An authentication circuit coupled to a plurality of memory bits includes a monitoring engine configured to provide a first data pattern to all the bits thereby causing each bit to be in a first data state, detect whether a transition from the first data state to a second data state occurs for each bit in response to a first reducing voltage applied to the plurality of bits, provide a second data pattern to all the bits thereby causing each bit to be in the second data state, and detect whether a transition from the second data state to the first data state occurs for each bit in response to a second reducing voltage applied to the plurality of bits, wherein the first data state is different from the second data state, and a PUF controller configured to generate a PUF signature based on the transitions of each bit.