The XCVU37P device U1 implements bitstream encryption key technology. The VCU128 board provides the encryption key backup battery circuit shown in the following figure.
Figure 1. Encryption Key Backup Circuit
The Seiko TS621E rechargeable 1.5V lithium button-type battery B1 is soldered to the board with the positive output connected to the XCVU37P device U1 VBATT pin BD13. The battery supply current IBATT specification is 150 nA maximum when the board power is off. B1 is charged from the VCC1V8_BUS 1.8V rail through a series diode with a typical forward voltage drop of 0.38V and 4.7 KΩ current limit resistor. The nominal charging voltage is 1.42V.