ST25R3910 datasheet(34/65 Pages) STMICROELECTRONICS

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Functional overview

ST25R3910

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DocID029768 Rev 2

Antenna calibration using command Calibrate Antenna

The calibration of LC tank resonance frequency is automatically done by running the direct

command Calibrate Antenna. During execution of this command the comparator at the

output of Phase Detector is used. In case the LC tank resonance frequency is higher than

the target 13.56 MHz, the Phase Detector output gets higher than VSP/2 and the comparator

output is high. In the opposite case, when the resonance frequency is lower, the Phase

Detector output gets lower than VSP/2 and the comparator output is low.

At the beginning of the command Calibrate Antenna execution the switches in all TRIMx_y

pads are turned off. As a consequence, all the trimming capacitors are disconnected so in

case the LC tank dimensioning is correct the resonance frequency has to be higher than the

target and the comparator output has to be high. In case the comparator output is low at this

initial state the resonance frequency is too low even when all the trimming capacitor are

disconnected, and adjustments of the resonance frequency are not possible. An error flag is

set and execution of the command is terminated.

In case the comparator output was high at the initial state, the LSB switches (TRIM1_0 and

TRIM2_0) are switched on and after 10 μs state of the comparator output is checked again.

This procedure is repeated until the comparator output transitions to low, or until the final

state with all switches turned on is reached. The switch state at which the comparator output

is transitional is the one at which the LC tank is in resonance.

In case the state with all switched turned on is reached while the comparator output is still

high, the resonance frequency is too high even when all the capacitors are connected and

adjustments are not possible. The error flag is set.

The result of the direct command Calibrate Antenna can be observed by reading the

Antenna Calibration Register, which displays the state of four bits representing the state of

the switches when the resonance was reached and the error flag.

After the execution of the direct command Calibrate Antenna the resonance can be checked

by running the direct command Check Antenna Resonance.

Antenna calibration using External Trim Register

There is also a possibility to control the position of the TRIM switches by writing the External

Trim Register. When bit trim_s is set to 1 position of the trim switches is controlled by bits

tre_3 to tre_0. Using this register and the direct command Check Antenna Resonance an

external trimming procedure can be implemented.

Another possible external trimming procedure is to use this register and the direct command

Measure RF. In this case the resonance is adjusted by looking for operating point with the

maximum amplitude.

Transparent mode

The AS3909/10 framing supports the ISO-14443 standard.

Other standards as well as custom 13.56 MHz RFID reader protocols can be implemented

by using the ST25R3910 AFE and framing implemented in the external microcontroller.

After sending the direct command Transparent Mode the external microcontroller directly

controls the transmission modulator and gets the Receiver output (the control logic

becomes “transparent”).

The Transparent mode is entered on falling edge of signal SEN after sending the command

Transparent Mode and is maintained as long as the signal SEN is kept low. Before sending

ST25R3910 Datasheet(PDF) 34 Page - STMicroelectronics

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