ST25R3920B datasheet(21/158 Pages) STMICROELECTRONICS

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DS13890 Rev 4

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ST25R3920B

Application information

157

Application information

4.1

Power-on sequence

Once powered, the device enters the Power-down mode where the content of all registers is

set to its default state.

The next steps are basic configurations of the IC:

The IO configuration register 1 and IO configuration register 2 must be properly

configured.

The internal voltage regulators have to be configured. It is recommended to use direct

command Adjust regulators to improve the system PSRR.

If AAT is used the tuning procedure must be performed.

After device power-on and direct command set default, release the direct command

Trigger RC calibration.

After the sequence of events mentioned above the devices are ready to operate.

4.2

Operating modes

The ST25R3920B operating mode is defined by the contents of the Operation control

this mode, the AFE static power consumption is minimized, as only the POR and part of the

bias are active. The regulator itself is disabled.

The SPI/I2C is still functional in this mode and all required settings on the configuration

registers can be done. The PT_memory and FIFO are not accessible in this mode.

Bit en (bit 7 of the Operation control register) is controlling the quartz crystal oscillator,

regulators and AAT control output pins. When this bit is set, the device enters in Ready

mode and the quartz crystal oscillator and regulators are enabled. An interrupt is sent to

inform the microcontroller when the oscillator amplitude and frequency is stable. The

PT_memory and FIFO are accessible in this mode.

The enable of the receiver and the transmitter block are separated, it is possible to operate

one without switching on the other (control bits rx_en and tx_en). This feature is used when

the reader field has to be maintained while no response from a tag is expected. Another

example is NFCIP-1 active communication in receive mode configuration. The RF field is

generated by the initiator on one side while the other side is only in receive operation.

Asserting the Operation control register bit wu while the other bits are set to 0 puts the

ST25R3920B into the Wake-up mode that is used to perform low power detection of card

presence. In this mode the low power RC oscillator and register configurable wake-up timer

are used to schedule periodic measurement(s). When a difference to the predefined

reference is detected an interrupt is sent to wake-up the MCU. Phase and amplitude

measurement are available to trigger the wake-up.

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