FLEX 10K Embedded Programmable Logic Device Family Data Sheet
Figure 7 shows how an n-bit full adder can be implemented in n + 1 LEs
with the carry chain. One portion of the LUT generates the sum of two bits
using the input signals and the carry-in signal; the sum is routed to the
output of the LE. The register can either be bypassed for simple adders or
be used for an accumulator function. The carry chain logic generates the
carry-out signal, which is routed directly to the carry-in signal of the next-
higher-order bit. The final carry-out signal is routed to an LE, where it can
be used as a general-purpose signal.
Figure 7. Carry Chain Operation (n-bit Full Adder)
Carry-In
s1
Register
a1
b1
LUT
Carry Chain
LE1
Register
s2
a2
b2
LUT
Carry Chain
LE2
Register
sn
an
bn
LUT
Carry Chain
LEn
Register
Carry-Out
LUT
Carry Chain
LEn + 1
16
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