Fast-carry arithmetic circuit using a multi-input look-up table
View Patent ↗In one embodiment of the invention, programmable circuits, such as FPGAs, may be used to implement different types of functions, such as a multi-bit adder, using look-up table (LUT) circuits as their building blocks. Efficient generation of carry-out signals and fast-carry generation signals using available SRAM cells in the various embodiments of the LUT circuit can reduce and/or eliminate area-inefficient look-ahead carry logic without a significant delay in signal generation.
1. Circuitry adapted to selectively operate in a look-up table (LUT) mode or an arithmetic mode, the circuitry comprising:
a LUT circuit having a plurality of memory cells and a decoder connected to receive signals based on data stored in the memory cells and having a plurality of multiplexers (muxes) controlled by LUT address signals A, B, C, and D and configured in three decoder stages, the first stage having four muxes controlled by address signals A and B in either mode, the second stage having two muxes controlled by address signal C in the LUT mode, and the third stage having one mux controlled by address signal D in the LUT mode; and
a control circuit connected to the LUT circuit and adapted to control whether the circuitry operates in the LUT mode as a LUT or the arithmetic mode as an adder, wherein the control circuit includes:
a memory cell adapted to store a control signal for the control circuit;
a first mux connected to receive the control signal at its selection input, address signal C at one signal input, and a carry-in signal CIN at another signal input, and having an output connected to the selection inputs of the two multiplexers in the second decoder stage; and
a second mux connected to receive the control signal at its selection input, address signal D at one signal input, and a constant value signal at another signal input, and having an output connected to the selection input of the multiplexer in the third decoder stage,
wherein in the arithmetic mode, the two muxes of the second decoder stage are controlled by carry-in signal CIN and the one mux of the third decoder stage is controlled by the constant value signal such that the sum of address signals A and B and carry-in signal CIN appears at the output of the one mux of the third decoder stage.
2. The invention of claim 1 , further comprising carry-out circuitry connected to the LUT circuit, the carry-out circuitry including a third mux having its signal inputs connected to two muxes of the first decoder stage and its selection input connected to receive the carry-in signal CIN.
3. The invention of claim 1 , wherein the circuitry is implemented as part of an FPGA.
4. The invention of claim 3 , wherein the FPGA comprises a plurality of programmable logic blocks (PLBs), at least one PLB having one or more instances of the circuitry.