mpc5744p pmc external voltage regulator

[luka_skoric]

Hello, petervlna

I have a question regarding implementation of the requirement SM_089 in Safety Manual for MPC5744P, Rev. 4, 02/2018.

"Assumption under certain conditions: [SM_089] When the system requires robustness
regarding 1.25 V over voltage failures, the external VREG mode is preferably selected.
The internal VREG mode uses a single pass transistor and, therefore, over voltage can not
be shut off redundantly."

What is the exact procedure for this implementation?

In our system we use MC33907/08 System Basis Chip that does what is described in Safety Requirement [SAG_MPC5744P_042] (from document Integrating the MPC5774P and MC33907/08 for Safety Applications Rev. 004/2015) but without any modifications done to PMC registers of MCP5744P controller.

Is the above mentioned Safety Manual requirement (SM_089) covered by the fact that the SBC monitors voltage levels? Is there anything to be done in relation to PMC_PMCCR register INT_REG_BYPASS flag?

When tried to modify that register in order to activate external voltage regulator, which was done by the following code:

        // Enable writing to PMC Control Register
        PMC.PMCCR.B.PMCCR_EN = 1u;
        // Disable internal regulator, external regulator is active
        PMC.PMCCR.B.INT_REG_BYPASS = 1u;

a series of resets crash the system.

Is there anything that has to be configured previous to disabling internal regulator? Does this register even have to be modified in order to satisfy requirement SM_089 from Safety Manual? What is the overall procedure for implementation of that requirement?

Thank you in advance.

Kind Regards

[petervlna]

Hello,

What is the exact procedure for this implementation?

Rationale: To enable system level measures to detect or shut down the supply voltage in case of an destructive (multiple point faults) 1.25 V over voltage incident.
Implementation hint: The digital (1.25 V) core supply voltage may be monitored externally and the power supply shut down in case of an over voltage.

so in short, use supplying 1.25 V with an external power supply like HW design guide explains:

Is there anything that has to be configured previous to disabling internal regulator? Does this register even have to be modified in order to satisfy requirement SM_089 from Safety Manual?

as above written:

It can be assumed that the internal regulator has no undesirable impact if the ballast transistor is not present (however, the   internal regulator can be switched off by setting PMC_PMCCR[INT_REG_BYPASS] to 1).

regards,

Peter

[luka_skoric]

Hello petervlna,

thank you for prompt answer.

In our case the Bipolar Junction Transistor (BJT) is connected on the BCTRL pin, and internal supply regulator is used for voltage level monitoring. There is no external Low Dropout Register (LDO) that would directly supply the core with 1.25V.

3.3V supply on VDD_HV_PMU is monitored and regulated by the SBC.

The only question remaining is, can we consider that the over voltage protection by the SBC on 3.3V supply simultaneously guarantees the protection of the 1.25V VDD_LV_COR?

[petervlna]

The only question remaining is, can we consider that the over voltage protection by the SBC on 3.3V supply simultaneously guarantees the protection of the 1.25V VDD_LV_COR?

No, you cannot. Because your 1.25V regulator is used as micro internal, and this one can go up to 3,3V if there is over voltage event. It wont destroy core or digital logic, but can affect its lifetime.

Of course there can be many ways how to achieve VDD_LV overvoltage protection, for example monitoring of VDD_LV_Core line externally and thus prevent this issue,etc...

But the easiest way is to use external power supply.

regards,

Peter