XC5000 Datasheet, PDF (13/48 Pages) Xilinx, Inc – High-density family of Field-Programmable Gate Arrays

R
Carry Function
The XC5200 family supports a carry-logic feature that
enhances the performance of arithmetic functions such as
counters, adders, etc. A carry multiplexer (CY_MUX)
symbol on a schematic is used to indicate the XC5200
carry logic. This symbol represents the dedicated 2:1
multiplexer in each LC that performs the one-bit high-
speed carry propagate per logic cell (four bits per CLB).
While the carry propagate is performed inside the LC, an
adjacent LC must be used to complete the arithmetic
function. Figure 6 represents an example of an adder
function. The carry propagate is performed on the CLB
shown, which also generates the half-sum for the four-bit
adder. An adjacent CLB is responsible for XORing the
half-sum with the corresponding carry-out. Thus an adder
or counter requires two LCs per bit. Notice that the carry
chain requires an initialization stage, which the XC5200
family accomplishes using the carry initialize (CY_INIT)
macro and one additional LC.
The XC5200 library contains a set of RPMs and arithmetic
functions designed to take advantage of the dedicated
carry logic. Using and modifying these macros makes it
much easier to implement customized RPMs, freeing the
designer from the need to become an expert on
architectures.
A3
or
B3
A3 and B3
to any two
DI
F4
F3
F2 XOR
F1
carry out
CO
CY_MUX
A2
or
DI
B2
A2 and B2
to any two
F4
F3
F2 XOR
F1
CY_MUX
A1
or
B1
A1 and B1
to any two
DI
F4
F3
F2 XOR
F1
CY_MUX
A0
or
B0
A0 and B0
to any two
0
DI
F4
F3
F2 ^XOR
F1
CY_MUX
CI
carry in
CE CK
DO ^carry3
D
Q
FD
X ^{half sum3}
LC3
DO ^carry2
D
Q
FD
X ^{half sum2}
LC2
DO ^carry1
D
Q
FD
half sum1
X
LC1
DO ^carry0
D
Q
FD
half sum0
X
CLR LC0
CY_MUX
F=0
Initialization of
carry chain (One Logic Cell)
Figure 6. XC5200 CY_MUX Used for Adder Carry Propagate
9
CO
DI
F4
F3
F2 XOR
F1
DI
F4
F3
F2 XOR
F1
DI
F4
F3
F2 XOR
F1
DI
F4
F3
F2 XOR
F1
CI
DO
D
Q
FD
X ^sum3
LC3
DO
D
Q
FD
X ^sum2
LC2
DO
D
Q
FD
sum1
X
LC1
DO
D
Q
FD
CE CK
sum0
X
CLR LC0
X5709