Logic Elements
Dynamic Arithmetic Mode
The dynamic arithmetic mode is ideal for implementing adders, counters,
accumulators, wide parity functions, and comparators. An LE in dynamic
arithmetic mode uses four 2-input LUTs configurable as a dynamic
adder/subtractor. The first two 2-input LUTs compute two summations
based on a possible carry-in of 1 or 0; the other two LUTs generate carry
outputs for the two chains of the carry select circuitry. As shown in
Figure 2–7, the LAB carry-in signal selects either the carry-in0or
carry-in1chain. The selected chain's logic level in turn determines
which parallel sum is generated as a combinatorial or registered output.
For example, when implementing an adder, the sum output is the
selection of two possible calculated sums:
data1 + data2 + carry-in0
or
data1 + data2 + carry-in1
The other two LUTs use the data1and data2signals to generate two
possible carry-out signals⎯one for a carry of 1 and the other for a carry of
0. The carry-in0signal acts as the carry select for the carry-out0
output and carry-in1acts as the carry select for the carry-out1
output. LEs in arithmetic mode can drive out registered and unregistered
versions of the LUT output.
The dynamic arithmetic mode also offers clock enable, counter enable,
synchronous up/down control, synchronous clear, synchronous load,
and dynamic adder/subtractor options. The LAB local interconnect data
inputs generate the counter enable and synchronous up/down control
signals. The synchronous clear and synchronous load options are
LAB-wide signals that affect all registers in the LAB. The Quartus II
software automatically places any registers that are not used by the
counter into other LABs. The addnsubLAB-wide signal controls
whether the LE acts as an adder or subtractor.
Altera Corporation
May 2008
2–9
Preliminary
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