Memory apparatus including programmable non-volatile multi-bit memory cell, and apparatus and method for demarcating memory states of the cell
Memory states of a multi-bit memory cell are demarcated by generating read reference signals having levels that constitute boundaries of the memory states. The read reference signals may be dependent upon the levels of programming reference signals used for controlling the programming of the memory cell. The memory cell can thus be programmed without reading out its memory state during the programming process, with programming margins being assured by the dependence of the read reference signals on the programming reference signals. Both sets of reference signals may be generated by reference cells which track variations in the operating characteristics of the memory cell with changes in conditions, such as temperature and system voltages, to enhance the reliability of memory programming and readout.
1. A non-volatile memory apparatus, comprising:
an electrically-alterable non-volatile memory cell having more than two predetermined memory states;
a programming signal source which applies a programming signal to said memory cell;
control circuitry which generates a plurality of programming reference parameters and selects among said plurality of programming reference parameters in accordance with information indicating a memory state to which said memory cell is to be programmed, each programming reference parameter corresponding to a different memory state of the memory cell, and said control circuitry controlling the application of said programming signal to said memory cell based on the selected programming reference parameter; and
read circuitry which reads the state of the memory cell by comparing a parameter corresponding to the state of the memory cell with a plurality of read reference parameters having values that are different from values of said programming reference parameters, and
wherein, for each read reference parameter and a corresponding programming reference parameter, the value of one of the read reference parameter and the programming reference parameter is shifted from and dependent upon the value of the other.
2. A non-volatile memory apparatus according to claim 1 , wherein said control circuitry includes a comparator which compares said parameter corresponding to the state of said memory cell with the selected programming reference parameter.
3. A non-volatile memory apparatus according to claim 2 , wherein said programming reference parameters and said parameter corresponding to the state of said memory cell are voltages.
4. A non-volatile memory apparatus according to claim 3 , wherein the voltage corresponding to the state of said memory cell is a voltage on a bit line of said memory cell.
5. A non-volatile memory apparatus according to claim 2 , wherein said comparator has a first input terminal coupled to a bit line terminal of said memory cell to receive said parameter corresponding to the state of said memory cell and a second input terminal which receives the selected programming reference parameter.
6. A non-volatile memory apparatus according to claim 2 , wherein an output of said comparator changes state after said memory cell reaches the memory state to which said memory cell is to be programmed.
7. A non-volatile memory apparatus according to claim 6 , wherein the application of said programming signal is stopped in response to the change of state of the output of said comparator.
8. A non-volatile memory apparatus according to claim 1 , wherein said memory cell includes a floating gate FET.
9. A non-volatile memory apparatus according to claim 1 , wherein one of said programming reference parameters and said read reference parameters is generated using a reference cell.
10. A non-volatile memory apparatus according to claim 9 , wherein said reference cell is fabricated using the same process as said memory cell.
11. A non-volatile memory apparatus according to claim 9 , wherein said reference cell is factory programmed.
12. A non-volatile memory apparatus according to claim 9 , wherein said reference cell is programmed from a ROM cell.
13. A non-volatile memory apparatus according to claim 9 , including a plurality of said memory cells arranged in an array of rows and columns, and each row of memory cells is provided with a corresponding set of reference cells for generating said programming reference parameters and/or said read reference parameters.
14. A non-volatile memory apparatus according to claim 1 , said memory cell has an erased state and three further states.
15. A non-volatile memory apparatus according to claim 1 , wherein said read reference parameters include a read reference parameter which has a value substantially midway between values of an adjacent pair of said programming reference parameters.
16. A non-volatile memory apparatus according to claim 1 , wherein said read reference parameters include a read reference parameter which has a value that is shifted relative to a value midway between values of an adjacent pair of said programming reference parameters.
17. A non-volatile memory apparatus according to claim 16 , wherein the shifted value is shifted toward a higher one of the values of said adjacent pair of said programming reference parameters.
18. A non-volatile memory apparatus according to claim 1 , wherein each read reference parameter is generated to have a value based on the value of at least one of said programming reference parameters.
19. A non-volatile memory apparatus according to claim 1 , wherein said read circuitry time shares common circuit components with said control circuitry.