Serial I/O Description - Xilinx RocketIO User Manual

R
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
complement (V
V
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
requirements. In this configuration, both legs of the driver, V
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
RocketIO™ Transceiver User Guide
UG024 (v2.3.2) June 24, 2004
) set of signals. The voltage difference represents the transferred data. Thus: V
N
= V . Differential switching is performed at the crossing of the two complementary signals.
N DATA
Figure 3-1.
Parameter
V Serial output differential
OUT
peak to peak (TXP/TXN)
V Output termination voltage
TTX
supply
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AVCCAUXTX
Pullup
Network
PMA
TXP
PMA
TXN
GNDA
Figure 3-1: Differential Amplifier
Figure 3-1. CML uses a positive supply and offers easy interface
Table 3-1, page
Min Typ
800
1.8
Chapter 3
) and a
P
TXP pin
50Ω or 75Ω
VTTX
50Ω or 75Ω
TXN pin
UG024_46_021704
and V , sink current, with one leg
P N
101.
Max Units Conditions
1600 mV Output differential voltage
is programmable
2.625 V
–
P
101