Programmer’s Guide

Initialization

The memory inside the SRAM controller can be used right away after a system reset. However, since the scrambling key defaults to a predefined value, it is recommended that SW performs the following initialization steps as early in the boot process as possible.

  1. Request an updated ephemeral scrambling key from OTP by writing 0x1 to CTRL.RENEW_SCR_KEY. SW should spin on STATUS.SCR_KEY_VALID to wait until the new key has been obtained. While this is not strictly needed since memory accesses to the SRAM will be stalled until the updated key has been obtained, the PC value upon a watchdog crash will be more informative when using a spin wait.

  2. (optional) Initialize the memory with pseudo random data by writing 0x1 to CTRL.INIT SW should spin on STATUS.INIT_DONE to wait until the memory has been initialized. While this is not strictly needed since memory accesses to the SRAM will be stalled until the initialization is done, the PC value upon a watchdog crash will be more informative when using a spin wait.

  3. (optional) Check the STATUS.SCR_KEY_SEED_VALID bit:

    • In case the scrambling key seeds have been fully provisioned to OTP, this bit should be set to 0x1. A value of 0x0 indicates that the OTP could be malfunctioning or has been tampered with.
    • If the scrambling seeds have not yet been provisioned to OTP, this bit is set to 0x0. The scrambling key will in that case still be ephemeral, but the key seed mixed in as part of the key derivation process will be set to a predefined netlist constant.

  4. (optional) Lock down write access to CTRL by writing to CTRL_REGWEN if future key renewals and initializations should be disallowed until the next system reset.

Note that before (re-)requesting an updated SRAM key it is imperative to make sure that:

  • The memory contents are not needed anymore. Requesting a key implicitly wipes all data in the SRAM.
  • The CSRNG and the entropy distribution network have been initialized. The key derivation mechanism in OTP needs to request a chunk of fresh entropy, and that request will block until the entropy distribution network responds.

It should also be noted that data and address scrambling is never entirely disabled - even when the default scrambling key is used.

Device Interface Functions (DIFs)