Hi,
The figure you're pointing out is the end of the HART communication which is the period between the valid carrier being present and the CD pin being disasserted.
The HART specifications allow a "Time from Carrier OFF to Carrier Detect Disassertion” spec of up to 6 bit times (HCF_SPEC-54 Revision 8.1, Page 34 Table 2). This is also covered in T5 of the datasheet (Table 3). See attached.
During this time the carrier is off but the CD is not disasserted yet. In this plot there is no HART signal being applied so the RXD is basically undefined (Random Signal Inputted, Random Signal Outputted).
But the AD5700 is disasserting the CD pin within the allowed time (t5). The microcontroller should not interpret RXD as a valid HART frame and ignore it (as long as the next layer of HART protocol is designed correctly).
Please let me know if anything is unclear.
Regards,
David