The external data bus can be scrambled to protect intellectual property data located in off-chip memories by means of data analysis at the package pin level of either the microcontroller or the memory device.
The scrambling and unscrambling are performed on-the-fly without additional wait states.
The scrambling/unscrambling function can be enabled or disabled by configuring the CSxSE bits in the SMC Off-Chip Memory Scrambling Register (SMC_OCMS).
When multiple chip selects are handled, the scrambling function per chip select is configurable using the CSxSE bits in the SMC_OCMS register.
The scrambling method depends on two user-configurable key registers, SMC_KEY1 and SMC_KEY2 plus a random value depending on device processing characteristics. These key registers cannot be read. They can be written once after a system reset.
The scrambling user key or the seed for key generation must be securely stored in a reliable non-volatile memory in order to recover data from the off-chip memory. Any data scrambled with a given key cannot be recovered if the key is lost.