The XCVM1802 device U1 implements bitstream encryption key technology. The VMK180 board provides the encryption key backup battery circuit shown in the following figure.
The Seiko TS621E rechargeable 1.5V lithium button-type battery B1 is soldered to the board with the positive output connected to the XCVM1802 device U1 VCC_BATT bank pin AG33. The battery supply current IBATT specification is 150 nA maximum when board power is off. Battery B1 is charged from the VCC1V8 1.8V rail through a 2 series diode with the first forward drop to yield between 0.24V to 0.46V over temperature per fixed 5 mA load, R1725, and limiting 1.56V max at the adaptive SoC pin, PSVBATT. The second diode and 4.7 kΩ current limit resistor allows the battery to trickle charge and prevent battery B1 from back powering R1725.