MCU Domain Reset Isolation: Introduction

Note

Reset isolation is applicable only for warm reset and not cold reset

In AM64x/AM243x devices the MCU domain can be reset isolated from the MAIN domain in cases where the MCU domain M4 core runs a safety application. The MCU domain can run a safety application when reset isolated to:

1. Monitor the MAIN domain and reset MAIN domain from MCU domain in case of unexpected errors
2. Run a safety critical firmware completely isolated accessing only MCU domain peripherals

When enabling reset isolation MCU domain can also be debug isolated, restricting JTAG access to MCU domain.

Reset Isolation: Developing a reset isolated safety application

MCU Domain

If MCU domain is isolated from accessing MAIN domain when enabling reset isolation (MCU domain running a safety critical firware accessing only MCU domain peripherals), the MCU firmware should not access any MAIN domain peripherals.

Note

Peripherals like Mailbox, SecureProxy is in the MAIN domain, so MCU firware should not use IPC or Sciclient calls if MCU domain is isolated from accessing MAIN domain.

When the MCU domain is monitoring the MAIN domain the MCU domain is not isolated from accessing MAIN domain peripherals and registers. In such a use case the MCU domain can access MAIN domain.

Note

In all reset isolation use case, on receiving reset request, the MCU domain before propogating the reset to MAIN domain, will enable isolation of MAIN domain from MCU domain and MCU domain from MAIN domain.

When running safety applications in MCU domain, the MCU domain will be isolated from the MAIN domain

MAIN domain

After reset isolation, the MAIN domain firmware should not access any MCU domain peripherals or registers. Before accessing the MCU domain registers the MAIN domain firmware should check the for the MCU MAGIC WORD in the mirrored MAIN domain register.

SBL running in MAIN domain also checks for the MAGIC WORD and only conditionally loads and runs MCU application. SBL also checks if reset isolation in not enabled and sends a TISCI message to change dev group to ALL so that MCU domain is also initialized by system firmware. SBL also turns on the MCU PLL if reset isolation is not enabled.

System Firmware & Board config

The board config dev group is changed to MAIN Domain only so that System Firmware does not access the MCU domain registers on bootup. The dev group is incremented to dev group ALL by SBL on sending a TISCI message, so that the system firmware iniializes the MCU domain