ST25R3920 datasheet(21/152 Pages) STMICROELECTRONICS

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DS13261 Rev 9

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ST25R3920

Application information

151

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.

To prevent the internal overheat protection to trigger below the junction temperature, the

3-byte frame FCh / 04h / 10h (register access / address / value) has to be sent after

power-on and Set default command.

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

4.2

Operating modes

The ST25R3920 operating mode is defined by the contents of the Operation control

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 can be 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

ST25R3920 into the Wake-up mode, 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. Capacitive sensor, phase and amplitude

measurement are available to trigger the wake-up.

4.2.1

Transmitter

The transmitter contains two identical push-pull driver blocks connected to pins RFO1 and

RFO2. These drivers are differentially driving the external antenna LC tank. It is also

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