MRF24XA_15 Datasheet, PDF (107/258 Pages) Microchip Technology – Low-Power, 2.4 GHz ISM-Band IEEE 802.15.4™ RF

MRF24XA
4.7.5
AUTHENTICATE AND ENCRYPT
BLOCK CIPHER MODE (CCM*)
This mode is a combination of CTR mode (encryption)
and CBC-MAC mode (authentication). Initially, CBC-
MAC is applied to compute the MIC tag. CTR mode
(encryption) is only performed on a selected portion of
the authenticated message and the MIC tag. Different
combination of authentication and encryption are
formed.
802.15.4-2003/2006 defines CCM* mode as follows:
P = {AUTHENTICATION FLAG, NONCE, LENGTH, MAC-
HEADER, MACPAYLOAD}
O₁ = EK(P₁)
Loop on each 16 byte block of plaintext
O_i = EK(P_i xor O_i-1 )
End
MICTAG is the leftmost M bit of O_end
AUTHENTICATION FLAG: Reserved || Adata || M || L
4.8 Buffer Processing in Non-Secured
Sending
General Frame processing as shown in Figure 4-8,
applies when AUTORPTEN = 0. Unsecured frame
(SecEn = 0) is not an Acknowledgement and
CRCSZ = 1. Acknowledgement is transmitted or
received by software (AUTOACKEN = 0) and CRCSZ
= 1 (ACK may contain piggyback data in the payload,
at the discretion of the host software).
In Figure 4-8, note the following:
• Transmit buffer is the buffer starting at 0x200,
whereas the Receive Buffer is the buffer starting
at address 0x300 (this also applies for Streaming
mode).
• In the Transmit Buffer, FCS is automatically
appended to the frame, and the LENGTH field is
also automatically incremented.
• The Receive buffer holds the FCS appended
frame, and the according LENGTH. RSV is
appended to the frame. Note the ordering of the
RSV fields. RSV appending does not affect the
LENGTH field.
The following cases are not described in this data
sheet:
• Processing of secured frames (SecEn = 1) is pre-
sented in Section 5.3 “Security Material”.
• When AckReq = 1 (implying TXRXMODE = 00,
CRCSZ = 1), then the Acknowledge frame is gen-
erated or accepted on-the-fly: without writing or
reading the buffers (If CRCSZ = 0 then the CRC
appending does not take place and Acknowledge
is always processed by software:
AUTOACKEN = 0).
• Repeater mode is described in Section 7.3
“Auto-Repeater”.
FIGURE 4-8:
GENERAL FRAME PROCESSING
TX buffer loaded:
TXST set by host
TX buffer processed:
Length MHR Payload
Length _{MHR Payload} CRC
RXIF interrupt received, RXBUFFUL is set. (RSV appending enabled in: RXCON2)
RX buffer holds
Length _{MHR Payload} CRC RSV
MAC Frame
/LENGTH up to payload
/LENGTH up to CRC
/ if CRCSZ = 1 in Transmitter
/LENGTH up to CRC
/ if CRCSZ = 1 in Receiver
 2015 Microchip Technology Inc.
Preliminary
DS70005023C-page 107