7.3.2 HART Demodulator

The HART demodulator converts the HART FSK input signals applied at the MOD_IN or MOD_INF pins, depending on whether an external filter is implemented, to binary data that is loaded into a receive FIFO in SPI mode. Data in the receive FIFO can then be read by the host controller via SPI serial interface. In UART mode received data is directly fed through to the UART interface.

When a valid carrier is detected on devices using the UART interfaces, the CD pin will toggle high. For devices using the SPI interface, the IRQ pin will toggle indicating an alarm condition. The MODEM STATUS register can then be read to determine the source of the interrupt, which includes a bit for carrier detection in DB1. Hysteresis is implemented with the carrier detect feature in order to prevent erroneous carrier detection signals. More details are explained in the respective Device Functional Modes sections.