Thermal monitoring on FPGAs using ring-oscillators

Abstract

In this paper, a temperature-to-frequency transducer suitable for thermal monitoring on FPGAs is presented. The dependence between delay and temperature is used to produce a frequency drift on a ring-oscillator. Different sensors have been constructed and characterized using XC4000 and XC3000 chips, obtaining typical sensibilities of 50 kHz per °C. In addition, the utility of the Xilinx OSC4 cell as thermal transducer has been demonstrated. Although a complete temperature verification system requires a control unit with a frequency counter, the use of ring-oscillators presents several advantages: minimum FPGA elements are required; no analog parts exists; the additional hardware needed (multiplexers, prescaler, etc.) can be constructed using the resources of an FPGA, the thermal-related signals can be routed employing the standard interconnection network of the board, and finally, the sensors can be dynamically inserted or eliminated.