S2-LP datasheet(30/90 Pages) STMICROELECTRONICS

30 / 90 page

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S2-LP

30/90

DocID029944 Rev 1

5.3.1

RF channel frequency settings

The channel center frequency can be programmed as follows:

Equation 3

– Center frequency setting

�� = �� + (

��

215

∙ ��) ∙ ��

The fbase sets the main channel frequency; the value depends on the value of fxo (the

frequency of the XTAL oscillator, typically 24-26 MHz or 48-52 MHz.

Equation 4

– Base frequency setting

�� =

��

2 ∙ ��

∙

��

220

where:

SYNT is a programmable 28-bits integer (SYNT[3:0] registers).

B is the out-of-loop SYNTH divider (BS field of the SYNT3 register):

Equation 5

– PLL divider

�� = {

4 �� ℎ�� ℎ��ℎ ��(860 �� 940 ��, �� = 0)

8 �� ℎ�� (430 �� 470 ��, �� = 0)

D is the reference divider (REFDIV bit of XO_RCO_CONFIG0 register)

Equation 6

– Reference divider

�� = {

1 �� = 0 (�� )

2 �� = 1 (�� )

The resolution in the programmed value of the base frequency depends on the actual band

selected.

Table 35: Resolution frequency

fxo [MHz]

High Band Resolution [Hz]

Low Band Resolution [Hz]

11.4

5.7

11.9

6.0

12.4

6.2

22.9

11.4

23.8

11.9

24.8

12.4

The fc is the frequency related to the channel specified. RF channels can be defined using

the CHSPACE and CHNUM registers. In this way, it is possible to change faster the

channel by changing just an 8-bits register, allowing the setting of 256 channels and

frequency-hopping sequences. The actual channel spacing is from 793 Hz to 202342 Hz in

793 Hz steps for the 26 MHz configuration and from 1587 to 404685 Hz in 1587 Hz steps

for the 52 MHz configuration.

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