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TMS320F28388D: About MEP step size

Wenjing
Prodigy 140 points
Part Number: TMS320F28388D

Hello,

We are using F28388D, the ePWM clock is 200MHz(5ns step).

Our Purpose is to ensure the accuracy of 1ns step resolution for Dead band high-resolution.

According to TRM manual, there are two ways to calculate in HR mode.

1. HR register is manually scaled by calculations in application software

2. HR register is automatically scaled by the MEP scale factor in the HRMSTEP register (SFO)

 

If using SFO in our system, we have concern that the MEP step size changing may cost unexpected behavior.

To avoid this, we may consider to calculate manually by software.

Here are my questions.

-        In TRM manual, I see the typical MEP step size is on the order of 150 ps, is it possible to set the MEP step size to 250ps?

-        If possible, is it guaranteed always calculating with this step size?

-        If there is a shift from original step size 250ps, how to confirm?

-        It is said using SFO can vary MEP step size based on worst-case process parameters, operating temperature, and voltage,

If scaling manually with fixed step size, what could happen in the worst-case process parameters, operating temperature, and voltage?

 

Thank you.

 

Best Regards,

Wenjing

  • 0
    TI__Guru 52010 points

    You cannot set the MEP step size. I urge you to use the SFO library. It will calculate the MEP step side for you, it does not set the step size. 

    You can continuously run the SFO to see the calculation results: the ratio between coarse steps to MEP HR steps.

    The number will not change by much with voltage. With temperature you see a small shift in the MEP step size but if the SFO is running periodically, the internal calculation will take care of the shift.

    If you TURN OFF AUTOCONVERT, you can see exactly what each MEP step size is, at different temperatures by incrementing the HR registers by one.

    Nima

  • 0 in reply to Nima Eskandari
    Prodigy 140 points

    Hello,

    Thank you very much for your answer.

    Could you please take a look at the following additional questions?

    1. If using SFO, are the values in the HRMSTEP register of each EPWM module the same?

    Or can different EPWM modules have different HRMSTEP register values?

    2. When using SFO, is the base step size a typical MEP step size of 150ps?

    Best Regards,

    Wenjing

  • 0 in reply to Wenjing
    TI__Guru 52010 points

    Absolutely!

    Wenjing said:
    1. If using SFO, are the values in the HRMSTEP register of each EPWM module the same?

    The HRMSTEP is approximated on EPWM1 and is the same for all other EPWMs.

    Wenjing said:
    2. When using SFO, is the base step size a typical MEP step size of 150ps?

    The SFO library is JUST ESTIMATING THE MEP step size by figuring out how many of the STEPs corrspond the COARSE step of an EPWM. Typically, the steps are around 150ps which is what the SFO estimates them to be.

    Nima