EP2AGX95EF29C6N Datasheet, PDF (43/380 Pages) Altera Corporation – Device Interfaces and Integration

Chapter 2: Logic Array Blocks and Adaptive Logic Modules in Arria II Devices
Adaptive Logic Modules
2–13
Shared Arithmetic Mode
In shared arithmetic mode, the ALM can implement a 3-input add in an ALM. 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 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 2–11 shows the ALM using this feature.
Figure 2–11. ALM in Shared Arithmetic Mode
shared_arith_in
carry_in
datae0
datac
datab
dataa
4-Input
LUT
4-Input
LUT
labclk
DQ
reg0
To general or
local routing
To general or
local routing
datad
datae1
4-Input
LUT
4-Input
LUT
DQ
reg1
To general or
local routing
To general or
local routing
carry_out
shared_arith_out
You can find adder trees in many different applications. For example, the summation
of the partial products in a logic-based multiplier can be implemented in a tree
structure. Another example is a correlator function that can use a large adder tree to
sum filtered data samples in a given time frame to recover or de-spread data that was
transmitted using spread-spectrum technology.
December 2010 Altera Corporation
Arria II Device Handbook Volume 1: Device Interfaces and Integration