Altera Stratix GX Transceiver - Manual (Page 22)

2–4 Altera Corporation
Stratix GX Transceiver User Guide January 2005
Transmitter Analog
You can set the differential V
OD
values statically during configuration or
dynamically adjust them in user mode. You select the static V
OD
value
through a list in the altgxb MegaWizard ® Plug-In Manager, which sets
the appropriate V
OD
setting in the configuration file. The disadvantage of
the static mode setting is that the V
OD
is set on a per transceiver block
basis and cannot be changed unless you regenerate another
programming file.
Alternatively, if you enable dynamic adjustment in the altgxb
MegaWizard Plug-In, you can dynamically configure the V
OD
setting by
the device during user mode. This configuration is done by asserting
encoded values on the tx_vodctrl bus, which is instantiated in the
altgxb
module when you select the dynamic adjustment option. This
option lets you make quick performance evaluations of the various
settings without having to recompile and regenerate multiple
configuration files. Another advantage of this option is that it allows the
V
OD
of each channel to be configured independently. Refer to the section
“MegaWizard Analog Features” on page 2–20 for further details.
Programmable Pre-Emphasis
The programmable pre-emphasis module in each transmit buffer boosts
the high frequencies in the transmit data signal, which may be attenuated
in the transmission media. This maximizes the data eye opening at the
far-end receiver. Pre-emphasis is particularly useful in lossy transmission
mediums.
The transfer function of a transmission line can be represented in the
frequency domain as a low-pass filter. Any frequency components below
the –3 dB frequency pass through with minimal losses. Frequency
components that are greater than the –3-dB frequency are attenuated.
This variation in frequency response yields data-dependant jitter and
other ISI effects. By applying pre-emphasis, the high frequency
components are boosted, or in other words, pre-emphasized. This
pre-emphasis equalizes the frequency response as seen at the receiver so
that the delta between the low-frequency and high-frequency
components is reduced, which in return minimizes the ISI effects from the
transmission medium.
In Stratix GX transceivers, the programmable pre-emphasis settings can
have one of six values (0 to 5). You should experiment with the
pre-emphasis values to determine the optimal setting based on your
system variables.