US Patent 7426134 Sense circuit for resistive memory

A memory includes a phase-change memory cell and a circuit. The phase-change memory cell can be set to at least three different states including a substantially crystalline state, a substantially amorphous state, and at least one partially crystalline and partially amorphous state. The circuit applies a first voltage across the memory cell to determine whether the memory cell is set at the substantially crystalline state and applies a second voltage across the memory cell to determine whether the memory cell is set at a partially crystalline and partially amorphous state.