Multi-state EEprom read and write circuits and techniques

A circuit for sensing a test current relative to a plurality of predetermined reference current levels, each of the plurality of reference current levels being scalable by a multiplication factor from that of a lowest reference current level thereof, comprising:

• a one-to-many current mirror means for reproducing a test current into one or more reproduced currents, said current mirror having a first leg for carrying the test current and a second leg comprising a plurality of branches, such that each branch is associated with a reference current level and a multiplication factor;
• a first current source at each branch for reproducing a reproduced current therein, said reproduced current being scalable from the test current in the first leg by the associated multiplication factor;
• a second current source at each branch for providing a highest reference current level from among said plurality of reference current levels;
• and
• means for simultaneously detecting in each branch a relatively high or low voltage at a node between first and second current sources, the relatively high or low voltage corresponding to whether the reproduced current scaled from the test current provided by the first current source has magnitude greater or less than that of the highest reference current provided by the second current source.

A circuit for sensing a test current relative to a plurality of predetermined reference current levels, each of the plurality of reference current levels being scalable by a multiplication factor from that of a given reference current level thereof, comprising:

• a one-to-many current mirror means for reproducing the given reference current level into one or more reproduced currents, said current mirror having a first leg for carrying the given reference current and a second leg comprising a plurality of branches, such that each branch is associated with a reference current level and a multiplication factor;
• a first current source at each branch for reproducing a reproduced current therein, said reproduced current being scalable from the given reference current level in the first leg by the associated multiplication factor;
• a second current source at each branch for providing the test current;
• and
• means for simultaneously detecting in each branch a relatively high or low voltage at a node between first and second current sources, the relatively high or low voltage corresponding to whether the reproduced current scaled from the lowest reference current level provided by the first current source has magnitude greater or less than that of the test current provided by the second current source.

In an integrated circuit memory system having an array of a plurality of addressable semiconductor electrically erasable and programmable memory (EEprom) cells of the type having a source, a drain, a control gate, a floating gate capable of retaining a charge level programmed into it during use of the memory system resulting in a definite memory state having a corresponding threshold of conduction current relative to one or more predetermined threshold current levels used to demarcate memory states, and an erase electrode capable of removing charge from said floating gate, and said memory system including a reading system for determining the programmed state of an addressed cell, said reading system comprising:

• one or more reference memory cells constituted from the array of EEprom cells that are each respectively programmed with a charge that corresponds to each of said one or more predetermined thresholds;
• and
• means responsive to said one or more reference memory cells for comparing the threshold current level of an addressed cell with that of said one or more reference memory cells, thereby determining relative to which of said one or more predetermined threshold levels the addressed cell lies, whereby one or more bits of data stored in the addressed cell is readable therefrom;
• and wherein said means for comparing the threshold current level further comprising:
• a one-to-many current mirror means for reproducing the threshold current level of an addressed cell into one or more reproduced currents, said current mirror having a first leg for carrying the threshold current level of an addressed cell and a second leg comprising a plurality of branches, such that each branch is associated with one of said one or more predetermined threshold levels;
• a first current source at each branch for reproducing a reproduced current therein, said reproduced current being substantially similar to the threshold current level in the first leg;
• a second current source at each branch for providing a reference current having one of the predetermined threshold levels;
• and
• means for simultaneously detecting in each branch a relatively high or low voltage at a node between first and second current sources, the relatively high or low voltage corresponding to whether the reproduced current similar to the threshold current level provided by the first current source has magnitude greater or less than that of one of the predetermined threshold levels provided by the second current source.