Rainbow Electronics AT86RF231 - Manual (Page 132)

132
8111A–AVR–05/08
AT86RF231
Note that IEEE 802.15.4-2006 standard MIC algorithm requires CBC mode encryption only, as it
implements a one-way hash function.
11.1.5 Data Transfer - Fast SRAM Access
The ECB and CBC modules including the AES core are clocked with 16 MHz. One AES opera-
tion takes 24 µs to execute, refer to parameter 12.4.15 in
Section 12.4 “Digital Interface Timing
Characteristics” on page 157 . That means that the processing of the data is usually faster than
the transfer of the data via the SPI interface.
To reduce the overall processing time the AT86RF231 provides a Fast SRAM access for the
address space 0x82 to 0x94.
Figure 11-5. Packet Structure - Fast SRAM Access Mode
Note: Byte 19 is the mirrored version of register AES_CON on SRAM address 0x94, see register
description AES_CON_MIRROR for details.
In contrast to a standard SRAM access, refer to Section 6.2.3 “SRAM Access Mode” on page
22 , the Fast SRAM access allows writing and reading of data simultaneously during one SPI
access for consecutive AES operations (AES run).
For each byte P0 transferred to pin 22 (MOSI) for example in "AES access #1", see Figure 11-5
on page 132 (lower part), the previous content of the respective AES register C0 is clocked out
at pin 20 (MISO) with an offset of one byte.
In the example shown in Figure 11-5 on page 132 the initial plaintext P0 - P15 is written to the
SRAM within "AES access #0". The last command on address 0x94 (AES_CON_MIRROR)
starts the AES operation ("AES run #0"). In the next "AES access #1" new plaintext data P0 -
P15 is written to the SRAM for the second AES run, in parallel the ciphertext C0 - C15 from the
first AES run is clocked out at pin MISO. To read the ciphertext from the last "AES run #(n)" one
dummy "AES access #(n+1)" is needed.
Note that the SRAM write access always overwrites the previous processing result.
The Fast SRAM access automatically applies to all write operations to SRAM addresses 0x82 to
0x94.
SRAM write MOSI
PHY_STATUS MISO
byte 0 (cmd)
address 0x83
XX XX
byte 1 (addr.) byte 2 (cfg)
P0[7:0]
XX
byte 3 byte 4 byte 18
(1)
byte 19 (start)
0x83 0x85 0x84 0x93 0x94 Address
MOSI
MISO
AES access #0
Address
P0 P15 ... cmd add cfg start
xx xx ... stat xx xx xx
0x83 0x94 ...
AES access #1
P0 P15 ... cmd add cfg start
xx C14 ... stat xx xx C15
0x83 0x94 ...
AES access #n+1
xx xx ... cmd add cfg start
xx C14 ... stat xx xx C15
0x83 0x94 ...
P1 P14
xx xx
P1 P14
C0 C13
xx xx
C0 C13
...
... C0[7:0]
P1[7:0]
C14[7:0] C15[7:0]
P15[7:0]
AES run #0 AES run #n
..
.