AS3933 datasheet(53/64 Pages) AMSCO

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[v1-08] 2015-Sep-02

AS3933 − Detailed Description

RC-Oscillator: Self Calibration. This procedure uses the

LC-tank (antenna) connected to the channel 1 (LF1P) not as

antenna but as resonator for an oscillator. The internal LC

oscillator is therefore connected through a multiplexer to the

external tank.

The LC-oscillator generates a clock which corresponds to the

resonance frequency of the LC-tank. In a typical application the

user designs the external resonators such to set the resonance

frequency of the external LC-tank as close as possible to the

carrier frequency. The mathematical relation between the

oscillation frequency and the LC time constant is:

Where:

L is the inductance and

C the capacitance of the external antenna

To start the calibration the direct command Calib_RCO_LC must

be sent over the SPI and as soon as the bit R14<7> is high, the

RC-oscillator will be calibrated. The calibrated frequency of the

RC-oscillator depends on the carrier frequency and is

automatically set to better perform the frequency detection,

according to the Figure 56.

External Clock Source

To clock the AS3933 with an external signal, the external clock

generator (R2<6> =1) and the crystal oscillator (R1<0>=1) need

to be enabled. As shown in the Figure 4 the clock can be directly

applied on the pin XOUT while the pin XIN must be connected

to VDD. The clock characteristics are summarized in Figure 60.

Figure 60:

Characteristics of External Clock

Note(s): In power down mode the external clock has to be set

to a definite potential (VDD or ground).

The frequency of the external clock source must be set

according to the Figure 56.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Low level

0.1* VDD

High level

0.9* VDD

VDD

Rise-time

μs

Fall-time

μs

(EQ1)

FLC

⋅

⋅⋅

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