Logic Elements
Another special packing mode allows the register output to feed back into
the LUT of the same LE so that the register is packed with its own fan-out
LUT, providing another mechanism for improved fitting. The LE can also
drive out registered and unregistered versions of the LUT output.
In addition to the three general routing outputs, the LEs within an LAB
have register chain outputs. Register chain outputs allow registers within
the same LAB to cascade together. The register chain output allows an
LAB to use LUTs for a single combinational function and the registers to
be used for an unrelated shift register implementation. These resources
speed up connections between LABs while saving local interconnect
resources. See “MultiTrack Interconnect” on page 2–10 for more
information on register chain connections.
LE Operating Modes
The Cyclone II LE operates in one of the following modes:
■
■
Normal mode
Arithmetic mode
Each mode uses LE resources differently. In each mode, six available
inputs to the LE—the four data inputs from the LAB local interconnect,
the LAB carry-in from the previous carry-chain LAB, and the register
chain connection—are directed to different destinations to implement the
desired logic function. LAB-wide signals provide clock, asynchronous
clear, synchronous clear, synchronous load, and clock enable control for
the register. These LAB-wide signals are available in all LE modes.
The Quartus® II software, in conjunction with parameterized functions
such as library of parameterized modules (LPM) functions, automatically
chooses the appropriate mode for common functions such as counters,
adders, subtractors, and arithmetic functions. If required, you can also
create special-purpose functions that specify which LE operating mode to
use for optimal performance.
Normal Mode
The normal mode is suitable for general logic applications and
combinational functions. In normal mode, four data inputs from the LAB
local interconnect are inputs to a four-input LUT (see Figure 2–3). The
Quartus II Compiler automatically selects the carry-in or the data3
signal as one of the inputs to the LUT. LEs in normal mode support
packed registers and register feedback.
2–4
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2007
ALTERA [ ALTERA CORPORATION ]
