Logic Elements
Figure 4–6. LE in Dynamic Arithmetic Mode
LAB Carry-In
Carry-In0
sload
sclear
aload
(LAB Wide)
(LAB Wide) (LAB Wide)
Carry-In1
Register chain
connection
addnsub
(LAB Wide)
(1)
ALD/PRE
data1
data2
data3
LUT
ADATA
D
Row, column, and
direct link routing
Q
LUT
LUT
LUT
Row, column, and
direct link routing
ENA
CLRN
clock (LAB Wide)
ena (LAB Wide)
aclr (LAB Wide)
Local routing
LUT chain
connection
Register
chain output
Register Feedback
Carry-Out0 Carry-Out1
Note to Figure 4–6:
(1) The addnsubsignal 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-in0and carry-in1signals 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. Because 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 GX
architecture to implement high-speed counters, adders, multipliers,
parity functions, and comparators of arbitrary width.
Figure 4–7 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
4–8
Stratix GX Device Handbook, Volume 1
Altera Corporation
February 2005
ALTERA [ ALTERA CORPORATION ]
