CC1000-RTR2 datasheet(23/59 Pages) TI2

23 / 59 page

CC1000

SWRS048A

Page 23 of 55

14. Frequency programming

RX mode:

LO (low-side)

LO (high-side)

(Receive frequency)

TX mode:

(Center frequency)

sep

(Lower FSK

frequency)

(Upper FSK

frequency)

vco

Figure 15. Relation between fvco, fif, and LO frequency

The

frequency

synthesiser

(PLL)

controlled by the frequency word in the

configuration registers. There are two

frequency words, A and B, which can be

programmed to two different frequencies.

One of the frequency words can be used

for RX (local oscillator frequency) and the

other for TX (transmitting frequency, f0).

This makes it possible to switch very fast

between RX mode and TX mode. They

can also be used for RX (or TX) on two

different

channels.

The

MAIN.F_REG

control bit performs selection of frequency

word A or B.

The frequency word, FREQ, is 24 bits (3

bytes)

located

FREQ_2A:FREQ_1A:FREQ_0A

and

FREQ_2B:FREQ_1B:FREQ_0B for the A

and B word, respectively.

The

frequency

word

FREQ

can

calculated from:

16384

8192

⋅

TXDATA

FSEP

FREQ

ref

VCO

where TXDATA is 0 or 1 in transmit mode

depending

the

data

bit

transmitted on DIO. In receive mode

TXDATA is always 0.

The reference frequency fref is the crystal

oscillator clock divided by PLL.REFDIV, a

number between 2 and 14 that should be

chosen such that:

1.0 MHz

≤ f

ref ≤ 2.46 MHz

Thus, the reference frequency fref is:

REFDIV

xosc

ref =

fVCO is the Local Oscillator (LO) frequency

in receive mode, and the f0 frequency in

transmit mode (lower FSK frequency). The

LO frequency must be fRF – fIF or fRF + fIF

giving low-side or high side LO injection

respectively. Note that the data on DIO will

be inverted if high-side LO is used.

The upper FSK transmit frequency is

given by:

f1 = f0 + fsep ,

where the frequency separation fsep is set

the

bit

separation

word

(FSEP1:FSEP0):

16384

FSEP

ref

sep

⋅

Clearing

PLL.ALARM_DISABLE

will

enable generation of the frequency alarm

bits PLL.ALARM_H and PLL.ALARM_L.

These bits indicate that the frequency

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