CC2500_06 Datasheet, PDF (29/84 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
15.3 Packet Filtering in Receive Mode
CC2500 supports three different packet-filtering
criteria: address filtering, maximum length
filtering and CRC filtering.
received packet must be read out of the FIFO
before receiving the current packet. The MCU
must not read from the current packet until the
CRC has been checked as OK.
15.3.1 Address Filtering
Setting PKTCTRL1.ADR_CHK to any other
value than zero enables the packet address
filter. The packet handler engine will compare
the destination address byte in the packet with
the programmed node address in the ADDR
register and the 0x00 broadcast address when
PKTCTRL1.ADR_CHK=10 or both 0x00 and
0xFF broadcast addresses when
PKTCTRL1.ADR_CHK=11. If the received
address matches a valid address, the packet is
received and written into the RX FIFO. If the
address match fails, the packet is discarded
and receive mode restarted (regardless of the
MCSM1.RXOFF_MODE setting).
If the received address matches a valid
address when the packet length is set to
infinite and address filtering is enabled, 0xFF
will be written into the RX FIFO followed by the
address byte and then the payload data.
15.3.2 Maximum Length Filtering
In the variable packet length mode the
PKTLEN.PACKET_LENGTH register value is
used to set the maximum allowed packet
length. If the received length byte has a larger
value than this, the packet is discarded and
receive mode restarted (regardless of the
MCSM1.RXOFF_MODE setting).
15.3.3 CRC Filtering
The filtering of a packet when CRC check fails
is enabled with PKTCTRL1.CRC_AUTOFLUSH.
The CRC auto flush function will flush the
entire RX FIFO if the CRC check fails. After
auto flushing the RX FIFO, the next state
depends on the MCSM1.RXOFF_MODE setting.
PKTCTRL0.CC2400_EN must be 0 (default)
for the CRC auto flush function to work
correctly.
When using the auto flush function, the
maximum packet length is 63 bytes in variable
packet length mode and 64 bytes in fixed
packet length mode. Note that the maximum
allowed packet length is reduced by two bytes
when PKTCTRL1.APPEND_STATUS is
enabled, to make room in the RX FIFO for the
two status bytes appended at the end of the
packet. Since the entire RX FIFO is flushed
when the CRC check fails, the previously
15.4 CRC Check
There are two different CRC implementations.
PKTCTRL0.CC2400_EN selects between the
2 options. The CRC check is different for the 2
options. Refer also to the CC2500 Errata Note.
15.4.1 PKTCTRL0.CC2400_EN = 0
If PKTCTRL0.CC2400_EN =0 it is possible to
read back the CRC status in 2 different ways:
1) Set PKTCTRL1.APPEND_STATUS=1 and
read the CRC_OK flag in the MSB of the
second byte appended to the RX FIFO after
the packet data. This requires double buffering
of the packet, i.e. the entire packet content of
the RX FIFO must be completely read out
before it is possible to check whether the CRC
indication is OK or not.
2) To avoid reading the entire RX FIFO,
another solution is to use the
PKTCTRL1.CRC_AUTOFLUSH feature. If this
feature is enabled, the entire RX FIFO will be
flushed if the CRC check fails. If
GDOx_CFG=0x06 the GDOx pin will be
asserted when a sync word is found. The
GDOx pin will be de-asserted at the end of the
packet. When the latter occurs the MCU
should read the number of bytes in the RX
FIFO from the RXBYTES.NUM_RXBYTES
status register. If RXBYTES.NUM_RXBYTES=0
the CRC check failed and the FIFO is flushed.
If RXBYTES.NUM_RXBYTES>0 the CRC check
was OK and data can be read out of the FIFO.
15.4.2 PKTCTRL0.CC2400_EN = 1
If PKTCTRL0.CC2400_EN = 1 the CRC can be
checked as outlined in 1) in Section 15.4.1 as
well as by reading the CRC_OK flag available
in the PKTSTATUS[7] register, in the LQI[7]
status register or from one of the GDO pins if
GDOx_CFG is 0x07 or 0x15.
The PKTCTRL1.CRC_AUTOFLUSH or data
whitening
cannot be used when
PKTCTRL0.CC2400_EN = 1.
15.5 Packet Handling in Transmit Mode
The payload that is to be transmitted must be
written into the TX FIFO. The first byte written
must be the length byte when variable packet
PRELIMINARY Data Sheet (Rev.1.2) SWRS040A
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