US Patent 9279850 Physically unclonable functions with enhanced margin testing

Apparatus for identifying stable physically unclonable function (PUF) cells includes an array of PUF cells, a bias control circuit, and a selector circuit. The bias control circuit has a plurality of bias control lines that apply one or more bias control signals to each PUF cell in the array of PUF cells. The selector circuit selects a subset of the PUF cells in the array of PUF cells based on whether outputs of the PUF cells in the array of PUF cells change in response to application of the bias control signals. A corresponding method is also disclosed.