Cryogenic testing was done using liquid nitrogen. We started the testing at room temperature of 24°C, having programmed the test chamber to proceed in steps to 10°C, 0°C, -10°C, all the way down to -150°C. Figure 2 charts Xilinx voltage currents vs. the temperature.
Figure 2 – Xilinx FPGA voltage currents vs. temperature
In the course of our testing, we made some interesting observations. For starters, we found that the 2.5V/2.5V auxiliary voltage currents remained stable over the test temperature range. Both of these voltages are used for system monitor and JTAG communication. All of the I/Os are tied to 3.3V.
Second, the internal 1.0V current was significantly reduced from 140 mA at +20°C to 81 mA at -150°C. This was no surprise, since a reduction in power is expected at low temperatures.
Finally, we found that the flash memory’s 1.8V current remained almost zero down to -50°C and then changed to 10 mA from -50 to -90°C. It dropped to zero from the -100 to -120°C temperature range, and went back to 10 mA from -130 to -150°C. We are not sure what to make of this finding; it could be due to test measurement errors.
Importantly, both clocks remained stable over the test temperature range (see Figure 3
). We used a PLL to both divide and multiply the master on-board 100-MHz oscillator so as to generate the 50- and 150-MHz clocks.
Figure 3 – Xilinx 50/150-MHz clock vs. temperature