Rainbow Electronics AT88SA100S - Manual (Page 17)

AT88SA100S [ Preliminary]
17
8558A–SMEM–03/09
4.5. LoadSram
Writes 256 bits into the battery backed SRAM and locks this memory against further modification. The value in the
battery backed SRAM cannot be read, it must be verified via the MAC command. If the SRAM is already valid then this
command will fail with an error response. The only way to unlock the SRAM is to remove power from the device.
The input data is always decrypted using the decryption digest previously generated by GenPersonalizationKey prior to
being written into the battery backed SRAM.
Note: Both the GenPersonalizationKey and LoadSram commands must be run consecutively within a single wake
cycle prior to the expiration of the watchdog timer. If any command is inserted between these two operations
then LoadSram will fail.
Table 17. Input Parameters
Name Size Notes
Opcode LOADSRAM 1 0x10
Param1 Zero1 1 Must be 0x00
Param2 Zero2 2 Must be 0x00 00
Data Map 32 Encrypted value to be written into the SRAM.
Table 18. Output Parameter
Name Size Notes
Success 1
Upon successful execution, a value of 0 will be returned by the AT88SA100S
chip.
The AT88SA100S chip executes the following sequence on receipt of this command.
1. If the internal flag (indicating that a personalization key has been loaded) is not set, then return error. If the
MemValid flag is set, return error.
2. Successively XOR each byte in the data parameter with the corresponding byte from the personalization key
generated by GenPersonalizationKey.
3. Transfer the bytes to the battery backed SRAM.
4. Set MemValid (internal flag) to 1.