Bit line discharge method for reading a multiple bits-per-cell flash EEPROM

An improved reading structure for performing a read operation in an array of two bits-per-cell flash EEPROM memory cells so as to render enhanced efficiency, said read structure comprising:

• a memory core array (12) having a plurality of memory cells arranged in rows of wordlines and columns of bit lines intersecting said rows of wordlines, each of said memory core cells including a floating gate array transistor having its control gate connected to one of said rows of wordlines, its source connected to one of said columns of bit lines, and its drain connected to a ground potential, each of said memory core cells being previously programmed to one of three memory states defined by memory core threshold voltages;
• row decoder means (14) responsive to row address signals and being operatively connected to said memory core array for selecting certain ones of said rows of wordlines;
• column decoder means (16,20) responsive to column address signals and being operatively connected to said memory core array for selecting certain ones of said columns of bit lines;
• reference cell array means (22) including a plurality of reference core cells arranged in rows of wordlines corresponding to the number of rows in said memory core array and in columns of reference bit lines corresponding to the three memory states and which are selected together with a selected core cell and providing selectively one of three reference cell bit line voltages defined by reference cell threshold voltages, each of said reference cells being previously programmed to a corresponding one of the three reference cell threshold voltages at the same time as said memory core cells are being programmed;
• each of said reference core cells including a reference cell transistor having its gate connected to the gates of the floating gate array transistors located in the same said rows of wordlines, its source connected to one of the columns of reference bit lines, and its drain connected to the ground potential;
• pre-charging means (36) for sequentially pre-charging said selected array bit lines in said memory core array and one of said reference bit lines in the columns to a predetermined potential;
• bandgap means (34) for generating bandgap reference derived voltages;
• a reference bit line level detector (28) having a first input connected to receive states of said bit line voltages of said three reference cells and a second input connected to receive said bandgap reference derived voltages for generating strobe signals when said bit line voltages of said reference cells have discharged down to corresponding ones of said bandgap reference derived voltages;
• reading circuit means including a differential pair amplifier (26) and a latch (27) and being responsive to said strobe signals for comparing the memory core threshold voltages of said selected memory core cells with each of said three reference cell threshold voltages so as to determine which one of the three memory states the selected memory core cell is storing;
• said differential pair amplifier having an inverting input coupled to said selected array bit line and a non-inverting input coupled to said reference bit line for comparing said memory core threshold voltage and said reference cell threshold voltage, and an output;
• said latch having an input connected to the output of said differential pair amplifier and an output providing a logic signal, said latch being turned on only when said strobe signals are generated by said reference bit line voltage level detector so as to store the output of said differential pair amplifier;
• and
• said latch storing said logic signal which is at a low logic level when said memory core threshold voltage of said selected memory core cells is less than said reference core threshold voltage and which is at a high logic level when said memory core threshold voltage is greater than said reference cell threshold voltage.

A method for performing a read operation in an array of multi-density flash EEPROM memory cells so as to render enhanced efficiency, said method comprising the steps of:

• providing a memory core array having a plurality of memory cells arranged in rows of wordlines and columns of bit lines intersecting the rows of wordlines, each of the memory core cells being previously programmed to one of a plurality of memory conditions defined by memory core threshold voltages;
• providing a reference cell array including a plurality of reference core cells arranged in rows of wordlines corresponding to the number of rows in the memory core array and in columns of reference bit lines corresponding to the number of storage levels and which are selected together with a selected core cell and providing selectively one of a plurality of reference cell bit line voltages defined by reference cell threshold voltages, each of the reference cells being previously programmed to the reference cell threshold voltages at the same time as the memory core cells are being programmed;
• precharging sequentially the array bit lines and one of the reference bit lines in the columns to a predetermined voltage potential;
• generating a strobe signal in response to the discharging of the selected reference bit line to a pre-selected level;
• comparing the memory threshold voltage of the selected memory core cells with each of the reference cell threshold voltages so as to determine which one of the plurality of memory conditions the selected memory core cell is storing;
• and
• storing a logic signal which is at a low logic level when the memory core threshold voltage of the selected memory core cell is less than the reference cell threshold voltage and which is at a high logic level when the memory core threshold voltage is greater than the reference cell threshold voltage only when the strobe signal has been generated.