Transceiver Digital Control and Communications
MKW01xxRM Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc.
7-17
•
On RX side the checksum is calculated on the received payload and compared with the two
checksum bytes received. The result of the comparison is stored in bit
CrcOk.
By default, if the CRC check fails then the FIFO is automatically cleared and no interrupt is generated.
This filtering function can be disabled via
CrcAutoClearOff
bit and in this case, even if CRC fails, the
FIFO is not cleared and only
PayloadReady
interrupt goes high. Please note that in both cases, the two
CRC checksum bytes are stripped off by the packet handler and only the payload is made available in the
FIFO.
The CRC is based on the CCITT polynomial as shown below. This implementation also detects errors due
to leading and trailing zeros.
Figure 7-13. CRC Implementation
7.5.8
DC-Free Data Mechanisms
The payload to be transmitted may contain long sequences of 1's and 0's, which introduces a DC bias in
the transmitted signal. The radio signal thus produced has a non uniform power distribution over the
occupied channel bandwidth. It also introduces data dependencies in the normal operation of the
demodulator. Thus it is useful if the transmitted data is random and DC free.
For such purposes, two techniques are made available in the packet handler: Manchester encoding and data
whitening.
NOTE
Only one of the two methods should be enabled at a time.
7.5.8.1
Manchester Encoding
Manchester encoding/decoding is enabled if
DcFree = 01
and can only be used in Packet mode.
The NRZ data is converted to Manchester code by coding '1' as "10" and '0' as "01".
In this case, the maximum chip rate is the maximum bit rate given in the specifications section and the
actual bit rate is half the chip rate.
Manchester encoding and decoding is only applied to the payload and CRC checksum while preamble and
Sync word are kept NRZ. However, the chip rate from preamble to CRC is the same and defined by
BitRate
in
RegBitRate
(Chip Rate = Bit Rate NRZ = 2 x Bit Rate Manchester).
Manchester encoding/decoding is thus made transparent for the user, who still provides/retrieves NRZ data
to/from the FIFO.
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