MRF24J40-I MICROCHIP [Microchip Technology], MRF24J40-I Datasheet - Page 22

• Manufacturer Part Number: MRF24J40-I
• Manufacturer: MICROCHIP [Microchip Technology]

5.2.1
When transmitting and receiving data with the
MRF24J40, the preamble and Start-of-Frame delimiter
bytes will automatically be generated or stripped from
the packets when they are transmitted or received. The
host controller does not need to concern itself with
them. Normally, the host controller will also not need to
concern itself with the CRC, which the MRF24J40 will
also be able to automatically generate when transmit-
ting and verify when receiving. The CRC fields will,
however, be written into the receive buffer when
packets arrive, so they may be evaluated by the host
controller if needed.
5.2.2
The length field is a 1-byte field which defines the size of
the packet excluding itself, the preamble and SFD, but
including all other bytes of the packet, including FCS.
5.2.3
The frame control field describes the format of this
packet. It defines the type of packet (beacon, data,
ACK, etc.) the addressing modes used, if the packet is
encrypted or not, if the packet requires an ACK and if
the packet is an intra-PAN network. This information is
used by the host controller to determine how to
decipher the data that follows the frame control field.
5.2.4
The sequence number field is a 1-byte sequence number
that distinguishes packets. The sequence number field is
used in the Acknowledgement process. An ACK packet
contains no addressing information, so the uniqueness of
the sequence number is the sole determining factor for
verifying that a packet reached its destination. The
MRF24J40 has an Auto-Acknowledgement feature that
is described in Section 7.1 “Transmitting Packets”.
5.2.5
The destination address fields of an IEEE 802.15.4
packet can change depending on the frame control
field of that packet. The frame control field can specify
that no destination address is present, or can specify
that the short address (2 bytes) or long address
(8 bytes) is present. In all cases where an address is
specified, the destination PAN identifier will also be
included. On incoming packets, the MRF24J40 will
filter out packets that do not match the preconfigured
addressing information for that radio. This eliminates
any software intervention for packets that do not meet
the addressing requirements. When transmitting the
host controller is required to write the appropriate
destination address into the transmit buffer.
DS39776A-page 20
MRF24J40
PREAMBLE/START-OF-FRAME
DELIMITER
LENGTH
FRAME CONTROL
SEQUENCE NUMBER
DESTINATION ADDRESS
INFORMATION
Advance Information
5.2.6
The source address fields of an IEEE 802.15.4 packet
can change depending on the frame control field of that
packet. The frame control field can specify that no
destination address is present, or can specify that the
short address (2 bytes) or long address (8 bytes) is
present. The frame control can also specify, by using
the intra-PAN bit, that the source PAN matches the
destination PAN and is thus, not included in the packet.
Long addresses consist of two portions. The first three
bytes are known as the Extended Organizationally
Unique Identifier (EUI). EUIs are distributed by the
IEEE 802.15.4. The last five bytes are address bytes
which can contain the needed requirements at the
discretion of the company that purchased the EUI.
When transmitting packets, the assigned source long or
short address, depending on the setting of the frame
control field, must be written into the transmit buffer by
the host controller. The MRF24J40 will not automatically
include the source address information.
5.2.7
The data section of the packet can vary in length from
0 bytes to 122 bytes. Packets that exceed 127 bytes,
including the frame control, source addressing, desti-
nation addressing, data and FCS fields, will be filtered
out by the MRF24J40.
5.2.8
The FCS field is a 2-byte field which contains an
industry standard, 16-bit CRC calculated with the data
from the frame control, sequence number, destination,
source, and data fields. When receiving packets, the
MRF24J40 will check the CRC of each incoming
packet. If the RXCRCEN bit (RXMCR<1>) is cleared,
packets with invalid CRCs will automatically be dis-
carded. If RXCRCEN is set, and the packet meets all
other receive filtering criteria, the packet will be written
into the receive buffer and the host controller will be
able to determine if the CRC was valid by reading the
receive status vector (see Section 7.3 “Receiving
Packets”).
When transmitting packets, the MRF24J40 will auto-
matically generate a valid CRC and transmit it attached
to the end of the packet if the TXCRCEN bit
(RXMCR<7>) is cleared.
SOURCE ADDRESS INFORMATION
DATA
FCS
© 2006 Microchip Technology Inc.