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TMS320F2809x Brownout Reset Problem

Robin Vice
Intellectual 950 points
Other Parts Discussed in Thread: TMS320F28069, CONTROLSUITE

Hi All,


Using a variac to decrease line voltage, as the raw DC supply drops below the 3.3V regulator's dropout voltage, ripple is presented to the Vddio pins on the device as you would expect.

When the troughs of the ripple approach 2.5V on the Vddio pins, the XRS starts its assertion to a low state, but only drops to about 2V before quickly rising back to 3V.  When that happens, the ripple in the power supply waveform disappears, probably because the processor is not pulling as much current.  No reset occurs, and the processor is no longer processing, even after increasing supply voltage to nominal.  Lowering the power supply further causes a repeat performance with the drup in current demand and restoration of a flat DC supply.

So, the brownout reset feature is not resetting the processor under these brownout conditions.

If I continue lowering the supply to the point where the XRS pin drops to less than 1V, which is significatnly lower than I went as described above, then a reset does occur after increasing the supply voltage.  This seems appropriate, but the marginal brownout condition stated above doesn't.

Forcing the XRS pin low (using a jumper to ground) initiates a perfectly controlled reset, and the processor works fine.


Comments, ideas?

  • 0
    TI__Guru 57255 points
    Hi Robin,

    The first questions I would ask are:
    1) What C2000 MCU are you using? The 'F2809x is not a C2000 device family, perhaps you mean a 'F28069?
    2) What circuitry is on the XRSn pin?


    Thank you,
    Brett
  • 0 in reply to Brett Larimore
    Intellectual 950 points
    Hi Brett,

    Sorry for the typo, you're right, it's a TMS320F28069.
    The XRS pin goes through a 1k resistor to an unused header, nowhere else.

    Thanks,
    Robin
  • 0 in reply to Robin Vice
    Guru 192875 points

    Robin Vice said:
    The XRS pin goes through a 1k resistor to an unused header, nowhere else.

    This is where you're going wrong..I feel. You need to pull up the XRS pin through a 2.2k resistor.

    Do the necessary changes... and keep us informed!

    Regards,

    Gautam

  • 0 in reply to Gautam Iyer
    Intellectual 950 points

    Hi Guatam,

    I followed your suggestion and tried tying XRS to +3.3V through a 2.2k resistor, but the results were the same.  I think it works without an external pull up resister by virtue of the internal weak pull up.

    Here's what it looks like when the XRS pin starts to assert low, but stops its assertion before making it low enough to initiate a reset:

    See the attached file for a more detailed look at the sequence of events.

    Thanks,

    RobinXRS Reset Slides.pdf

  • 0 in reply to Robin Vice
    TI__Guru 57255 points

    Robin,

    Could a share the relevant portion of your schematic (the MCU and its 'device support' circuitry)?  I'm particularly interested in the capacitance you have on your rails, whether you're using the internal Vreg to generate 1.8V (VDD), etc.

    Could you run an experiment and remove what was your 1K ohm resistor (which ties XRSn to your unconnected header) and see if anything changes?  We'd like to completely remove that connector as a potential root cause.  As you've estimated, in most situations, the weak internal pull-up should be enough to pull XRSn high.  We recommend having the additional pull-up external to the chip for greater noise immunity in the system.


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Intellectual 950 points
    OK, now we're getting somewhere.

    I found there was a 20uF cap from XRS (pin 9) to ground. That was filtering out the power supply ripple, and preventing the pin from going low quickly enough to get to reset before the current dropped. When the current dropped, the power supply ripple went away and released the reset assertion before it completed.

    What is the suggested capacitance? Is there an app note on the subject. I've searched and can't find anything more specific than what's found in the data sheet.

    Thanks for your help!
  • 0 in reply to Robin Vice
    TI__Guru 57255 points

    Hi Robin,

    You can use the F28069 controlCARD as a reference if you've installed controlSUITE:
    \controlSUITE\development_kits\~controlCARDs\CC2806xHWdevPkg\R1_1\

    I'd recommend no more than 100nF on XRSn.  No capacitance actually should be fine (as in the cCARD), but something like 1nF may provide a bit more robustness. 


    Thank you,
    Brett

  • 0 in reply to Brett Larimore
    Intellectual 950 points
    Thanks Brett,

    You led me to the problem.

    The XRS pin had a 20uF cap that prevented the reset line to get low quickly enough to fully assert a reset.

    By changing the cap 0.01uF, the brown out reset now works properly.

    Thanks a million!