The diagnosis is carried out during the charge phase to detect whether the full bridge and the antenna are working. When the full bridge drives the antenna, the voltage across the coil exceeds the supply voltage so that the voltage at the input of the RF amplifier is clamped by the ESD-protection diodes. For diagnosis, the SENSE pin is loaded on-chip with a switchable resistor to ground so that the internal switchable resistor and the external SENSE resistor form a voltage divider, while the internal resistor is switched off in read mode. When the voltage drop across the internal resistor exceeds a certain value, the diagnosis block passes the frequency of its input signal to the digital demodulator. The frequency of the diagnosis signal is accepted if eight subsequent times can be detected, all with their counter state within the range of 112 to 125, during the diagnosis time (at most 0.1 ms). The output signal is used only during the charge phase, otherwise it is ignored.

When the short-circuit protection switches off one of the full-bridge drivers, the diagnosis also indicates an improper operation of the antenna by sending the same diagnostic byte to the microcontroller as for the other failure mode.

During diagnosis, the antenna drivers are active. In synchronous mode the antenna drivers remain active up to 1 ms after the diagnosis is performed, without any respect to the logic state of the signal at TXCT (thus enabling the microcontroller to clock out the diagnosis byte).