1093
6437E–ATARM–23-Apr-13
SAM9M11
Note:
Warning: In PDC mode, reading to the Output Data registers before the last data transfer may lead to unpredictable results.
45.4.4
Security Features
45.4.4.1
Countermeasures
The TDES also features hardware countermeasures that can be useful to protect data against
Differential Power Analysis (DPA) attacks.
These countermeasures can be enabled through the CTYPE field in the TDES Mode Register.
This field is write-only, and all changes to it are taken into account if, at the same time, the Coun-
Enabling countermeasures has an impact on the TDES encryption/decryption throughput. The
best throughput is achieved with all the countermeasures disabled. On the other hand, the best
protection is achieved with all of them enabled. By default, all the countermeasures are enabled.
The LOADSEED bit in the TDES Control Register (TDES_CR) allows to restart the countermea-
sure generator.
45.4.4.2
Unspecified Register Access Detection
When an unspecified register access occurs, the URAD bit in the Interrupt Status Register
(TDES_ISR) raises. Its source is then reported in the Unspecified Register Access Type field
(URAT). Only the last unspecified register access is available through the URAT field.
Several kinds of unspecified register accesses can occur:
Input Data Register written during the data processing in PDC mode
Output Data Register read during the data processing
Mode Register written during the data processing
Write-only register read access
The URAD bit and the URAT field can only be reset by the SWRST bit in the TDES_CR control
register.
Table 45-3.
Last Output Mode Behavior versus Start Modes
Manual and Auto Modes
PDC Mode
LOD=0
LOD=1
LOD=0
LOD=1
DATRDY Flag
At least one Output
Data Register must be
read
At least one Input Data
Register must be written
Not used
Managed by the PDC
Encrypted/Decrypted
Data Result Location
In the Output Data
Registers
In the Output Data
Registers
At the address
specified in the
Receive Pointer
Register
(TDES_RPR)
In the Output Data
Registers
End of
Encryption/Decryption
DATRDY
ENDRX (or RXBUFF)
ENDTX (or TXBUFE)
then DATRDY