ST25R3912 datasheet(20/130 Pages) STMICROELECTRONICS

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Functional overview

ST25R3912

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DS11794 Rev 9

Squelch

This feature is designed for operation of the receiver in noisy conditions. The noise can

come from tags (caused by the processing of reader commands), or it can come from a

noisy environment. This noise may be misinterpreted as start of transponder response,

resulting in decoding errors.

During execution of the Squelch procedure the output of the digitizing comparator is

observed. In case there are more than two transitions on this output in a 50 μs time period,

the receiver gain is reduced by 3 dB, and the output is observed during the next 50 μs. This

procedure is repeated until the number of transitions in 50 μs is lower or equal to two, or

until the maximum gain reduction is reached. This gain reduction can be cleared sending

the direct command Reset Rx gain.

There are two possibilities of performing squelch: automatic mode and using the direct

command Squelch.

Automatic mode is enabled in case bit sqm_dyn in the Receiver configuration register 2

is set. It is activated automatically 18.88 μs after end of Tx and is terminated when the

Mask receive timer expires. This mode is primarily intended to suppress noise

generated by tag processing during the time when a tag response is not expected

(covered by Mask receive timer).

Command Squelch is accepted in case it is sent when signal rx_on is low. It can be

used when the time window in which noise is present is known by the controller.

AGC (automatic gain control)

AGC is used to reduce gain to keep the receiver chain out of saturation. With gain properly

adjusted the demodulation process is also less influenced by system noise.

AGC action starts when signal rx_on is asserted high and is reset when it is reset to low. At

the high to low transitions of the rx_on signal the state of the receiver gain is stored in the

Gain reduction state register. Reading this register at a later stage gives information on the

gain setting used during last reception.

When AGC is switched on the receiver gain is reduced so that the input to the digitizer stage

is not saturated. The AGC system comprises a comparator with a window 3.5 times larger

than that of the digitizing window comparator. When the AGC function is enabled the gain is

reduced until there are no transitions on the output of its window comparator. This

procedure ensures that the input to the digitizing window comparator is less than 3.5 times

larger than its threshold.

AGC operation is controlled by the control bits agc_en, agc_m and agc_fast in the Receiver

configuration register 2. Bit agc_en enables the AGC operation, bit agc_m defines the AGC

mode, and bit agc_alg defines the AGC algorithm.

Two AGC modes are available. The AGC can operate during the complete Rx process (as

long as signal rx_on is high), or it can be enabled only during the first eight sub-carrier

pulses.

Two AGC algorithms are available. The AGC can either start by presetting code 4h (max

digitizer window, max gain) or by resetting the code to 0h (min digitizer window, max gain).

The algorithm with preset code is faster, therefore it is recommended for protocols with short

SOF (like ISO14443A fc / 128).

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