Specifications

Page 57
WT1231H
ADVANCED COMMUNICATIONS & SENSING DATASHEET
5.5.7.2. Addr
ess Based
Address filtering can be enabled via the AddressFiltering bits. It adds another level of filtering, above Sync word (i.e. Sync
must match first), typically useful in a multi-node networks where a network ID is shared between all nodes (Sync word)
and each node has its own ID (address).
T
wo a
ddress based filtering options are available:

AddressFiltering = 01: Received address field is compared with internal register NodeAddress. If they match then
the
packet is accepted and processed, otherwise it is discarded.
 AddressFiltering = 10: Received address field is compared with internal registers NodeAddress and
BroadcastAddress.If either is a match, the received packet is accepted and processed, otherwise it is discarded.
This additional check with a constant is useful for implementing broadcast in a multi-node networks
Plea
se note that
the received address byte, as part of the payload, is not stripped off the packet and is made available in
the FIFO. In addition, NodeAddress and AddressFiltering only apply to Rx. On Tx side, if address filtering is expected, the
address byte should simply be put into the FIFO like any other byte of the payload.
As
addr
ess filtering requires a Sync word match, both features share the same interrupt flag SyncAddressMatch.
5.5.7.3.
Length Based
In variable length Packet mode, PayloadLength must be programmed with the maximum payload length permitted. If
received length byte is smaller than this maximum then the packet is accepted and processed, otherwise it is discarded.
Please
note that the receiv
ed length byte, as part of the payload, is not stripped off the packet and is made available in the
FIFO.
To
disable this function the user should set the value of the PayloadLength to 255.
5.5.7.4. CRC Based
The
CRC check is enabled by setting bit CrcOn in RegPacketConfig1. It is used for checking the integrity of the message.

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
defa
ult, 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.
Th
e CRC
is based on the CCITT polynomial as shown below. This implementation also detects errors due to leading and
trailing zeros.