Si102x/3x
14.2.4. Using the DMA to unwrap the extended Key
When used with the DMA, the address offset sfr DMANAOH/L may be manipulated to store the extended
key in the desired order. This requires two DMA transfers for the AES0YOUT channel. When using a 192-
bit key, the DMA0NSZ can be set to 24 bytes and the DMA0NA0 set to 16. This will place the last 8 bytes
of the 192-bit key in the desired location as shown in Table 14.2. The Yout arrow indicates the address off-
set position after each 8 bytes are transferred. Enabling the WRAP bit in DMA0NMD will reset the
DMA0NAO value after byte 23. Then the DMA0NZ can be reset to 16 for the remaining sixteen bytes.
Table 14.2. 192-Bit Key DMA Usage
Yout
Yout
K16...23
Yout
K0...7
K16...23
Yout
K0...7
K8...15
K16...23
When using a 256-bit key, DMA0NSZ can be set to 32 and DMA0NAOL set to 16 This will place the last 16
bytes of the 256-bit key in the desired location as shown in Table 14.3. Enabling the WRAP bit in
DMA0NMD will reset the DMA0NAO value after byte 31. Then DMA0NZ can be set to 16 for the remaining
sixteen bytes.
Table 14.3. 256-bit Key DMA Usage
Yout
?
Yout
K16...23
Yout
K16...23
Yout
K0...7
K16...23
Yout
K0...7
K8...15
K16...23
K24...31
K24...31
K24...31
184
Rev. 0.3
SILABS [ SILICON LABORATORIES ]
