ST25R3911B datasheet(6/116 Pages) STMICROELECTRONICS

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1.2.1

Operating modes

The ST25R3911B operating mode is defined by the contents of the Operation control register.

At power-up all bits of the Operation control register are set to 0, the ST25R3911B is in Power-down mode. In this

mode AFE static power consumption is minimized, only the POR and part of the bias are active, while the

regulators are transparent and are not operating. The SPI is still functional in this mode so all settings of ISO

mode definition and configuration registers can be done.

Control bit en (bit 7 of the Operation control register) is controlling the quartz crystal oscillator and regulators.

When this bit is set, the device enters in Ready mode. In this mode the quartz crystal oscillator and regulators are

enabled. An interrupt is sent to inform the microcontroller when the oscillator frequency is stable.

Enable of receiver and transmitter are separated so it is possible to operate one without switching on the other

(control bits rx_en and tx_en). In some cases this may be useful, if the reader field has to be maintained and there

is no transponder response expected, the receiver can be switched-off to save current. Another example is the

NFCIP-1 active communication receive mode in which the RF field is generated by the initiator and only the

receiver operates.

Asserting the Operation control register bit wu while the other bits are set to 0 puts the ST25R3911B into the

Wake-Up mode that is used to perform low power detection of card presence. In this mode the low power RC

oscillator and register configurable Wake-Up timer are used to schedule periodic measurement(s). When a

difference of the measured value vs. the predefined reference is detected an interrupt is sent to wake-up the

microcontroller.

1.2.2

Transmitter

The transmitter contains two identical push-pull driver blocks connected to the pins RFO1 and RFO2. These

drivers are differentially driving the external antenna LC tank. It is also possible to operate only one of the two

drivers by setting the IO configuration register 1 bit single to 1. Each driver is composed of eight segments having

binary weighted output resistance. The MSB segment typical ON resistance is 2 Ω, when all segments are turned

on; the output resistance is typically 1 Ω. All segments are turned on to define the normal transmission (non-

modulated) level. It is also possible to switch off certain segments when driving the non-modulated level to reduce

the amplitude of the signal on the antenna and/or to reduce the antenna Q factor without making any hardware

changes. The RFO normal level definition register defines which segments are turned on to define the normal

transmission (non-modulated) level. Default setting is that all segments are turned on.

Using the single driver mode the number of the antenna LC tank components (and therefore the cost) is halved,

but also the output power is reduced. In single mode it is possible to connect two antenna LC tanks to the two

RFO outputs and multiplex between them by controlling the IO configuration register 1 bit rfo2.

In order to transmit the data the transmitter output level needs to be modulated. Both AM and OOK modulation

are supported. The type of modulation is defined by setting the bit tr_am in the Auxiliary definition register.

During the OOK modulation (for example ISO14443A) the transmitter drivers stop driving the carrier frequency. As

consequence the amplitude of the antenna LC tank oscillation decays, the time constant of the decay is defined

with the LC tank Q factor. The decay time in case of OOK modulation can be shortened by asserting the Auxiliary

definition register bit ook_hr. When this bit is set to logic one the drivers are put in tristate during the OOK

modulation.

AM modulation (for example ISO14443B) is done by increasing the output driver impedance during the

modulation time. This is done by reducing the number of driver segments that are turned on. The AM modulated

level can be automatically adjusted to the target modulation depth by defining the target modulation depth in the

AM modulation depth: definition and calibration and sending the Calibrate Modulation Depth direct command.

Refer to Section 1.2.20 AM modulation depth: definition and calibration for further details.

Slow transmitter ramping

When the transmitter is enabled it starts to drive the antenna LC tank with full power, the ramping of the field

emitted by antenna is defined by antenna LC tank Q factor.

However there are some reader systems where the reader field has to ramp up with a longer transition time when

it is enabled. The STIF (Syndicat des transports d'Ile de France) specification requires a transition time from 10%

to 90% of field longer than or equal to 10 μs.The ST25R3911B supports that feature. It is realized by collapsing

VSP_RF regulated voltage when transmitter is disabled and ramping it when transmitter is enabled. Typical

transition time is 15 μs at 3 V supply and 20 μs at 5 V supply.

Procedure to implement the slow transition:

When transmitter is disabled set IO configuration register 2 bit slow_up to 1. Keep this state for at least 2 ms

to allow discharge of VSP_RF.

ST25R3911B

Application information

DS11793 - Rev 6

page 6/116

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