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Revision 2.3

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AS3910

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

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

comparator output is high. In the opposite case, when the resonance frequency is lower, the Phase Detector output gets lower then VSP/2 and

the comparator output is low.

At the beginning of the command Calibrate Antenna execution the switches in all TRIM pads are turned off. As consequence all the trimming

capacitors are disconnected so in case the LC tank dimensioning is correct the resonance frequency has to be higher then 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 adjusting of the resonance frequency is 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 10µs 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 transitioned is the one at which the LC tank is in resonance.

In case the state with all switched turned on was reached and the comparator output is still high, the resonance frequency is too high even when

all the capacitors are connected and the adjusting is not possible. The error flag is set.

The result of the direct command Calibrate Antenna can be observed by reading the Antenna Calibration Register (#0E). This register displays

the state of four bits representing state of the switches when resonance was reached and the error flag.

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

Resonance.

External Trim Register (#0F). When the bit trim_s of this register 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 a trimming procedure may be implemented from externally.

Another possibility of external trimming procedure is using this register and the direct command Measure RF. In this case the resonance is

adjusted by looking for operating point with the maximum amplitude.

8.17 Transparent Mode

The AS3910 framing supports the ISO-14443 standard. Other standard and custom 13.56MHz RFID reader protocols can be realized using the

AS3910 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 (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 the direct command Transparent Mode the Transmitter and Receiver have to be turned on, the AFE have

to be configured properly.

While the AS3910 is in the Transparent Mode the AFE is controlled directly through SPI interface:

The Transmitter modulation is controlled by pin SDATAI (high is modulator on)

Signal rx_on is controlled by pin SCLK (high enables RSSI and AGC)

The Receiver output is sent to pin SDATAO

By controlling the rx_on advanced Receiver features like the RSSI and AGC can be used.

Configuration bits related to the ISO mode, framing and FIFO are of course meaningless in Transparent Mode, all other configuration bits are

respected.

For communication reader to transponder the OOK and AM modulation are supported. Type of the modulation is defined by writing the bit 6 (am)

of the Configuration Register 5 (#05). The direct command Calibrate Modulation Depth supports modulation depths up to 30%, by writing the AM

Modulated Level Definition Register (#12) also definition of deeper am modulation is possible.

The Receiver filters support the subcarrier frequencies from 212 kHz to 848 kHz. The filter characteristics are defined by writing the bits fs2 to fs0

in the Receiver Configuration Register (#6).