Method of handling reference cells in NVM arrays
A memory chip includes memory cells storing data to be read, at least one reference cell having a reference cell current level, at least one reference gate voltage memory cell storing a reference gate voltage value and a read circuit to read the memory cells with a fixed gate voltage with respect to at least one reference cell activated at a voltage having its associated stored reference gate voltage value.
1. A memory chip comprising:
memory cells storing data to be read;
at least one reference cell having a reference cell current level;
at least one reference gate voltage memory cell storing a reference gate voltage value; and
a read circuit to read said memory cells with a fixed gate voltage with respect to at least one reference cell activated at a voltage having its associated said stored reference gate voltage value.
2. The memory chip according to claim 1 and also comprising a reference gate voltage determiner to determine, for each said reference cell, a difference between a shifted reference cell current level and an initial state and to set said reference gate voltage value as a function of said difference.
3. The memory chip according to claim 2 and wherein said reference gate voltage determiner comprises a unit to change said reference gate voltage level from said fixed gate voltage level in steps and to stop changing said reference gate voltage level when a predefined margin has been achieved.
4. The memory chip according to claim 1 and wherein said memory cells are one-time programmable cells.
5. The memory chip according to claim 1 and wherein said memory cells are multi-level cells.
6. The memory chip according to claim 5 and wherein said multi-levels are 4 levels.
7. The memory chip according to claim 5 and wherein said multi-levels are 8 levels.
8. The memory chip according to claim 5 and wherein said multi-levels are 16 levels.
9. The memory chip according to claim 1 and also comprising a reference gate voltage determiner to determine a difference between a shifted reference cell current level and an edge of a distribution of cell currents and to set said reference gate voltage value as a function of said difference.
10. The memory array according to claim 2 and wherein said memory cells are programmable and erasable cells.
11. A method for reading a memory array, the method comprising:
reading one or more reference gate voltage memory cells for reference gate voltage values stored therein;
providing a fixed gate voltage to at least one memory cell to be read;
providing reference gate voltages at said reference gate voltage values to associated reference cells when reading said at least one memory cell to be read; and
comparing the output of said at least one memory cell to be read with the output of said reference cells.
12. The method according to claim 11 and also comprising determining, for each said reference cell, a difference between a shifted reference cell current level and an initial state and setting said reference gate voltage value as a function of said difference.
13. The method according to claim 12 and wherein said determining comprises changing said reference gate voltage level from said fixed gate voltage level in steps and stopping to change said reference gate voltage level when a predefined margin is achieved.
14. The method according to claim 11 and also comprising determining, for each said reference gate voltage cell, a difference between a shifted reference cell current level and an edge of a distribution of cell currents and setting said reference gate voltage value as a function of said difference.