In SPI plus UART mode, the UART data are transmitted and received at 1200 baud, which is matched to the HART FSK input and output signals. Both SDO and UARTOUT pins are used; therefore, the IRQ functionality is not available in SPI plus UART mode. FIFO_H2U level monitoring is not required because any HART data received by the demodulator and enqueued into FIFO_H2U are automatically dequeued and transmitted on UARTOUT. FIFO_U2H level monitoring is also not required if HART bus communication activity is interfaced to the host controller through the CD and RTS pins. The host controller can properly time the RTS pin to transmit the HART data when no carrier is detected on the bus. If the CD and RTS pins are not used in SPI plus UART mode, the host controller can periodically poll the MODEM_STATUS register through the SPI to detect when the carrier is not present on the HART bus, and assert the request to send by setting MODEM_CFG.RTS bit = 1.

In UBM, the UART data are transmitted and received at 9600 baud. The HART data characters are interleaved with break commands for register map access or interrupt reporting; see also Section 7.5.3.1.1. Similar to SPI plus UART mode, monitoring of FIFO_H2U and FIFO_U2H levels is not required. The CD and RTS pins are available to interface the HART bus activity with the microcontroller. IRQ functionality is also available on the SDO pin. If the SDO pin is connected to the microcontroller, the IRQ event based on CD_ASSERT can be set to report when the carrier is detected. In this case, CD pin connection to the microcontroller is not required. Similarly, RTS pin connection is not required if MODEM_CFG.RTS is used to issue a request to send. The SDO pin connection to the microcontroller is also not required if the microcontroller can periodically poll the MODEM_STATUS register using break commands, and monitor all the required flags.