ST25R3916 datasheet(29/157 Pages) STMICROELECTRONICS

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DS12484 Rev 4

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ST25R3916/7

Application information

156

Receiver configuration register 4. Sending of direct command Reset RX Gain is necessary

to initialize the AGC, Squelch and RSSI block. Sending this command clears the current

Squelch setting and loads the manual gain reduction from Receiver configuration register 4.

Second stage has a second order low-pass filtering characteristic, the pass band is adjusted

according to subcarrier frequency using the bits lp2 to lp0 of the Receiver configuration

Digitizing stage

The digitizing stage produces a digital representation of the sub-carrier signal coming from

the receiver. This digital signal is then processed by the receiver framing logic. The digitizing

stage consists of a window comparator with adjustable digitizing window (five possible

settings, 3 dB steps, adjustment range from ±33 to ±120 mV). The adjustment of the

digitizing window is included in the AGC and Squelch loops. The digitizing window can also

be set manually in the Receiver configuration register 4.

AGC, Squelch and RSSI

As mentioned above, the second and third gain stage gain and the digitizing stage window

are included in the AGC and Squelch loops. Eleven settings are available. The default state

features minimum digitizer window and maximum gain. The first four steps increase the

digitizer window in 3 dB steps, the next six steps reduce the gain in the second and third

gain stage, again in 3 dB steps. The initial start setting for Squelch and AGC is defined in

Receiver configuration register 4. The Gain reduction state register displays the actual state

of gain resulting from Squelch, AGC and initial settings in Receiver configuration register 4.

Squelch

This feature is designed for operation in noisy environments. The noise may be

misinterpreted as the start of tag response, resulting in decoding errors.

Automatic squelch is enabled by option bit sqm_dyn in the Receiver configuration register 2.

It is activated automatically 18.88 µs after end of TX and is terminated at the moment the

Mask receive timer (MRT) reaches the value defined in the Squelch timer register. This

mode is primarily intended to suppress noise generated by tag processing during the time

when the tag response is not expected (covered by MRT).

Squelch can operate in two modes, namely with ratios 1 and 6, selectable by pulz_61 bit in

the Receiver configuration register 2.

Squelch ratio 1 means that system observes the subcarrier signal from the main digitizer

and decrease the system gain to decrease the frequency of transitions. If there are more

than two transitions on this output in a 50 μs time period, gain is reduced by 3 dB and output

is observed during the following 50 μs. This procedure is repeated until number of

transitions in 50 μs is lower or equal to 2 or until the maximum gain reduction is reached.

This mode is intended for protocols where digitized subcarrier outputs are used.

Squelch ratio 6 means the system similarly observes and decreases the frequency seen at

the window comparator set to 6 times the digitizing window. This mode is intended for

protocols where output from correlators are used (ISO-A, ISO-B correlated reception).

The gain setting acquired by squelch is cleared by sending direct command Reset RX gain.

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