US Patent 7518934 Phase change memory with program/verify function

A phase change memory includes a plurality of cells for storing data in the form of respective resistance levels, addressing circuits for addressing cells to be programmed, and the resistance levels are determined from comparison of cell currents of addressed cells with a reference current. A reference generator provides the sense amplifier with the reference current. The reference generator is provided with a reference select circuit to select the reference current from a plurality of verify currents based on program data to be stored in the cell.