Serial I/O Description - Xilinx RocketIO User Manual

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Analog Design Considerations

Serial I/O Description

The RocketIO transceiver transmits and receives serial differential signals. This feature
operates at a nominal supply voltage of 2.5 VDC. A serial differential pair consists of a true
(V
transferred data. Thus: V
crossing of the two complementary signals. Therefore, no separate reference level is
needed. A graphical representation of this concept is shown in
The RocketIO transceiver is implemented in Current Mode Logic (CML). A CML output
consists of transistors configured as shown in
offers easy interface requirements. In this configuration, both legs of the driver, V
sink current, with one leg always sinking more current than its complement. The CML
output consists of a differential pair with 50Ω (or, optionally, 75Ω) source resistors. The
signal swing is created by switching the current in a common-drain differential pair.
The differential transmitter specification is shown in
Table 3-1: Differential Transmitter Parameters
V
V
RocketIO™ Transceiver User Guide
UG024 (v3.0) February 22, 2007
) and a complement (V ) set of signals. The voltage difference represents the
P N
– V
P
Figure 3-1: Differential Amplifier
Parameter
Serial output differential
OUT
peak to peak (TXP/TXN)
Output termination
TTX
voltage supply
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= V . Differential switching is performed at the
N DATA
AVCCAUXTX
Pullup
Network
50Ω or 75Ω
PMA
TXP
PMA
50Ω or 75Ω
TXN
GNDA
Figure 3-1. CML uses a positive supply and
Table 3-1, page
Min Typ Max
800 1600
1.8 2.625
Chapter 3
Figure 3-1.
TXP pin
VTTX
TXN pin
UG024_46_021704
and V
P
103.
Units Conditions
mV Output differential
voltage is programmable
V
,
N
103