CC1101
SWRS061H Page 41 of 98
The modulator will first send the programmed
number of preamble bytes. If data is available
in the TX FIFO, the modulator will send the
two-byte (optionally 4-byte) sync word followed
by the payload in the TX FIFO. If CRC is
enabled, the checksum is calculated over all
the data pulled from the TX FIFO, and the
result is sent as two extra bytes following the
payload data. If the TX FIFO runs empty
before the complete packet has been
transmitted, the radio will enter
TXFIFO_UNDERFLOW state. The only way to
exit this state is by issuing an SFTX strobe.
Writing to the TX FIFO after it has underflowed
will not restart TX mode.
If whitening is enabled, everything following
the sync words will be whitened. This is done
before the optional FEC/Interleaver stage.
Whitening is enabled by setting
PKTCTRL0.WHITE_DATA=1.
If FEC/Interleaving is enabled, everything
following the sync words will be scrambled by
the interleaver and FEC encoded before being
modulated. FEC is enabled by setting
MDMCFG1.FEC_EN=1.
15.5 Packet Handling in Receive Mode
In receive mode, the demodulator and packet
handler will search for a valid preamble and
the sync word. When found, the demodulator
has obtained both bit and byte synchronization
and will receive the first payload byte.
If FEC/Interleaving is enabled, the FEC
decoder will start to decode the first payload
byte. The interleaver will de-scramble the bits
before any other processing is done to the
data.
If whitening is enabled, the data will be de-
whitened at this stage.
When variable packet length mode is enabled,
the first byte is the length byte. The packet
handler stores this value as the packet length
and receives the number of bytes indicated by
the length byte. If fixed packet length mode is
used, the packet handler will accept the
programmed number of bytes.
Next, the packet handler optionally checks the
address and only continues the reception if the
address matches. If automatic CRC check is
enabled, the packet handler computes CRC
and matches it with the appended CRC
checksum.
At the end of the payload, the packet handler
will optionally write two extra packet status
bytes (see Table 27 and Table 28) that contain
CRC status, link quality indication, and RSSI
value.
15.6 Packet Handling in Firmware
When implementing a packet oriented radio
protocol in firmware, the MCU needs to know
when a packet has been received/transmitted.
Additionally, for packets longer than 64 bytes,
the RX FIFO needs to be read while in RX and
the TX FIFO needs to be refilled while in TX.
This means that the MCU needs to know the
number of bytes that can be read from or
written to the RX FIFO and TX FIFO
respectively. There are two possible solutions
to get the necessary status information:
a) Interrupt Driven Solution
The GDO pins can be used in both RX and TX
to give an interrupt when a sync word has
been received/transmitted or when a complete
packet has been received/transmitted by
setting IOCFGx.GDOx_CFG=0x06. In addition,
there are two configurations for the
IOCFGx.GDOx_CFG register that can be used
as an interrupt source to provide information
on how many bytes that are in the RX FIFO
and TX FIFO respectively. The
IOCFGx.GDOx_CFG=0x00 and the
IOCFGx.GDOx_CFG=0x01 configurations are
associated with the RX FIFO while the
IOCFGx.GDOx_CFG=0x02 and the
IOCFGx.GDOx_CFG=0x03 configurations
are associated with the TX FIFO. See Table
41 for more information.
b) SPI Polling
The PKTSTATUS register can be polled at a
given rate to get information about the current
GDO2 and GDO0 values respectively. The
RXBYTES and TXBYTES registers can be
polled at a given rate to get information about
the number of bytes in the RX FIFO and TX
FIFO respectively. Alternatively, the number of
bytes in the RX FIFO and TX FIFO can be
read from the chip status byte returned on the
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