XC3S100E_06 Datasheet, PDF (26/231 Pages) Xilinx, Inc – Configurable Logic Block (CLB)

Functional Description
R
ing LUTs or by using the wide function multiplexers that are
described later.
The output of the LUT can connect to the wide multiplexer
logic, the carry and arithmetic logic, or directly to a CLB out-
put or to the CLB storage element. See Figure 18.
4
G[4:1]
A[4:1] ^D
G-LUT
Y
YQ
FFY
Wide Multiplexers
Wide-function multiplexers effectively combine LUTs in
order to permit more complex logic operations. Each slice
has two of these multiplexers with F5MUX in the bottom por-
tion of the slice and FiMUX in the top portion. The F5MUX
multiplexes the two LUTs in a slice. The FiMUX multiplexes
two CLB inputs which connect directly to the F5MUX and
FiMUX results from the same slice or from other slices. See
Figure 19.
4
F[4:1]
A[4:1] _D
F-LUT
X
XQ
FFX
DS312-2_33_111105
Figure 18: LUT Resources in a Slice
FXINA
FXINB
BY
FiMUX
1
0
DQ
FX (Local Feedback to FXIN)
Y (General Interconnect)
YQ
F[4:1]
G[4:1]
BX
F5MUX
LUT
1
LUT
0
DQ
F5 (Local Feedback to FXIN)
X (General Interconnect)
XQ
x312-2_34_021205
Figure 19: Dedicated Multiplexers in Spartan-3E CLB
Depending on the slice, FiMUX takes on the name F6MUX,
F7MUX, or F8MUX. The designation indicates the number
of inputs possible without restriction on the function. For
example, an F7MUX can generate any function of seven
inputs. Figure 20 shows the names of the multiplexers in
each position in the Spartan-3E CLB. The figure also
includes the direct connections within the CLB, along with
the F7MUX connection to the CLB below.
Each mux can create logic functions of more inputs than
indicated by its name. The F5MUX, for example, can gener-
ate any function of five inputs, with four inputs duplicated to
two LUTs and the fifth input controlling the mux. Because
each LUT can implement independent 2:1 muxes, the
F5MUX can combine them to create a 4:1 mux, which is a
six-input function. If the two LUTs have completely indepen-
dent sets of inputs, some functions of all nine inputs can be
implemented. Table 11 shows the connections for each mul-
tiplexer and the number of inputs possible for different types
of functions.
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DS312-2 (v3.4) November 9, 2006
Product Specification