RXD is the interface to the MCU’s LIN protocol controller or SCI/UART, which reports the state of the LIN bus voltage. LIN recessive (near battery) is represented by a high level on RXD and LIN dominant (near ground) is represented by a low level on RXD. The RXD output structure is an open-drain output stage. This allows the device to be used with 3.3-V and 5-V I/O microcontrollers. If the microcontroller’s RXD pin does not have an integrated pullup, an external pullup resistor to the microcontroller I/O supply voltage is required. In standby mode the RXD pin is driven low to indicate a wake-up request from LIN or NWake.