SX1231
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
On Tx side a two byte CRC checksum is calculated on the payload part of the packet and appended to the end of the
message
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.
data input
CRC Polynomial =X16 + X12 + X5 + 1
X15
X14
X13
X12
X5
X0
X11
X4
* * *
* * *
Figure 33. CRC Implementation
5.5.7. 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.
5.5.7.1. 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.
Rev 2 - Nov 2009
Page 55
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
