Programmable Pre-Emphasis
The V
OD
setting and the output impedance of the driver set the output current limit of a high-speed
transmission signal. At a high frequency, the slew rate may not be fast enough to reach the full V
OD
level
before the next edge, producing pattern-dependent jitter. With pre-emphasis, the output current is boosted
momentarily during switching to increase the output slew rate.
Pre-emphasis increases the amplitude of the high-frequency component of the output signal, and thus helps
to compensate for the frequency-dependent attenuation along the transmission line. The overshoot introduced
by the extra current happens only during a change of state switching to increase the output slew rate and
does not ring, unlike the overshoot caused by signal reflection. The amount of pre-emphasis required depends
on the attenuation of the high-frequency component along the transmission line.
Figure 5-11: Programmable Pre-Emphasis
This figure shows the LVDS output with pre-emphasis.
OUT
OUT
V
OD
V
P
V
P
Voltage boost
from pre-emphasis
Differential output
voltage (peak–peak)
Table 5-26: Quartus II Software Assignment Editor—Programmable Pre-Emphasis
This table lists the assignment name for programmable pre-emphasis and its possible values in the Quartus II software
Assignment Editor.
Assignment (Default setting in bold)
Field
tx_out
To
Programmable Pre-emphasis
Assignment name
0 (disabled) and 1 (enabled)
Allowed values
Related Information
Programmable IOE Features in Cyclone V Devices
on page 5-29
Programmable Differential Output Voltage
The programmable V
OD
settings allow you to adjust the output eye opening to optimize the trace length
and power consumption. A higher V
OD
swing improves voltage margins at the receiver end, and a smaller
V
OD
swing reduces power consumption. You can statically adjust the V
OD
of the differential signal by
changing the V
OD
settings in the Quartus II software Assignment Editor.
I/O Features in Cyclone V Devices
Altera Corporation
CV-52005
Programmable Pre-Emphasis
5-32
2014.01.10