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Revision 1.2

27 - 33

AS3932

8.9.1 Crystal Oscillator

8.9.2 RC-Oscillator

To trim the RC-Oscillator, set the chip select (CS) to high before sending the direct command trim_osc over SDI. Then 65 digital clock cycles of

the reference clock (e.g. 32.768 kHz) have to be sent on the clock bus (SCL), as shown in Figure 22. 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 22. RC-Oscillator Calibration via SDI

Table 22. Characteristics of XTAL

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Crystal accuracy

(initial)

Overall accuracy

±120

p.p.m.

Crystal motional resistance

60

K

Ω

Frequency

32.768

kHz

Contribution of the oscillator to the

frequency error

±5

p.p.m

Start-up Time

Crystal dependent

1

s

Duty cycle

45

50

55

%

Current consumption

1

µA

Table 23. Characteristics of RCO

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Frequency

If no calibration is performed

27

32.768

42

kHz

If calibration is performed

31

32.768

34.5

kHz

Calibration time

Periods of reference clock

65

cycles

Current consumption

200

nA