R
Virtex-II Platform FPGAs: Functional Description
Shift Registers
Each function generator can also be configured as a 16-bit
shift register. The write operation is synchronous with a
clock input (CLK) and an optional clock enable, as shown in
Figure 21. A dynamic read access is performed through the
4-bit address bus, A[3:0]. The configurable 16-bit shift regis-
ter cannot be set or reset. The read is asynchronous, how-
ever the storage element or flip-flop is available to
implement a synchronous read. The storage element
should always be used with a constant address. For exam-
ple, when building an 8-bit shift register and configuring the
addresses to point to the 7th bit, the 8th bit can be the
flip-flop. The overall system performance is improved by
using the superior clock-to-out of the flip-flops.
1 Shift Chain
in CLB
DI
SRLC16
MC15
D
IN
FF
DI
SRLC16
D
FF
MC15
SLICE S3
SHIFTOUT
SHIFTIN
D
SRLC16
MC15
DI
FF
SRLC16
SHIFTIN
SHIFT-REG
4
DI
Output
D
D
FF
A[3:0]
D(BY)
A[4:1]
SRLC16
MC15
Registered
Output
MC15
D
Q
SLICE S2
DI
WS
SHIFTOUT
(optional)
WSG
CE (SR)
CLK
WE
CK
SHIFTIN
DI
D
FF
FF
SRLC16
MC15
SHIFTOUT
DS031_05_110600
Figure 21: Shift Register Configurations
DI
SRLC16
D
An additional dedicated connection between shift registers
allows connecting the last bit of one shift register to the first
bit of the next, without using the ordinary LUT output. (See
Figure 22.) Longer shift registers can be built with dynamic
access to any bit in the chain. The shift register chaining
and the MUXF5, MUXF6, and MUXF7 multiplexers allow up
to a 128-bit shift register with addressable access to be
implemented in one CLB.
MC15
SLICE S1
SHIFTOUT
FF
SHIFTIN
D
SRLC16
MC15
DI
DI
D
FF
SRLC16
MC15
SLICE S0
CLB
OUT
CASCADABLE OUT
DS031_06_110200
Figure 22: Cascadable Shift Register
DS031-2 (v3.5) November 5, 2007
Product Specification
www.xilinx.com
Module 2 of 4
16
XILINX [ XILINX, INC ]
