A High-Resolution Nanosecond-Scale On-Chip Voltage Sensor for FPGA Applications

As field programmable gate array (FPGA) becomes an increasingly important digital circuit platform, the voltage fluctuations on its power distribution network (PDN) play an unignorable role in impacting circuit operation and performance. Voltage fluctuations induced by one circuit block can affect other blocks on the same chip through the PDN, causing performance degradation or even functionality loss by critical timing fault. This also opens the door to numerous potential attacks that maliciously create transient voltage drops on the PDN in multi-user FPGA environments. Therefore, a compact, location-flexible, and high-speed ON-chip voltage sensor is needed to understand or monitor the voltage fluctuations on the PDN at runtime. Limited by the available logic resources of FPGA, in this article, the design possibility of such sensors using tapped delay line (TDL) based time-to-digital converter (TDC) technique is explored, and two types of resolution enhancement methods are presented to achieve its optimal performance. Through a rigorous calibration process, the performance of the proposed sensors is quantitively evaluated and compared. Using Xilinx Kintex-7 FPGA, the voltage sensors can achieve 0.9 mV measurement resolution with 0.9 mVrms precision at a 554 MHz sampling rate. The achievable resolution is comparable to the built-in 12-bit analog-to-digital converter (ADC), but the sampling rate is increased by several hundred times. Through experimental measurements, the nanosecond-scale transients detecting capability of the sensors are also demonstrated.