ST25R3912 datasheet(21/130 Pages) STMICROELECTRONICS

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DS11794 Rev 9

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ST25R3912

Functional overview

Default AGC settings are:

•

AGC is enabled

•

AGC operates during complete Rx process

•

algorithm with preset is used.

RSSI

The receiver also performs the RSSI (received signal strength indicator) measurement for

both channels. The RSSI measurement is started after the rising edge of rx_on. It stays

active as long as signal rx_on is high, it is frozen while rx_on is low. The RSSI is a peak hold

system, and the value can only increase from the initial zero value. Every time the AGC

reduces the gain the RSSI measurement is reset and starts from zero. Result of RSSI

measurements is a 4-bit value that can be observed by reading the RSSI display register.

The LSB step is 2.8 dB, and the maximum code is Dh (13d).

Since the RSSI measurement is of peak hold type the RSSI measurement result does not

follow any variations in the signal strength (the highest value will be kept). In order to follow

RSSI variations it is possible to reset the RSSI bits and restart the measurement by sending

the direct command Clear RSSI.

Receiver in NFCIP-1 active communication mode

There are several features built into the receiver to enable reliable reception of active

NFCIP-1 communication. All these settings are automatically preset by sending the direct

command Analog preset after the NFCIP-1 mode has been configured. In addition to the

filtering options, there are two NFCIP-1 active communication mode specific configuration

bits stored in the Receiver configuration register 3.

Bit lim enables clipping circuits that are positioned after the first and second gain stages.

The function of the clipping circuits is to limit the signal level for the following filtering stage

(when the NFCIP-1 peer is close the input signal level can be quite high).

Bit rg_nfc forces gain reduction of second and third filtering stage to -6 dB while keeping the

digitizer comparator window at maximum level.

1.2.4

Wake-up mode

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

ST25R3912 in Wake-up mode, used to perform low power detection of card presence. The

ST25R3912 includes several possibilities of low power detection of a card presence ( phase

measurement, amplitude measurement). An integrated low power 32 kHz RC oscillator and

a register configurable Wake-up timer are used to schedule periodic detection.

Usually the presence of a card is detected by a so-called polling loop. In this process the

reader field is periodically turned on and the controller checks whether a card is present

using RF commands. This procedure consumes a lot of energy since the reader field has to

be turned on for 5 ms before a command can be issued.

Low power detection of card presence is performed by detecting a change in the reader

environment, produced by a card. When a change is detected, an interrupt is sent to the

controller. As a result, the controller can perform a regular polling loop.

In the Wake-up mode the ST25R3912 periodically performs the configured reader

environment measurements and sends an IRQ to the controller when a difference to the

configured reference value is detected.

ST25R3912 Datasheet(PDF) 21 Page - STMicroelectronics

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