Replacing the LS1043ARDB RTC and TA_BB_RTC using an off SoC RTC will break the TrustZone WDOG?

[tracysmith]

Replacing the LS1043ARDB RTC and TA_BB_RTC using an off SoC RTC will break the TrustZone WDOG?

Note on the LS1043ARDB the WDOG receives a clock which is the Ored version of the TA_BB_RTC and RTC.  See the LS1043A RM Fig. 4-7 below for the clock subsystem block diagram.  On the LS1043ARDB the 32 KHx clock is applied to both inputs TA_BB_RTC and RTC. The WDOG monitors internal system operation and forces a reset in case of failure. It operates on RTC 32 KHz clock. The number of instances supported in LS1043A are five. Out of five WDOGs, one is dedicated for Trustzone support and other four are for each A53 core(one for each core in the cluster). LS1043ARDB RM

31.4.6 TrustZone Watchdog (TrustZone WDOG) LS1043 RM p. 2033
The TrustZone WDOG is an additional watchdog timer instance. The TrustZone WDOG protects against TZ non-secure world software preventing a switch back to the Secure World, thereby starving security services of access to GPP resources. Once the TrustZone WDOG is activated, it must be serviced by TZ secure world software on a periodic basis. If servicing does not take place before the configured time-out, the TrustZone WDOG asserts a secure interrupt that forces a switch to the Secure World. If it is still not served, the TrustZone WDOG asserts a security violation alarm. The TrustZone WDOG cannot be programmed or deactivated from TZ non-secure world. The TrustZone WDOG requires a non-gateable clock source, and should be capable of counting up to 256 seconds. It may sleep during deep-sleep modes.

[tracysmith]

I see all WDOG modules use 32 KHz clock, driven at device input RTC ORed with TA_BB_RTC pin for their counters, but as long as the RTC signal is provided by the CPLD, will the WDOG for the TrustedZone function?  Or, does it require RTC ORed with TA_BB_RTC pin?

[alexander_yakov]

The device does not require "RTC ORed with TA_BB_RTC pin", this ORing is performed internally. Simply applying 32KHz clock to RTC pin and pulling down TA_BB_RTC pin will be enough.

[Gandalf-kern]

Please explain why you believe it will not be operable?  What is the basis for the concern?

[alexander_yakov]

I believe RTC module can not operate without RTC clock input. Concern is - watchdog timeout is measured in clock ticks, to count these ticks reference clock must be present. Counting RTC clock ticks is not possible, if RTC input clock is not applied.

[alexander_yakov]

Yes, I do not think Trust Zone watchdog will remain operable, if you remove RTC clock input and replace it by external watchdog circuit.

Have a great day,
Alexander

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