Logic Elements
Figure 2–8 shows the carry-select circuitry in an LAB for a 10-bit full
adder. One portion of the LUT generates the sum of two bits using the
input signals and the appropriate carry-in bit; the sum is routed to the
output of the LE. The register can be bypassed for simple adders or used
for accumulator functions. Another portion of the LUT generates carry-
out bits. An LAB-wide carry in bit selects which chain is used for the
addition of given inputs. The carry-in signal for each chain, carry-in0
or carry-in1, selects the carry-out to carry forward to the carry-in
signal of the next-higher-order bit. The final carry-out signal is routed to
an LE, where it is fed to local, row, or column interconnects.
The Quartus II Compiler automatically creates carry chain logic during
design processing, or you can create it manually during design entry.
Parameterized functions such as LPM functions automatically take
advantage of carry chains for the appropriate functions.
The Quartus II Compiler creates carry chains longer than 10 LEs by
linking LABs together automatically. For enhanced fitting, a long carry
chain runs vertically allowing fast horizontal connections to TriMatrix™
memory and DSP blocks. A carry chain can continue as far as a full
column.
2–12
Stratix Device Handbook, Volume 1
Altera Corporation
July 2005
ALTERA [ ALTERA CORPORATION ]
