35.6.9 Scrambling/Unscrambling Function

The scrambling/unscrambling function cannot be performed on devices other than memories. Data is scrambled when written to memory and unscrambled when data is read.

The external data lines 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 the micro-controller or the QSPI Client device (e.g. memory).

The scrambling/unscrambling function can be enabled by writing a '1' to the ENABLE bit in the Scrambling Control register (SCRAMBCTRL.ENABLE).

The scrambling and unscrambling are performed on-the-fly without impacting the throughput.

The scrambling method depends on the user-configurable Scrambling User Key in the Scrambling Key register (SCRAMBKEY.KEY). This register is only accessible in write mode.

By default, the scrambling and unscrambling algorithm includes the scrambling user key, plus a device-dependent random value. This random value is not included when the Scrambling/Unscrambling Random Value Disable bit in the Scrambling Mode register (SCRAMBCTRL.RANDOMDIS) is written to ‘1’.

The random value is neither user configurable nor readable. If SCRAMBCTRL.RANDOMDIS=0, data scrambled by a given circuit cannot be unscrambled by a different circuit.

If SCRAMBCTRL.RANDOMDIS=1, the scrambling/unscrambling algorithm includes only the scrambling user key, making it possible to manage data by different circuits. Note that the same key must be used by the different circuits.

The scrambling user key 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.