Hello,
Following datasheet, to set pin9 as clkin I have done the following:
uint8_t data;
write_byte(ICM42688_REG_BANK_SEL, 1); // bank sel 1
read_byte(ICM42688_INTF_CONFIG5, &data); // read INTF_CONFIG5
data |= 0b00000100; // [2:1] PIN9_FUNCTION: CLKIN: 10
write_byte(ICM42688_INTF_CONFIG5, data); // write INTF_CONFIG5