ST25R3910 datasheet(14/65 Pages) STMICROELECTRONICS

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Functional overview

ST25R3910

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RSSI

The receiver also performs the RSSI (Received Signal Strength Indicator) measurement of

the modulated signal that is superimposed on the 13.56 MHz carrier. 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 Receiver State Display Register. The LSB step is

2.15 dB.

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.

1.2.4

A/D converter

The ST25R3910 contains an 8-bit successive approximation A/D converter. Inputs to the

A/D converter can be multiplexed from different sources to be used in several direct

commands and adjustment procedures. The result of the last A/D conversion is stored in the

A/D Output Register.

The A/D converter has two operating modes, absolute and relative.

•

In absolute mode the low reference is 0 V and the high reference is 2 V. This means

that A/D converter input range is from 0 to 2 V, 00h code means input is 0 V or lower,

FFh means that input is 2 V - 1 LSB or higher (LSB is 7.8125 mV).

•

In relative mode low reference is 1/6 of VSP_A and high reference is 5/6 of VSP_A, so

the input range is from 1/6 to 5/6 VSP_A.

Relative mode is only used in phase measurement (phase detector output is proportional to

power supply). In all other cases absolute mode is used.

The A/D converter input can also be accessed externally. When the direct command AD

Convert is sent, an A/D conversion of voltage present on pin AD_IN is performed in absolute

mode, result is stored in A/D Output Register. AD_IN pin should be left non-connected in

case A/D conversion is not needed in application.

1.2.5

Phase and amplitude detector

Phase detector

The phase detector is observing phase difference between the transmitter output signals

(RFO1 and RFO2) and the receiver input signals RFI1 and RFI2, which are proportional to

the signal on the antenna LC tank. These signals are first elaborated by digitizing

comparators, then digitized signals are processed by a phase detector. Filter characteristics

of the phase antenna are adapted to one of the two possible operation modes. For antenna

tuning check, a strong low power filter is used to get average phase difference, for PM

demodulation a low pass filter having 1 MHz corner frequency is used to pass the subcarrier

frequency.

The phase detector output is inversely proportional to the phase difference between the two

inputs. The 90° phase shift (ideal antenna LC tank tuning) results in VSP/2 output voltage.

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