The PS-side MIO 26 is reserved as an input to the PMU for indicating a warm boot. PS bank 501 MIO26 (U1.G25) is connected to the I2C0 U22 TCA6416A bus expander (port P02 U22.6) through a 0Ω series resistor R92. See the Zynq UltraScale+ Device Technical Reference Manual (UG1085) [Ref 3] for more details about the PMU interface.