摘要:
The controlled erase method includes supplying at least one erase pulse to cells of a memory array; comparing the threshold voltage of the erased cells with a low threshold value; selectively soft-programming the erased cells which have a threshold voltage lower than the low threshold value; and verifying whether the erased cells have a threshold voltage lower than a high threshold value, which is higher than the low threshold value. If at least one predetermined number of erased cells has a threshold voltage which is higher than the high threshold value, an erase pulse is applied to all the cells and the steps of comparing, selectively soft-programming and verifying are repeated.
摘要:
The reading circuit comprises a current source, which, via a current reflection circuit, supplies a constant predetermined current to a cell to be read, an operational amplifier with a non-inverting input connected to the drain terminal of the cell, and an output connected to the gate terminal of the cell. The source terminal of the cell is connected to ground. Thereby the output voltage of the operational amplifier supplies directly (at the set current) the threshold voltage of the cell, and the drain terminal of the cell is biased to a positive voltage.
摘要:
A programming method comprises the steps of applying a ramp voltage having a first slope to the gate terminal of a selected memory cell to rapidly bring the threshold voltage of the selected cell to an intermediate value; then applying a ramp voltage having a second slope lower than the first, to end programming to the desired final threshold value with high precision. Thereby, when a high threshold value is to be programmed, programming time is reduced; on the other hand, if a low threshold value is to be programmed, the slower ramp voltage is applied right from the start, to prevent possible overprogramming of the cell.
摘要:
A programming method for a nonvolatile memory includes the steps of: a) determining a current value of the threshold voltage; b) acquiring a target value of the threshold voltage; c) calculating a first number of gate voltage pulses necessary to take the threshold voltage from the current value to the target value; d) applying a second number of consecutive voltage pulses to the gate terminal of the cell, the second number being correlated to the first number and having a uniformly increasing amplitude; e) then measuring a current value of the threshold voltage; and repeating steps c) to e) until a final threshold value is obtained.
摘要:
For each memory cell to be programmed, the present threshold value of the cell is determined; the desired threshold value is acquired; the analog distance between the present threshold value and the desired threshold value is calculated; and a programming pulse is then generated, the duration of which is proportional to the analog distance calculated. The programming and reading cycle is repeated until the desired threshold is reached. By this means a time saving is obtained, owing to the reduction of the number of intermediate reading steps. The method permits programming in parallel and simultaneously of a plurality of cells of a memory array which is connected to a single word line and to different bit lines, each with a programming pulse the duration of which is proportional to the analog distance calculated for the same cell. The programming process is thus very fast, owing to parallel application of the programming and the saving in the intermediate reading cycles.
摘要:
A device for analog programming is disclosed. The device comprises a current mirror circuit connected to drain terminals of a cell to be programmed and of a MOS reference transistor. An operational amplifier has inputs connected to the drain terminals of the cell and respectively of the MOS transistor and an output connected to the control terminal of the MOS transistor. During programming, the control and drain terminals of the cell are biased at corresponding programming voltages and the output voltage of the operational amplifier, which is correlated to the current threshold voltage level of the cell, is monitored and the programming is interrupted when this output voltage becomes at least equal to a reference voltage correlated to the threshold value desired for the cell.
摘要:
The programming method comprises the steps of applying a programming pulse to a first cell and simultaneously verifying the present threshold value of at least a second cell; then verifying the present threshold value of the first cell and simultaneously applying a programming pulse to the second cell. In practice, during the entire programming operation, the gate terminal of both the cells is biased to a same predetermined gate voltage and the source terminal is connected to ground; the step of applying a programming pulse is carried out by biasing the drain terminal of the cell to a predetermined programming voltage and the step of verifying is carried out by biasing the drain terminal of the cell to a read voltage different from the programming voltage. Thereby, switching between the step of applying a programming pulse and verifying is obtained simply by switching the drain voltage of the cells.
摘要:
An analog reading circuit having a current mirror circuit forcing two identical currents into a cell to be read and into a reference cell. An operational amplifier has an inverting input connected to the drain terminal of the cell to be read, a non-inverting input connected to the drain terminal of the reference cell, and an output connected to the gate terminal of the reference cell. The reference cell therefore forms part of a negative feedback loop which maintains the overdrive voltages of the cell to be read and the reference cell constant, irrespective of temperature variations. The reading circuit is also of high precision and has a high reading speed.
摘要:
A reading circuit comprises a current mirror circuit connected, at a first and a second output node, to the drain terminals of an array cell and of a reference cell; a comparator whose inputs are connected to the output nodes of the current mirror circuit; a ramp generator having an enabling input connected to the output of the comparator and an output connected to the control terminal of the reference cell. Biasing the gate terminal of the array cell to a constant voltage, when the currents flowing in the array cell and in the reference cell are equal, the value assumed by the ramp voltage is proportional to the threshold value of the array cell; at that time the comparator is triggered and discontinues the ramp increase, supplying as output the desired threshold value.
摘要:
A method of programming a plurality of memory cells are connected in parallel between first and second supply references and having their gate terminals connected together and, through row decoding means, also connected to an output terminal of an operational amplifier that is adapted to generate a word voltage signal, the first voltage reference being provided by a charge pump circuit. The programming method uses a program loop that includes the cells to be programmed and the operational amplifier, the charge pump circuit thus outputting a voltage ramp whose slope is a function of the cell demand. A programming circuit adapted to implement the method is also provided.