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EP1C12F100I7 Datasheet(PDF) 11 Page - Altera Corporation |
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EP1C12F100I7 Datasheet(HTML) 11 Page - Altera Corporation |
11 / 94 page ![]() Altera Corporation 11 Preliminary Information Cyclone FPGA Family Data Sheet 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 addnsub signal 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 addnsub signal 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-in0 and carry-in1 from 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 preset/load, synchronous clear, synchronous load, and clock enable control for the register. These LAB-wide signals are available in all LE modes. The addnsub control signal is allowed in arithmetic mode. 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, the designer can also create special-purpose functions that specify which LE operating mode to use for optimal performance. |
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