Figure 1-8: ALM in Arithmetic Mode for Cyclone V Devices
datae0
carry_in
carry_out
dataa
datab
datac
datad
datae1
4-Input
LUT
4-Input
LUT
4-Input
LUT
4-Input
LUT
adder1
dataf0
dataf1
adder0
reg1
To General or
Local Routing
reg0
reg3
reg2
Carry Chain
The carry chain provides a fast carry function between the dedicated adders in arithmetic or shared arithmetic
mode.
The two-bit carry select feature in Cyclone V devices halves the propagation delay of carry chains within
the ALM. 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.
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 use either the top half or bottom half of the LAB
before connecting to the next LAB. This leaves the other half of the ALMs in the LAB available for
implementing narrower fan-in functions in normal mode. Carry chains that use the top five ALMs in the
first LAB carry into the top half of the ALMs in the next LAB in the column. Carry chains that use the bottom
five ALMs in the first LAB carry into the bottom half of the ALMs in the next LAB within the column. You
can bypass the top-half of the LAB columns and bottom-half of the MLAB columns.
The Quartus II Compiler creates carry chains longer than 20 ALMs (10 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 the TriMatrix memory and DSP blocks. A carry chain can
continue as far as a full column.
Shared Arithmetic Mode
The ALM in shared arithmetic mode can implement a 3-input add in the ALM.
This mode configures the ALM 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 using a dedicated
connection called the shared arithmetic chain.
Altera Corporation
Logic Array Blocks and Adaptive Logic Modules in Cyclone V Devices
1-9
Shared Arithmetic Mode
CV-52001
2014.01.10