EP2SGX130GF1508C4 Datasheet, PDF (66/316 Pages) Altera Corporation – Section I. Stratix II GX Device Data Sheet

Adaptive Logic Modules
Carry Chain
The carry chain provides a fast carry function between the dedicated
adders in arithmetic or shared arithmetic mode. Carry chains can begin in
either the first ALM or the fifth ALM in a LAB. The final carry-out signal
is routed to an ALM, where it is fed to local, row, or column interconnects.
The Quartus II Compiler automatically creates carry chain logic during
compilation, 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 16 (8 ALMs in arithmetic or
shared arithmetic mode) 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. To avoid routing congestion in
one small area of the device when a high fan-in arithmetic function is
implemented, the LAB can support carry chains that only utilize either
the top half or the bottom half of the LAB before connecting to the next
LAB. The other half of the ALMs in the LAB is available for implementing
narrower fan-in functions in normal mode. Carry chains that use the top
four ALMs in the first LAB will carry into the top half of the ALMs in the
next LAB within the column. Carry chains that use the bottom four ALMs
in the first LAB will carry into the bottom half of the ALMs in the next
LAB within the column. Every other column of the LABs are top-half
bypassable, while the other LAB columns are bottom-half bypassable.
Refer to “MultiTrack Interconnect” on page 2–63 for more information on
carry chain interconnect.
Shared Arithmetic Mode
In shared arithmetic mode, the ALM can implement a three-input add. In
this mode, the ALM is configured with four 4-input LUTs. Each LUT
either computes the sum of three inputs or the carry of three inputs. The
output of the carry computation is fed to the next adder (either to adder1
in the same ALM or to adder0 of the next ALM in the LAB) using a
dedicated connection called the shared arithmetic chain. This shared
arithmetic chain can significantly improve the performance of an adder
tree by reducing the number of summation stages required to implement
an adder tree. Figure 2–43 shows the ALM in shared arithmetic mode.
2–58
Stratix II GX Device Handbook, Volume 1
Altera Corporation
October 2007