Look up table implementation of fast carry for adders and counters
AssigneeAltera Corporation
InventorsRichard G. Cliff, L. Todd Cope, Kerry Veenstra, Bruce B. Pedersen
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Abstract
Look up tables for use in programmable logic devices are modified to facilitate use of those tables to provide adders (including subtracters) and various types of counters. Each look up table is effectively partitioned into smaller look up tables when an adder or counter is required. One portion of the partitioned table is used to provide a sum out signal, while the other portion of the partitioned table is used to provide a fast carry out signal for application to the next stage of the adder or counter.
Claims23
In programmable look table apparatus which includes a plurality of programmable data storage cells, each of which produces a cell output signal indicative of the data stored in that cell, and means for normally selecting from all of said cell output signals any one of said cell output signals as a normal output signal on a normal output lead of said look up table apparatus, said means for normally selecting being responsive to a plurality of first input signals such that each of said first input signals normally controls a respective one of a plurality of successive selection means which collectively comprise said means for selecting, a first of said selection means selecting one of two mutually exclusive and collectively exhaustive subsets of said cell output signals, and each succeeding selection means selecting one of two mutually exclusive and collectively exhaustive subsets of the cell output signals selected by the preceding selection means until a final one of said selection means produces said normal output signal on said normal output lead, an improvement for allowing said look up table apparatus to produce on an auxiliary output lead, which is separate from said normal output lead, an auxiliary output signal so that when said look up table apparatus is used to perform addition, one of said normal and auxiliary output signals can be used to generate a sum out digit signal, while the other of said normal and auxiliary output signals can be used to generate a carry out digit signal, said improvement comprising:
- means for selectively applying a second input signal to a first portion of a predetermined one of said selection means in lieu of the first input signal that would normally be applied to said first portion and a remaining second portion of said predetermined selection means so that said second input signal can control said first portion to select from among the cell output signals applied to said first portion independently of the selection made by said second portion from among the cell output signals applied to said second portion in response to the first input signal which is still applied to said second portion;
- and
- means for connecting said auxiliary output lead to a predetermined one of said first and second portions for providing as said auxiliary output signal on said auxiliary output lead a cell output signal selected by said predetermined one of said portions.
Programmable logic apparatus comprising:
- a look up table for using a plurality of look up table inputs to select a plurality of look up table outputs from among a plurality of predetermined values stored in said look up table;
- means for storing a first of said look up table outputs;
- means for receiving a plurality of input signals;
- means for receiving a carry in signal;
- a first switch for selectively applying either a first of said input signals or the output of said means for storing to a first of said look up table inputs;
- a second switch for selectively applying either said first input signal or a second of said input signals to a second of said look up table inputs, said second input signal also being applied to a third of said look up table inputs;
- a third switch for applying either a third of said input signals or said carry in signal to a fourth of said look up table inputs;
- and
- means for applying a fourth of said input signals to a fifth of said look up table inputs.
Dependencies are inferred from the claim text.