ep1sgx25d Altera Corporation, ep1sgx25d Datasheet - Page 74

• Manufacturer Part Number: ep1sgx25d
• Description: Stratix Gx Device Family Data Sheet
• Manufacturer: Altera Corporation

Logic Elements
Figure 4–6. LE in Dynamic Arithmetic Mode
Note to
(1)
4–8
Stratix GX Device Handbook, Volume 1
The addnsub signal is tied to the carry input for the first LE of a carry chain only.
Figure
LAB Carry-In
Carry-In0
Carry-In1
data1
data2
data3
(LAB Wide)
addnsub
4–6:
(1)
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. 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
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
Carry-Out0
LUT
LUT
LUT
LUT
Carry-Out1
shows the carry-select circuitry in an LAB for a 10-bit full
Register chain
connection
clock (LAB Wide)
(LAB Wide)
ena (LAB Wide)
aclr (LAB Wide)
sload
(LAB Wide)
Register Feedback
sclear
(LAB Wide)
ADATA
ENA
D
ALD/PRE
aload
CLRN
Q
Altera Corporation
Row, column, and
direct link routing
Row, column, and
direct link routing
Local routing
LUT chain
connection
Register
chain output
February 2005