AS3933 datasheet(52/64 Pages) AMSCO

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ams Datasheet

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[v1-08] 2015-Sep-02

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AS3933 − Detailed Description

In case the pattern detection and the Manchester decoder are

not enabled (R1<1>=0 and R1<3>=1) the calibration on the

RC-oscillator is not needed. Should this not be the case, the

RC-oscillator has to be calibrated. The calibration of the

RC-oscillator can be done in two different ways:

• Over SPI, the host system (MCU) has to be able to provide

65 clock pulses of a reference clock. In this case the host

has to have a precise reference clock (quartz, resonator

etc.).

• Using the internal calibration procedure based on the

antenna resonator. Using this calibration method the

RC-oscillator is automatically trimmed to the proper

frequency, according to the operating frequency band.

The precision of the calibration depends on the tolerances

of the resonator of the first channel (LC connected to

LF1P).

RC-Oscillator: Calibration via SPI. The calibration gets started

with the Calib_RCosc direct command. Since no non-volatile

memory is available on the chip, the calibration must be done

every time after battery replacement. Since the Clock Generator

defines the time base of the frequency detection, the selected

frequency depends on the carrier frequency. The choice of the

reference clock frequency delivered by the host (MCU) is the

same as the choice of the frequency in case the crystal oscillator

is used and it is shown in the Figure 56.

To trim the RC-Oscillator, set the chip select (CS) to high before

sending the direct command Calib_RCosc over SPI. Then 65

digital clock cycles of the reference clock (e.g.

125kHz/4=31.25kHz) have to be sent on the clock bus (SCLK),

as shown in Figure 59. After that the signal on the chip select

(CS) has to be pulled down.

The calibration is effective after the 65th reference clock edge

and it will be stored in a volatile memory. In case the

RC-oscillator is switched OFF or a power-on-reset happens (e.g.

battery change) the calibration has to be repeated.

Figure 59:

RC-Oscillator Calibration via SPI

SCLK

SDI

65 clock cycles

DIRECT COMMAND

Trim_osc

REFERENCE CLOCK

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