Abstract
In this paper, we show how a deep-submicron field-programmable gate array (FPGA) can be operated more stably at extremely low temperatures through special firmware design techniques. Stability at low temperatures is limited through long power supply wires and reduced performance of various printed circuit board components commonly employed at room temperature. Extensive characterization of these components shows that the majority of decoupling capacitor types and voltage regulators are not well behaved at cryogenic temperatures, asking for an ad hoc solution to stabilize the FPGA supply voltage, especially for sensitive applications. Therefore, we have designed a firmware that enforces a constant power consumption, so as to stabilize the supply voltage in the interior of the FPGA. The FPGA is powered with a supply at several meters distance, causing significant resistive voltage drop and thus fluctuations on the local supply voltage. To achieve the stabilization, the variation in digital logic speed, which directly corresponds to changes in supply voltage, is constantly measured and corrected for through a tunable oscillator farm, implemented on the FPGA. The impact of the stabilization technique is demonstrated together with a reconfigurable analog-to-digital converter (ADC), completely implemented in the FPGA fabric and operating at 15 K. The ADC performance can be improved by at most 1.5 bits (effective number of bits) thanks to the more stable supply voltage. The method is versatile and robust, enabling seamless porting to other FPGA families and configurations.
Highlights
The cryogenic operation of electronic circuits in general and field-programmable gate arrays (FPGAs) in particular has been extensively studied over the past years.1–7 The majority of the FPGA building blocks, in Xilinx Artix 7 series, for instance, has been shown to operate rather stably over the temperature range from 4 K to 300 K.5–7 For example, the delay change in both look-up tables (LUTs) and carry elements has been shown to change by less than 10%. the performance of FPGAs operating at deepcryogenic temperatures is shown to be stable, this is only demonstrated in specific cases, e.g., when building blocks were tested individually
The performance of FPGAs operating at deepcryogenic temperatures is shown to be stable, this is only demonstrated in specific cases, e.g., when building blocks were tested individually
The implemented system consists of our 1.2 GSa/s analogto-digital converter (ADC)19 combined with the voltage regulation circuit as discussed in Sec
Summary
The cryogenic operation of electronic circuits in general and field-programmable gate arrays (FPGAs) in particular has been extensively studied over the past years. The majority of the FPGA building blocks, in Xilinx Artix 7 series, for instance, has been shown to operate rather stably over the temperature range from 4 K to 300 K.5–7 For example, the delay change in both look-up tables (LUTs) and carry elements has been shown to change by less than 10%. The performance of FPGAs operating at deepcryogenic temperatures is shown to be stable, this is only demonstrated in specific cases, e.g., when building blocks were tested individually These tests ignored the influence of voltage drop over the long power supply wires into the cryogenic environment and ignored the influence the blocks might have on one another. While a static voltage drop would not cause significant problems, the dynamic voltage drop continuously alters the internal delays of the FPGA, causing potential glitches and irregularities These problems are mitigated using decoupling capacitors, but the performance of capacitors at cryogenic temperatures is not sufficient to compensate for these fluctuations. The technique is based on real-time measurements of the variation of cell delay in the carry chain This fluctuation is mainly caused by voltage drop of the logic supply voltage and by temperature fluctuations.
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