Method and device for refreshing reference cells
AssigneeSTMicroelectronics S.r.l.
InventorsRichard Fournel, Leila Sedjai Aitouarab
Life of this patent
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Abstract
Reference cells are refreshed in a non-volatile memory that includes a plurality of memory cells. A selected reference cell and a non-used memory cell are read simultaneously, and a signal read from the reference cell is compared to a signal read from the non-used memory cell. A refresh signal for refreshing the reference cell is supplied when the signal read therefrom is less than the signal read from the non-used memory cell.
Claims11
Method of refreshing at least one reference memory cell (Cref) in a non-volatile type of memory (10), said reference cell (Cref) exhibiting a characteristic curve (A, A′) which corresponds to a characteristic curve of an “erased” memory cell, but shifted towards the characteristic curve of a “programmed” memory cell, characterized in that it comprises the following steps:
- (a) selecting at least a first non-used memory cell (10) as a checking cell (Cveri) and positioning its characteristic curve (VERI1g, VERI1d, VERI2d) between that of the reference cell and that of a “programmed” cell,
- (b) simultaneously selecting in readout a reference cell (Cref) and the checking cell (Cveri),
- (c) comparing the signals read in step (b),
- (d) supplying a refresh signal (Sr1, Sr2, Sr3) for refreshing the reference cell (Cref) when the signal read on the latter is less than the signal read on the checking cell (Cveri).
Method according to any one of the preceding claims, characterized in that steps (a) to (d) are repeated to the refresh the checking cell(s) (Cveri) by choosing a second checking cell.
Device for refreshing at least one reference memory cell (Cref) in a non-volatile type of memory, said reference cell (Cref) exhibiting a characteristic curve (A, A′) which corresponds to a characteristic curve of an “erased” memory cell but shifted towards the characteristic curve of a “programmed” memory cell, characterized in that it comprises:
- means (12, 14, 16, 18) for selecting a non-used memory cell as a checking cell (Cveri) and positioning the latter according to a characteristic curve (VERI1g, VERI1d, VERI2d) between that of the reference cell and that of a programmed cell,
- means for simultaneously selecting (VERI, 30) in readout a reference cell (Cref) and the checking cell,
- means (22, 28, 30) for comparing the signals read on the reference cell (Cref) and on the checking cell (Cveri), and supplying a refresh signal (Sr1, Sr2, Sr3) of the reference cell when the signal read on the reference cell is less than the signal read on the checking cell, and
- means for modifying, in the presence of the refresh signal (Sr1, Sr2, Sr3), the characteristic curve of the reference cell (Cref) in order to return it to a state close to the initial state.
Dependencies are inferred from the claim text.