Redundancy decoder

An integrated circuit, comprising:

• a plurality of primary memory cells, arranged in a plurality of primary array blocks;
• means for accessing a primary memory cell responsive to a column address signal presented thereto, comprising:
• a column predecoder, having a plurality of block select outputs, for decoding a first portion of the column address signal and for generating a block select signal at its block select outputs to select one of the plurality of primary array blocks corresponding to the value of the first portion of the column address signal;
• and
• a column decoder, for decoding a second portion of the column address signal, and for selecting a column in the selected primary array block corresponding to the second portion of the column address signal;
• a redundant memory array comprising a plurality of redundant memory cells;
• and
• a redundant decoder, for selecting a redundant memory cell responsive to an address signal presented thereto corresponding to a programmed value in said redundant decoder, said redundant decoder comprising:
• an enable circuit, comprising a fuse, having an output indicating whether or not the redundant decoder is enabled responsive to the state of its fuse;
• a first plurality of compare circuits, each having an input for receiving an associated one of the block select outputs of said column predecoder, and each having an output, the outputs of the first plurality of compare circuits being connected together;
• a second plurality of compare circuits, each having an input coupled to an address line corresponding to the second portion of the column address signal, and having an outputs, each of said first and second pluralities of compare circuits comprising (a) a pass gate having a conduction path, and having a control terminal coupled to the output of said enable circuit so that said pass gate is conductive responsive to the enable circuit indicating that the redundant decoder is enabled, and (b) a fuse, connected in series with the conduction path of the pass gate between the input and output of its compare circuit, for disconnecting, when opened, the input of its compare circuit from the output of its compare circuit;
• a conjunctive logic circuit coupled to receive the connected-together outputs of said first plurality of compare circuits, and to receive the outputs of said second plurality of compare circuits, said conjunctive logic circuit having an output coupled to enable selection of a redundant memory cell in said redundant memory array responsive to the column address signal matching a programmed value corresponding to a pattern of opened fuses in the first and second pluralities of compare circuits;
• and
• a plurality of bias transistors, each having a conduction path coupled between the output of one of said first and second pluralities of compare circuits and a bias voltage, and each having a control terminal coupled to the output of said enable circuit in such a manner that, when the redundant decoder is not enabled, each of the inputs to said conjunctive logic circuit is biased to said bias voltage to cause the output of said conjunctive logic circuit not to enable selection of a redundant memory cell in said redundant memory array.