Cyclone Device Handbook, Volume 1
Figure 2–7. LE in Dynamic Arithmetic Mode
LAB Carry-In
Carry-In0
Carry-In1
sload
sclear
aload
(LAB Wide)
(LAB Wide) (LAB Wide)
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 2–7:
(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 dynamic 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 0 and
carry-in of 1 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 a 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 delays between LAB carry-in generation (LE 5 and LE 10) are
now part of the critical path. This feature allows the Cyclone architecture
to implement high-speed counters, adders, multipliers, parity functions,
and comparators of arbitrary width.
2–10
Preliminary
Altera Corporation
May 2008
ALTERA [ ALTERA CORPORATION ]
