The external data bus can be scrambled in order to prevent intellectual property data located in off-chip memories from being easily recovered by analyzing data at the package pin level of either microcontroller or 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 SDR_SE bit in the OCMS register (SDRAMC_OCMS). This bit cannot be reconfigured as long as the external memory device is powered.
The scrambling method depends on two user-configurable key registers, SDRAMC_OCMS_KEY1 and SDRAMC_OCMS_KEY2 plus a random value depending on device processing characteristics. These key registers are only accessible in Write mode.
The scrambling user key or the seed for key generation must be securely stored in a reliable nonvolatile 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.
When multiple chip selects are handled, it is possible to configure the scrambling function per chip select using the OCMS field in the SDRAMC_OCMS registers.