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3.3.2. Configuration lockdown

The AHB5 TrustZone memory protection controller provides a configuration lockdown feature that prevents malicious software from changing the security configuration. Writing 0x1 to the security lockdown bit, CTRL[31], enables the configuration lockdown feature.

Once the configuration lockdown feature is enabled:

  • It can only be disabled by a component reset which resets CTRL[31] to 0x0.

  • The following registers are read-only:

    • CTRL.

    • BLK_LUT.

    • INT_EN.


ARM recommends that you write 0x1 to the LUT autoincrement bit, CTRL[8] before enabling the configuration lockdown feature. When the feature is enabled only LUT reading is available which is simpler when BLK_IDX increments automatically during the read sequence.