Logic Elements
functions. 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. This provides another mechanism for improved
fitting. The LE can also drive out registered and unregistered versions of
the LUT output.
LUT Chain and Register Chain
In addition to the three general routing outputs, the LEs within a LAB
have LUT chain and register chain outputs. LUT chain connections allow
LUTs within the same LAB to cascade together for wide input functions.
Register chain outputs allow registers within the same LAB to cascade
together. The register chain output allows a LAB to use LUTs for a single
combinatorial 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. “MultiTrack
Interconnect” on page 2–12 for more information on LUT chain and
register chain connections.
addnsub Signal
The LE's dynamic adder/subtractor feature saves logic resources by
using one set of LEs to implement both an adder and a subtractor. This
feature is controlled by the LAB-wide control signal addnsub. The
addnsubsignal sets the LAB to perform either A + B or A −B. The LUT
computes addition; subtraction is computed by adding the two's
complement of the intended subtractor. The LAB-wide signal converts to
two's complement by inverting the B bits within the LAB and setting
carry-in = 1 to add one to the least significant bit (LSB). The LSB of an
adder/subtractor must be placed in the first LE of the LAB, where the
LAB-wide addnsubsignal automatically sets the carry-in to 1. The
Quartus II Compiler automatically places and uses the adder/subtractor
feature when using adder/subtractor parameterized functions.
LE Operating Modes
The Cyclone LE can operate in one of the following modes:
■
■
Normal mode
Dynamic arithmetic mode
Each mode uses LE resources differently. In each mode, eight available
inputs to the LE⎯the four data inputs from the LAB local interconnect,
carry-in0and carry-in1from the previous LE, 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, asynchronous
Altera Corporation
May 2008
2–7
Preliminary
ALTERA [ ALTERA CORPORATION ]
