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Specific Guidelines for I/O Supported Standards
HyperTransport™ Protocol (HT)
The HyperTransport protocol (HT) also known as Lightning Data Transport (LDT), is a
low-voltage standard for high speed interfaces. Its differential signaling based interface is
very similar to LVDS. Virtex-5 FPGA IOBs are equipped with HT buffers. Table 6-38
summarizes all the possible HT I/O standards and attributes supported.
Table 6-37:
Allowed Attributes of the HT I/O Standard
Primitives
Attributes
IBUFDS/IBUFGDS
OBUFDS/OBUFTDS
IOSTANDARD
HT_25
DIFF_TERM
TRUE, FALSE
N/A
Reduced Swing Differential Signaling (RSDS)
Reduced Swing Differential Signaling (RSDS) is similar to an LVDS high-speed interface
using differential signaling. RSDS has a similar implementation to LVDS in Virtex-5
devices and is only intended for point-to-point applications.
Table 6-38:
Allowed Attributes of the RSDS I/O Standard
Primitives
Attributes
IBUFDS/IBUFGDS
IOSTANDARD
OBUFDS/OBUFTDS
RSDS_25
DIFF_TERM
TRUE, FALSE
N/A
BLVDS (Bus LVDS)
Since LVDS is intended for point-to-point applications, BLVDS is not an EIA/TIA standard
implementation and requires careful adaptation of I/O and PCB layout design rules. The
primitive supplied in the software library for bidirectional LVDS does not use the Virtex-5
FPGA LVDS current-mode driver, instead, it uses complementary single-ended differential
drivers. Therefore, source termination is required. Figure 6-88 shows the BLVDS
transmitter termination.
BLVDS_25
IOB
IOB
RS
Z0 = 50Ω
IN
165Ω
BLVDS_25
RS
RDIV
140Ω
BLVDS_25
-
RDIFF = 100Ω
+
Data in
Z0 = 50Ω
INX
165Ω
ug190_6_83_030506
Figure 6-88:
Virtex-5 FPGA User Guide
UG190 (v4.4) December 2, 2008
BLVDS Transmitter Termination
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