IGLOO DC and Switching Characteristics
2- 66
Advance v0.5
B-LVDS/M-LVDS
Bus LVDS (B-LVDS) and Multipoint LVDS (M-LVDS) specifications extend the existing LVDS standard
to high-performance multipoint bus applications. Multidrop and multipoint bus configurations
may contain any combination of drivers, receivers, and transceivers. Actel LVDS drivers provide the
higher drive current required by B-LVDS and M-LVDS to accommodate the loading. The drivers
require series terminations for better signal quality and to control voltage swing. Termination is
also required at both ends of the bus since the driver can be located anywhere on the bus. These
configurations can be implemented using the TRIBUF_LVDS and BIBUF_LVDS macros along with
appropriate terminations. Multipoint designs using Actel LVDS macros can achieve up to 200 MHz
with a maximum of 20 loads. A sample application is given in
Figure 2-14. The input and output
Example: For a bus consisting of 20 equidistant loads, the following terminations provide the
required differential voltage, in worst-case Industrial operating conditions, at the farthest receiver:
RS =60 Ω and RT =70 Ω, given Z0 =50 Ω (2") and Zstub =50 Ω (~1.5").
LVPECL
Low-Voltage Positive Emitter-Coupled Logic (LVPECL) is another differential I/O standard. It
requires that one data bit be carried through two signal lines. Like LVDS, two pins are needed. It
also requires external resistor termination.
The full implementation of the LVDS transmitter and receiver is shown in an example in
Figure 2-15. The building blocks of the LVPECL transmitter-receiver are one transmitter macro, one
receiver macro, three board resistors at the transmitter end, and one resistor at the receiver end.
The values for the three driver resistors are different from those used in the LVDS implementation
because the output standard specifications are different.
Figure 2-14 B-LVDS/M-LVDS Multipoint Application Using LVDS I/O Buffers
...
R
T
R
T
BIBUF_LVDS
R
+
-
T
+
-
R
+
-
T
+
-
D
+
-
EN
Receiver
Transceiver
Receiver
Transceiver
Driver
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
R
S
Z
stub
Z
stub
Z
stub
Z
stub
Z
stub
Z
stub
Z
stub
Z
stub
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Z
0
Figure 2-15 LVPECL Circuit Diagram and Board-Level Implementation
187 W
100
Ω
Z0 = 50 Ω
100
Ω
100
Ω
+
–
P
N
P
N
INBUF_LVPECL
OUTBUF_LVPECL
FPGA
Bourns Part Number: CAT16-PC4F12