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Two-LUT-based synthesizable temperature sensor for Virtex-6 FPGA devices

This paper proposes a new synthesizable oscillator-based temperature sensor with minimal footprint for use in contemporary Xilinx FPGA devices. In contrast to previously published ring-oscillator architectures, based on inverters mapped onto single LUTs, the proposed oscillator uses an asynchronous Gray-coded 4-bit counter requiring only two 6-input LUTs. Due to its reduced hardware requirements, the feedback path can be implemented using local routing signals only. Therefore, the impact of the routing on the oscillator frequency is slightly reduced making the oscillator less prone to placement-caused routing deviations. The proposed temperature sensor is calibrated using a two-point calibration approach, resulting in a mean accuracy of ±0.71° C with a mean resolution of 0.0081° C. As a further case study, a methodology to characterize on-chip semiconductor variations between identical FPGAs and within the same FPGA chip is presented using a sensor array, including up to 196 of the proposed two-LUT-based oscillators.