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CC112X/CC1175
SWRU295C
Page 26 of 108
4.1.3
Minimum Shift Keying
When using MSK
4
, the complete transmission (preamble, sync word, and payload) will be MSK
modulated.
MSK modulation is configured by
set to 2-(G)FSK modulation and
frequency deviation set to ¼ of data rate. Then phase shifts are performed with a constant transition
time. Table 13 shows what the frequency deviation should be programmed to for different data rates
to achieve a modulation index ~0.5.
Data Rate [ksps]
Frequency Deviation [kHz]
Actual Modulation Index
1.0
0.25
0.49999
1.2
0.3
0.50000
2.4
0.6
0.50000
4.8
1.2
0.50041
9.6
2.4
0.49958
19.6
4.8
0.50010
38.4
9.6
0.49963
50
12.5
0.50048
76.8
19.2
0.49966
100
25.0
0.50048
125
31.25
0.50047
Table 13: MSK Parameters
4.1.4
Custom Frequency Modulation/Analog FM
CC112X
supports a simple scheme to do custom frequency modulation/analog FM (e.g. communication
with analog legacy voice devices, N-FSK systems). This feature utilizes the high resolution PLL and
lets the user in a simple way directly control/read the instantaneous frequency without SPI overhead.
Custom frequency modulation is enabled by setting
SOFT_TX_DATA_CFG.SOFT_TX_DATA_EN = 1
One register (
) is used to set the carrier frequency offset and another register
) is used to read the instantaneous frequency offset. The two registers have the
same format
(two‟s complement) to simplify SW control in both TX and RX. Accessing these registers
should be done using burst mode combined with setting
EXT_CTRL.BURST_ADDR_INCR_EN = 0
continuously access the same address without any SPI address overhead.
can be output on a GPIO by setting
IOCFGx.GPIOx_CFG = 30
.
This signal will be asserted every time the
register should be written and should
be used as an interrupt to the MCU to synchronize the SPI data to the internal modulation rate. The
signal runs at 16x the programmed data rate.
should be output on a GPIO (
IOCFGx.GPIOx_CFG = 29
) and used as a
trigger to read the
samples.
4
Identical to offset QPSK with half-sine shaping (data coding may differ).