ST25R3910 datasheet(31/65 Pages) STMICROELECTRONICS

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ST25R3910

Functional overview

Table 6 shows setting of the mod bits for some often used modulation indexes.

Execution of direct command Calibrate Modulation Depth

The modulation level is adjusted by increasing the RFO1 and RFO2 driver output

resistance. The RFO drivers are composed of 8 binary weighted segments. Usually all these

segments are turned on to define the normal, non-modulated level, there is also a possibility

to increase the output resistance of the non-modulated state by writing the Antenna Driver

Non-Modulated Level Definition Register.

Before sending the direct command Calibrate Modulation Depth the oscillator and

regulators have to be turned on. When the direct command Calibrate Modulation Depth is

sent the following procedure is executed:

The transmitter is turned on, non-modulated level is established.

The amplitude of the non-modulated carrier level established on the inputs RFI1 and

RFI2 is measured by the A/D converter and stored.

Based on the measurement of the non-modulated level and the target modulated level

defined by the bits mod5 to mod0 the target modulated level is calculated.

The output driver control is taken over by an internal register with initial level defined in

the Antenna Driver Non-Modulated Level Definition Register. Content of the internal

carrier level. The reduced carrier level, in turn, is measured by the A/D converter and

compared with the target modulation level.

The procedure from previous point is repeated as long as the measured level is higher

than the target modulation level.

When the measured carrier level is equal to or lower than the target modulation level,

the state of the internal register is copied into the Modulation Depth Display Register,

whose content is used to define the AM modulated level.

Note:

After the calibration procedure is finished, the content of the Antenna Driver Non-Modulated

Level Definition Register should not be changed. Modifications of the content of this register

will change the non-modulated amplitude and therefore the ratio between the modulated

and non-modulated level.

Note:

In case the calibration of antenna resonant frequency in used, the command Calibrate

Antenna has to be run before AM modulation depth adjustment.

Table 6. Setting mod bits

Modulation Index (%)

a/b (dec)

a/b (bin)

mod5 … mod0

1.1739

1.001011

001011

1.2222

1.001110

001110

1.3256

1.010100

010100

1.5000

1.100000

100000

1.8571

1.110111

110111

1.9843

1.111111

111111

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