
Altera Corporation
2–11
July 2005
Stratix Device Handbook, Volume 1
Stratix Architecture
Figure 2–7. LE in Dynamic Arithmetic Mode
(1)
The addnsub signal is tied to the carry input for the first LE of a carry chain only.
Carry-Select Chain
The carry-select chain provides a very fast carry-select function between
LEs in arithmetic mode. The carry-select chain uses the redundant carry
calculation to increase the speed of carry functions. The LE is configured
to calculate outputs for a possible carry-in of 1 and carry-in of 0 in
parallel. The carry-in0 and carry-in1 signals from a lower-order bit
feed forward into the higher-order bit via the parallel carry chain and feed
into both the LUT and the next portion of the carry chain. Carry-select
chains can begin in any LE within an LAB.
The speed advantage of the carry-select chain is in the parallel pre-
computation of carry chains. Since the LAB carry-in selects the
precomputed carry chain, not every LE is in the critical path. Only the
propagation delay between LAB carry-in generation (LE 5 and LE 10) are
now part of the critical path. This feature allows the Stratix architecture to
implement high-speed counters, adders, multipliers, parity functions,
and comparators of arbitrary width.
data1
LUT
data2
data3
addnsub
(LAB Wide)
clock (LAB Wide)
ena (LAB Wide)
aclr (LAB Wide)
ALD/PRE
CLRN
D
Q
ENA
ADATA
Register chain
connection
LUT
Carry-Out1
Carry-Out0
LAB Carry-In
Carry-In0
Carry-In1
(1)
sclear
(LAB Wide)
sload
(LAB Wide)
LUT chain
connection
Register
chain output
Row, column, and
direct link routing
Row, column, and
direct link routing
Local routing
aload
(LAB Wide)
Register Feedback