CC112X/CC1175
SWRU295C
Page 27 of 108
When using custom frequency modulation there are 129 values (referred to as f
OFFSET
)
between −f
DEV
and +f
DEV
that can be used (see Equation 1 and Equation 2 in Section 4.1.1 for details on how to
program the frequency deviation). f
OFFSET
is given by Equation 3.
64
_
_
_
IN
DATA
TX
SOFT
f
f
DEV
OFFSET
Equation 3: f
OFFSET
5
The modulator writes values to the PLL at 16x the programmed data rate. Between the modulator and
the PLL there is an optional linear upsampler configured through the
register field.
4.1.5
DSSS PN Mode
CC112X
supports DSSS PN mode for applications requiring high sensitivity. Preamble and sync word is
unchanged, but the payload data bit is spread with a fixed PN gold sequence initialized at the
beginning of each packet. The spreading factor is set to 4 hence the effective data rate in kbps is
reduced by 4 compared to the programmed data rate (in ksps). The PN gold sequence is generated
from a combination of two 7-bits LFSR registers with generator polynomial given by Equation 4 and
Equation 5. h1(p) is initialized to 0x04 and h2(p) is initialized to 0x0B.
1
)
(
1
2
3
7
p
p
p
p
p
h
Equation 4: h1(p)
1
)
(
2
3
7
p
p
p
h
Equation 5: h2(p)
The resulting bit is then XOR
‟ed with the transmitted bits, where each of the input data bits is mapped
into 4 consecutive symbols, as shown in the following Figure 11. The figure shows what is sent on the
air when the input data is 101b.
+
+
+
+
+
+
Input Data
@ ¼ Symbol Rate
Gold
Sequence
TX Data @ Symbol Rate
Gold Sequence
Input Data @ ¼ Symbol Rate
TX Data @ Symbol Rate
Bit n
Bit n + 2
Bit n + 1
Figure 11: Gold Sequence Generation
The resulting sequence has good autocorrelation properties.
5
This equation is only valid when -64
≤ SOFT_TX_DATA_IN ≤ +64. SOFT_TX_DATA_IN > 64
corresponds to
+
f
DEV
while
SOFT_TX_DATA_IN < -64
gives a frequency
of −f
DEV
.
SOFT_TX_DATA_IN = -128
is the same as setting
SOFT_TX_DATA_IN = 0
.