Non-volatile memory device with plural reference cells, and method of setting the reference cells
View Patent ↗A non-volatile memory device has an array of non-volatile memory cells, a first plurality of non-volatile memory reference cells, with each reference cell capable of being programmed to a reference level different from the other reference cells; and a second plurality of comparators. Each of the comparators is connectable to one of the first plurality of non-volatile memory reference cells and to one of a third plurality of memory cells from among the array of non-volatile memory cells.
1. A method of programming a first plurality of reference cells in a memory device having an array of non-volatile memory cells and a second plurality of comparators comprising:
(i) programming one of said first plurality of reference cells;
(ii) programming a third plurality of non-volatile memory cells, wherein said third plurality is associated with one of said second plurality of comparators;
(iii) reading said third plurality of non-volatile memory cells using one of said second plurality of comparators and a reference voltage;
(iv) continuing the programming of said third plurality of non-volatile memory cells based upon said reading;
(v) measuring the read signals of the third plurality of non-volatile memory cells;
(vi) calculating the median value of the signals from the measured signals of the third non-volatile memory cells; and
(vii) continuing the programming of said one of said first plurality of reference cells based upon said calculation.
2. The method of claim 1 further comprising:
wherein the programming step (i) programs said first plurality of reference cells in parallel;
wherein the reading step (iii) reads each of said plurality of non-volatile memory cells using a different one of said second plurality of comparators
wherein the calculating step (vi) calculates the median value of the signals from the measured signals of the third non-volatile memory cells associated with a different one of each of said first plurality of reference cells;
wherein said method further comprising: deselecting reference cells from said first plurality of reference cells based upon said calculation, after step (vi); and
wherein the continuing step (viii) continues the programming of reference cells from the first plurality of reference cells which have not been deselected.
3. The method of claim 1 further comprising:
repeating the steps of (i)-(vii) for a different one of said first plurality of reference cells.
4. The method of claim 1 wherein said continuing of programming is done by adjusting program conditions so as to maintain a substantially constant rate of electron injection.
5. The method of claim 1 wherein said one of said first plurality of reference cells is associated with a third plurality of comparators, wherein said third plurality of comparators is a subset of said second plurality of comparators, and said reading step reads said programmed reference cells and the read signal is compared to a reference signal for each one of said third plurality of comparators.
6. The method of claim 1 , wherein each reference cell from the first plurality of reference cells has an associated different comparator from the second plurality of comparators.