A physical unclonable function (PUF) is a digital circuit that can generate a die specific unique and stable response, which can be used for authentication and key generation. Since no major design or manufacturing modifications are required, exploitation of SRAMs to implement PUFs is a promising option. When initially powered up, in dividual SRAM cells acquire unique logic states based on the inherent bias of the cell. At advanced technology nodes, this bias is primarily due to unavoidable random manufacturing process variations, which are unpredictable and vary randomly from cell to cell, as well as chip to chip. When an SRAM is read out, these power-up states provide a unique output that is largely consistent during repeated power-up cycles for a given SRAM, but varies for different copies of the same part, as required of a PUF. However, this powerup state of SRAMs cannot be directly used (e.g. in cartographic key generation), due to unpredictability in some of the SRAM cells caused by electrical and electromagnetic noise and temperature fluctuations. We show in this paper that power-up states are also influenced by the power supply ramp rate at power-up, which can be yet another source of cell instability. To address the general problem of instability in SRAM power-up states that can result in inconsistent responses from SRAM PUFs, we present an effective stable cell selection method to identify the cells in the SRAM that are strongly biased, thereby resistant to circuit noise, voltage and temperature changes, and also aging. The data from the Silicon experiments presented here shows that the selected stable SRAM cells are highly reliable over temperature and voltage variations, with a bit error rate (BER) close to zero.
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