The Zynq UltraScale+ RFSoC ZU49DR U1 implements bit stream encryption key technology. The ZCU216 board provides the encryption key backup battery circuit shown in the figure below.
Figure 1. Encryption Key Battery 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 ZU49DR RFSoC U1 VCC_PSBATT pin AE29. The battery supply current IBATT specification is 150 nA maximum when board power is off. B1 is charged from the VCC1V8 1.8V rail through a series diode with a typical forward voltage drop of 0.38V and 4.7 ΩK current limit resistor. The nominal charging voltage is 1.42V.