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CC112X/CC1175
SWRU295C
Page 28 of 108
At the receiver, the PN gold sequence is known and is initialized at the beginning of each packet. For
every group of 4 incoming symbols, two accumulated distance computation are performed; one
assuming that a „0‟ was sent and the other assuming that a „1‟ was sent, and the most likely
transmitted bit is chosen.
DSSS PN mode is enabled by setting
MODCFG_DEV_E.MODEM_MODE = 10b
6
.
When using this mode both FIFO mode and synchronous serial mode are supported
(
PKT_CFG2.PKT_FORMAT = 00b
or
01b
).
4.1.6
DSSS Repeat Mode
CC112X
supports DSSS repeat mode for applications requiring high sensitivity. In DSSS repeat mode,
preamble is unchanged. The payload data bits are spread using the sync word meaning that the
complete sync word is sent for every 1 in the payload and the inverted sync word is sent for every 0 in
the payload. Only
SYNC_CFG0.SYNC_MODE = 001b
and
010b
are supported (11 or 16 bits).
DSSS repeat mode is enabled by setting
In TX mode all packet handling features are supported, but the packet format must be set to FIFO
mode (
PKT_CFG2.PKT_FORMAT = 00b
).
In RX mode, none of the packet handling features are supported and synchronous serial mode must
be
selected
=
01b,
MDMCFG1.FIFO_EN
=
0
and
MDMCFG0.TRANSPARENT_MODE_EN = 0
).
It is the MCU‟s responsibility to extract the demodulated
data, which are available on GPIO by configuring
IOCFGx.GPIOx_CFG = 18
(GPIO3/1 =
, GPIO2 =
) will run at a
frequency twice the programmed data rate and will start as soon as the radio enters RX state.
The receiver uses two correlation filters (see Section 5.6 for more details) to search for sync word and
inverted sync word. Each output is connected directly to
. After
strobing RX the sync search starts, and when a sync word or inverted sync word is detected the
corresponding serial data line will be asserted high, otherwise the data line is low. The output from the
correlation filter is high as long as the sync word/inverted sync word is detected, so the MCU needs to
do edge detect on the data in order to not duplicate the demodulated data bit. Figure 12 shows how
the DSSS signal will look like on the GPIO pins when the following packet is sent on the air using
DSSS repeat mode: 0x03, 0x55, 0x55, 0x55. Data rate is 1.2 ksps.
Figure 12. DSSS Repeat Mode
6
DSSS PN mode and DSSS repeat mode are not supported for 4‟ary modulation formats.
within the first 5
edges after
entering RX should be ignored.