREFCLK
Mux
PCSCLK
PLL
Data Clock
Transmit
32,40
Parallel Data
Slew Rate Control
There are several attributes that control the pre-emphasis characteristics. These are shown
in
Table 4-1: Attributes Controlling Pre-Emphasis Characteristics
The effect of these three taps are shown in
start of the pulse on the receiver. This degradation is small, which is why
TXPRE_TAP_DAC is recommended to be small compared to the other driver settings. The
post-cursor (post-driver) improves the "tail" of the pulse at the receiver.
TXPOST_TAP_DAC should be less than TXDAT_TAP_DAC, so as not to destroy the pulse
140
Optional
Divider
Phase
Comparator
Bit Slip
Datapath
Divider
32/40:1
-1
Z
Mux
Figure 4-2: 3-Tap Pre-Emphasis
Table 4-1.
Attribute
TXPOST_TAP_PD
TXDAT_TAP_DAC
TXPOST_TAP_DAC
TXHIGHSIGNALEN
TXPRE_TAP_DAC
TXPRE_TAP_PD
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Figure
Pre-
Cursor
Cursor
Post-
-1
Z
Cursor
Disables the POST driver which disables the pre-emphasis.
Controls the output swing of the data driver. This value should
be much larger (~4 times) than TXPOST_TAP_DAC and (~8
times) TXPRE_TAP_DAC.
Controls the output swing of the post data driver. This value is
usually twice the value of TXPRE_TAP_DAC.
This doubles the current level of the driver when set to a logic 1.
When set to a logic 0, the current level is set for applications
interfacing to an XFP or similar device that requires low signal
strength.
Controls the output swing of the pre-data driver. This value
should be very small.
Disables the PRE driver. Used to disable the pre-emphasis.
Figure 4-3. The pre-cursor (pre-driver) helps the
4-2). This allows a data driver, pre-
V
TTX
Differential
Transmit
Serial Data
ug076_ch6_08_101404
Definition
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
R