HC4GX15 Datasheet, PDF (122/668 Pages) Altera Corporation – HardCopy IV Device Handbook

8–12
Figure 8–11. Output Differential Voltage
Chapter 8: High-Speed Differential I/O Interfaces and DPA in HardCopy IV Devices
Differential I/O Termination
Overshoot
OUT
OUT
VOD
Undershoot
Pre-emphasis is an important feature for high-speed transmission. Without
pre-emphasis, the output current is limited by the VOD setting and the output
impedance of the driver. At high frequency, the slew rate might not be fast enough to
reach the full VOD before the next edge, producing a pattern dependent jitter.
With pre-emphasis, the output current is boosted momentarily during switching to
increase the output slew rate. The overshoot introduced by the extra current happens
only during switching 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.
You can adjust pre-emphasis in HardCopy IV devices to create the right amount of
overshoot at different transmission conditions. There are four settings for
pre-emphasis: zero, low, medium, and high. The default setting is low. For a particular
design, simulation with an LVDS buffer and transmission line can be used to
determine the best pre-emphasis setting. The VOD can also be adjusted to any of the
four settings: low, medium low, medium high, and high. The default setting is
medium low.
Differential I/O Termination
HardCopy IV devices provide a 100-, on-chip differential termination option on
each differential receiver channel for LVDS standards. On-chip termination (OCT)
saves board space by eliminating the need to add external resistors on the board. You
can enable OCT in the Quartus II Assignment Editor.
On-chip differential termination is supported on all row I/O pins and
serial/deserializer (SERDES) block clock pins: clk[0,2,9,11]. It is not supported
for column I/O pins, high speed clock pins clk[1,3,8,10], or the corner PLL clock
inputs. Figure 8–12 illustrates device OCT.
HardCopy IV Device Handbook, Volume 1
© January 2010 Altera Corporation