Programmed reference

摘要

A programmable reference used to identify a state of an array cell in a multi-density or low voltage supply flash EEPROM memory array. The programmable reference includes one or more reference cells, each reference cell having a floating gate which is programmed to control its threshold value. The array cells are read by applying an identical voltage to the gate of the array cell and the reference cell and comparing outputs to determine the array cell state. During read of an array cell, the programmable reference cell is biased the same as the array cell, so that the difference in threshold values between reference cells and array cells remain constant with a change in VCC. Circuitry is included for programming the reference cells utilizing a simple resistor ratio. Programming is performed at test time, preferably by the manufacturer, to assure VCC remains within strict tolerances. The array cells are programmed and read without resistor biasing and under looser tolerances using the reference cells at a later time.