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3.2.2. The AMBA3 APB peripheral bus

One of the most useful features of the TrustZone architecture is the ability to secure peripherals, such as interrupt controllers, timers, and user I/O devices. This enables the security environment to be extended so that it can solve some of the wider security issues which need more than just a secure data processing environment. A secure interrupt controller and timer allows a non-interruptible secure task to monitor the system, a secure clock source enables robust DRM, and a securable keyboard peripheral enables secure entry of a user password.

The AMBA3 specification includes a low gate-count low-bandwidth peripheral bus known as the Advanced Peripheral Bus (APB), which is attached to the system bus using an AXI-to-APB bridge. The APB bus does not carry an equivalent of the NS bits. This ensures that existing AMBA2 APB peripherals are compatible with systems implementing TrustZone technology. The AXI-to-APB bridge hardware is responsible for managing the security of the APB peripherals; the bridge must reject transactions of inappropriate security setting and must not forward these requests to the peripherals.

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