# KEY - Set Activation KEY

The KEY instruction is used for communicating KEY bytes to the UPDI, opening up for executing protected features on the device. See Table 1for an overview over functions that are activated by KEYs. For the KEY instruction, only 64bit KEY size is supported. If the System Information Block (SIB) field of the KEY instruction is set, the KEY instruction returns the SIB instead of expecting incoming KEY bytes. Maximum supported size for SIB is 128bits.

Figure 1. KEY Instruction Operation

The Figure above shows the transmission of a KEY, and the reception of a SIB. In both cases, the SIZE_C field in the opcode determines the number of frames to be sent or received. There is no response after sending a KEY to the UPDI. When requesting the SIB, data will be transmitted from the UPDI according to the current Guard Time setting.