Multiple bit per cell non volatile memory apparatus and system having polarity control and method of programming same
View Patent ↗A Multiple-bit per Cell (MBC) non-volatile memory apparatus, method, and system wherein a controller for writing/reading data to/from a memory array controls polarity of data by selectively inverting data words to maximize a number of bits to be programmed within (M−1) virtual pages and selectively inverts data words to minimize a number of bits to be programmed in an M th virtual page where M is the number of bits per cell. A corresponding polarity control flag is set when a data word is inverted. Data is selectively inverted according the corresponding polarity flag when being read from the M virtual pages. A number of the highest threshold voltage programming states in reduced. This provides tighter distribution of programmed cell threshold voltage, reduced power consumption, reduced programming time, and enhanced device reliability.
1. A Solid-State Disk (SSD) comprising:
a multiple-bit per cell (MBC) non-volatile memory apparatus including:
a memory array including an electrically erasable block;
the block including a reprogrammable page;
the reprogrammable page comprising upper and lower pages sharing common word-lines;
the upper and lower pages including respective upper and lower data fields;
the upper and lower data fields including respective virtual upper and lower cells of MBC memory cells;
the MBC memory cells having respective threshold voltages programmable to a selected one of first level, second level, third level, or fourth level in order from the lowest voltage level,
wherein programming the lower cells comprises programming the respective threshold voltages from the first threshold voltage level to the second threshold voltage level, and
programming upper cells comprises programming the respective threshold voltages from the first threshold voltage level to the fourth threshold voltage level or from the second threshold voltage level to the third threshold voltage level; and
a controller for writing data to the memory array, wherein the controller controls polarity by selectively inverting data to maximize a number of the bits within a lower page to be programmed and selectively inverting data to minimize a number of bits to be programmed in the respective upper page.
2. The SSD as claimed in claim 1 wherein the upper and lower pages comprise respective upper and lower spare fields.
3. The SSD as claimed in claim 2 wherein the upper and lower spare fields comprise respective upper and lower page polarity flags.
4. The SSD as claimed in claim 1 wherein the first level, the second level, the third level, and the fourth level of threshold voltages are defined as ‘11’, ‘10’, ‘00’, and ‘01’ combinations of the upper and lower cells each respectively.
5. The SSD as claimed in claim 4 wherein the first level threshold voltage represents an erased state.
6. The SSD as claimed in claim 4 wherein the first level threshold voltage represents an unprogrammed upper cell and an unprogrammed lower cell.
7. The SSD as claimed in claim 1 wherein the second level threshold voltage represents an unprogrammed upper cell and a programmed lower cell.
8. The SSD as claimed in claim 1 wherein the third level threshold voltage represents a programmed upper cell and a programmed lower cell.
9. The SSD as claimed in claim 1 wherein the second level threshold voltage represents a programmed upper cell and an unprogrammed lower cell.
10. The SSD as claimed in claim 1 wherein the upper and lower pages further comprise respective upper and lower spare fields.
11. The SSD as claimed in claim 10 wherein the upper and lower spare fields include respective upper and lower polarity flags.
12. The SSD as claimed in claim 1 wherein the controller comprises means for reading data from the memory array wherein the controller includes means for sensing threshold voltages of MBC cells within a page, means for providing an upper data word by comparing the threshold voltages to a predetermined voltage reference, and means for inverting the upper data word if a upper page polarity flag is set.
13. The SSD as claimed in claim 1 wherein the controller comprises means for reading data from the memory array wherein the controller includes means for sensing threshold voltages of MBC cells within a page, means for providing a lower data word by comparing the threshold voltages to two predetermined voltage references, and means for inverting the lower data word if a lower page polarity flag is set.
14. A Solid-State Disk (SSD) including a multi-bit per cell (MBC) non-volatile memory including means for reading data comprising:
means for sensing threshold voltages of MBC cells within a page;
means for providing an upper data word by comparing the threshold voltages to a predetermined voltage reference; and
means for inverting the upper data word if an upper page polarity flag is set.
15. The SSD as claimed in claim 14 wherein the means for inverting the upper data word comprises means for reading the upper page polarity flag from a spare field.