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TMS320F28035: No external pullup resistor to XRSn pin in design

Patrick Verkennis
Expert 4290 points
Part Number: TMS320F28035


Hi Champs, A customer contacted me on following:

"We have a design in production that does not have a pullup resistor connected to XRSn pin.

What are the risks? Is the internal weak pullup sufficient?

Can we verify the design robustness on this issue in any way?"

One of the changes in the most recent changes in the TMS320F28035 specification is about the /XRS pin. The latest revision of the specification requires a pullup resistor on this pin.

SPRS584J, october 2013, table 2-2: states An R-C circuit must be connected to this pin for noise immunity reasons.

SPRS584K, june 2016, table 4-1: this was made more explicit: A resistor between 2.2 kΩ and 10 kΩ should be placed between XRS and VDDIO. If a capacitor is placed between XRS and VSS for noise filtering, it should be 100 nF or smaller.

Does this mean that the C is not mandatory but the R is mandatory? Should customer retrofit? Or how to asses this?

Thanks and best regards, Patrick

ps: 

  • 0
    TI__Guru* 95315 points

    Patrick,

                Guidance for the R & C values were added based on our experience fielding questions over the last few years. This also aids in properly distinguishing a POR and a WD initiated reset. Details on http://processors.wiki.ti.com/index.php/WDFlag_on_Piccolo

     

    The internal pull-up resistor on the -XRS pin is indeed weak, so we had always recommended an external pull-up, by way of the mandated R-C circuit. In rev K of the datasheet, guidelines for the R & C values were provided, making it more explicit.

     

    Hard to comment one way or the other about the impact to the current design, since the noise is not easily quantified.