The Start mode field in the Control A Register (CTRLA.STARTMODE) allows the selection of encryption start mode.
The Auto Start Mode is similar to the manual one, except in this mode, as soon as the correct number of input data registers is written, processing is automatically started without setting the START bit in the Control B Register. DMA operation uses this mode.
This mode is used to generate message authentication code (MAC) on data in CCM mode of operation. The CCM mode combines counter mode for encryption and CBC-MAC generation for authentication.
When LOD is disabled in CCM mode then counter mode of encryption is performed on the input data block.
When LOD is enabled in CCM mode then CBC-MAC generation is performed. Zero block is used as the initialization vector by the hardware. Also software read from the Output Data Register (DATA) is not required to clear the ENCCMP flag. The ENCCMP flag is automatically cleared by writing into the Input Data Register (DATA). This allows retrieval of only the last data in several encryption/decryption processes. No output data register reads are necessary between each block of encryption/decryption process.
Note that assembling message depending on the security level identifier in CCM* has to be done in software.