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AS3935

Datasheet - Detailed Descri p ti on

8.10 Antenna Tuning

The AS3935 uses a loop antenna based on a parallel LC resonator. The antenna has to be designed to have its resonance frequency at 500kHz

and a quality factor of around 15. With a register setting it is possible to display on the IRQ pin the resonance frequency of the antenna as a

digital signal with the register REG0x08[7] =1. The external unit can measure this frequency and tune the antenna adding or removing the

internal capacitors with the register REG0x08[3:0]. It is necessary to tune the antenna with an accuracy of ±3.5% to optimize the performance of

the signal validation and distance estimation. The resonance frequency is internally divided by a factor, which is programmable with the register

REG0x03[7:6]. Table 20 shows the division ratio.

8.11 Clock Generation

The clock generation is based on two different RC oscillators: a system RCO (SRCO) and a timer RCO (TRCO). The SRCO will run at about

1.1MHz and provides the main clock for the whole digital part. The TRCO is a low power low frequency oscillator and runs at 32.768 kHz.

Frequency variations in these two oscillators, due to temperature change, are automatically compensated.

The output frequency of those oscillators can be displayed on the IRQ pin with register setting (REG0x08[5] =1 TRCO, while REG0x08[6] =1

SRCO). Due to process variations, the frequency of both oscillators can be different from the nominal frequency. Therefore, it is possible to

calibrate both with a direct command. The precision of the calibration will depend on the accuracy of the resonance frequency of the antenna. It

is recommended to first trim the receiver antenna before the calibration of both oscillators is done.

The result of calibration of the 3 oscillators is stored in a volatile memory and needs to be done every time after POR (e.g. battery change) but all

oscillators are internally compensated in temperature and voltage supply variations.

If the AS3935 is set in power-down mode, the TRCO needs to be recalibrated using the following procedure:

1. Send Direct command CALIB_RCO

2. Modify REG0x08[5] = 1

3. Wait 2ms

4. Modify REG0x08[5] = 0

Table 20. Frequency Division Ratio for the Antenna Tuning

Division Ratio

REG0x03[7]

REG0x03[6]

16

0

0

32

0

1

64

1

0

128

1

1