AS3911 datasheet(16/149 Pages) AMSCO

16 / 149 page

AS3911 – 16

ams Datasheet, Confidential: 2013-Oct [2-03]

A p pl i c at io n I n f o r m at i o n

SPI Interface

A 4-wire Serial Peripheral Interface (SPI) is used for

communication between external microcontroller and the

AS3911.

Operating Modes

The AS3911 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 AS3911 is in Power-down mode. In this mode AFE static

power consumption is minimized, only the POR and part of the

bias are active, 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, in case

the reader field has to be maintained and there is no

transponder response expected receiver can be switched-off to

save current. Another example is NFCIP-1 active

communication receive mode in which RF field is generated by

the initiator and only Receiver operates.

Asserting the Operation Control Register bit wu while the other

bits are set to 0 puts the AS3911 into the Wake-up mode which

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 to the predefined reference is detected an

interrupt is sent to wake-up the micro. Capacitive sensor, phase

measurement and amplitude measurement are available.

Transmitter

The Transmitter contains two identical push-pull driver blocks

connected to the pins RFO1 and RFO2. These drivers are

differentially driving 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. Each driver is composed of

8 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. Usually all

segments are turned on to define the normal transmission

(non-modulated) level. It is also possible to switch off certain

Application Information

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