8.10.3 Send Checksum

To write data to user zones that require authentication or encryption for write access (ER = 0, AM[1:0] = 00, 01 or 10 in the access register), the host must first carry out the write command $B0. At this point, the memory remains unchanged and the device is waiting for the host to provide a valid checksum before initiating the write cycle. The host must immediately send the checksum it has computed using the System Write command with P1 = $02. Only if the checksum is valid will the device initiate the write cycle. Furthermore, if the device receives an incorrect checksum, it will clear the authentication privilege. After this command, the host must perform ACK polling.

Important: When a Write User Zone command is sent in Authentication mode or Encryption mode, the data are saved in a buffer until a cryptographic checksum is received. The host must send the checksum it has computed immediately after the Write User Zone command. If the checksum is valid, CryptoMemory® writes the data; if the checksum is incorrect, the data are discarded and the cryptographic engine is reset.

If there is any activity on the TWI bus between the Write User Zone command and the Send Checksum command, the EEPROM write may be aborted and the data in the user zone will remain unchanged.