A circuit includes 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 for reading bits stored in the first reference cell and the second reference cell.