MRF39RA Datasheet, PDF (38/72 Pages) Microchip Technology – Low-Power, Integrated UHF Receiver

MRF39RA
4.5.5 HANDLING LARGE PACKETS
When Payload length exceeds FIFO size (66 bytes)
whether in fixed, variable or unlimited length packet
format, in addition to PayloadReady or CrcOk in Rx, the
FIFO interrupts/flags can be used as follows:
FIFO must be unfilled on the fly during Rx to prevent
FIFO overrun.
1. Start reading bytes from the FIFO when
FifoNotEmpty or FifoThreshold becomes set.
2. Suspend reading from the FIFO if FifoNotEmpty
clears before all bytes of the message have
been read
3. Continue to step 1 until PayloadReady
4. Read all remaining bytes from the FIFO either in
Rx or Sleep/Standby mode.
Note:
AES decryption is not feasible on large
packets, since all Payload bytes need to
be in the FIFO at the same time to perform
decryption.
4.5.6 PACKET FILTERING
MRF39RA’s packet handler offers several mechanisms
for packet filtering, ensuring that only useful packets
are made available to the uC, reducing significantly
system power consumption and software complexity.
4.5.6.1 Sync Word-Based
Sync word filtering/recognition is used for identifying
the start of the payload and also for network
identification. As previously described, the sync word
recognition block is configured (size, error tolerance,
value) in RegSyncValue registers. This information is
used to filter packets in Rx.
Every received packet which does not start with this
locally configured sync word is automatically discarded
and no interrupt is generated.
When the sync word is detected, payload reception
automatically starts and SyncAddressMatch is
asserted.
Note: Sync word values containing 0x00 byte(s)
are forbidden.
4.5.6.2 Address-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).
Two address-based filtering options are available:
1. 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.
2. 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, oth-
erwise it is discarded. This additional check with
a constant is useful for implementing broadcast
in a multi-node networks.
As address filtering requires a sync word match, both
features share the same interrupt flag
SyncAddressMatch.
Note that the received address byte, as part of the
payload, is not stripped off the packet and is made
available in the FIFO.
4.5.6.3 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.
Note that the received 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 must set the value of
the PayloadLength to 255.
DS40001778B-page 38
 2015 Microchip Technology Inc.