Chapter 7: High-Speed Differential Interfaces in the Cyclone III Device Family
7–15
True Output Buffer Feature
Figure 7–13 shows the differential HSTL Class II interface.
Figure 7–13. Differential HSTL Class II Interface
VTT
VTT
VTT
VTT
50 Ω
50 Ω
50 Ω
50 Ω
Output Buffer (1)
Receiver
Z
= 50 Ω
= 50 Ω
0
Z
0
Note to Figure 7–13:
(1) PLL output clock pins do not support differential HSTL Class II I/O standard.
True Output Buffer Feature
Cyclone III device family true differential transmitters offer programmable
pre-emphasis—you can choose to turn it on or off. The default setting is on.
Programmable Pre-Emphasis
The programmable pre-emphasis boosts the high frequencies of the output signal to
compensate the frequency-dependent attenuation of the transmission line to
maximize the data eye opening at the far-end receiver. Without pre-emphasis, the
output current is limited by the VOD specification and the output impedance of the
transmitter. At high frequency, the slew rate may not be fast enough to reach full VOD
before the next edge; this may lead to pattern dependent jitter. With pre-emphasis, the
output current is momentarily boosted during switching to increase the output slew
rate. The overshoot produced by this extra switching current is different from the
overshoot caused by signal reflection. This overshoot happens only during switching,
and does not produce ringing.
Figure 7–14 shows the differential output signal with pre-emphasis.
Figure 7–14. The Output Signal with Pre-Emphasis
Overshoot
Positive channel (p)
Negative channel (n)
VOD
Undershoot
December 2011 Altera Corporation
Cyclone III Device Handbook
Volume 1
ALTERA [ ALTERA CORPORATION ]
