A circuit includes a cell segment, first and second reference cells, and a current sense amplifier. The first and second reference cells are configured to store opposite logic values, respectively. The current sense amplifier is configured with a first node and a second node for currents therethrough to be compared with each other. The current sense amplifier includes a multiplexer configured to couple the first reference cell or the second reference cell to the first node of the current sense amplifier, and couple the second reference cell or the first reference cell to the second node of the current sense amplifier in a first mode, and couple a cell of the cell segment to the first node of the current sense amplifier, and couple the first and second reference cells to the second node of the current sense amplifier in a second mode.