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gpio safe mode

leon zhang
Prodigy 70 points

Other Parts Discussed in Thread: AM3715

Dear team,

My project is using am3715 and linux operation system, we found a strange thing happened on the gpio(more specific, gpio129). it seems the gpio will change into the safe mode for a very short time until the gpio module restore its original values  when the system wakeup from sleep.

we tried to found more detail from the spec,but we only found:The safe mode avoids any risk of electrical contention by configuring the pin as an input with no functionalinterface mapped to it. The safe mode is used mainly as the default mode for all pins containing nomandatory interface at the release of POR.

I 'd like to clarify the following things:

1.  does a wakeup operation generate the release of POR?

2. The gpio is changed to safe mode at the release of POR, this transition is done by sw or by internal hardware? The default safe mode is input with pulllup or pulldown?

3. Through my experiment, we also found if the gpio is in reserve mode(mode 0) or safe mode(mode 7), it will not be changed to safe mode. it only happened when the gpio is in mode4.

I'm sorry it seems a quite long question, thanks a lot for your effort. it would be great if you can share more detail about gpio safe mode.

  • 0
    TI__Guru**** 393215 points
    Hi Leon,
     
    Is this an input or output? This GPIO is a special pin. Have you seen section 13.4.5 and the Caution in section 25.2 of the AM37X TRM Rev. R?
  • 0 in reply to Biser Gatchev-XID
    Prodigy 70 points

    Hi Biser,

    yes, the gpio129 is a special pin. We use this gpio as an output pin. I read the sections you mentioned, but i still have no idea about the safemode of gpio. Could you explain more about the safe mode?

    Another question: in section25.2, the TRM highlights the CATUTION: 

    Due to buffer strength, an external serial resistor must be connected to the balls corresponding to gpio_120 to gpio_129 pads.

    I didnot understand this sentence very clear, what will happen if there is no external serial resistor? thanks a lot.

  • 0 in reply to leon zhang
    TI__Guru**** 393215 points
    Hi Leon,
     
    It seems to me that this is not related to safe mode, but rather to GPIO_129 being an extended darin I/O pin. Section 13.4.5 in the TRM mentions that there are conditions which may cause this pin to float.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 70 points

    so, maybe the problem i describle:

    it seems the gpio will change into the safe mode for a very short time until the gpio module restore its original values  when the system wakeup from sleep.

    should be:"it seems the gpio will got a float state for a very short time until the gpio module restore its original values  when the system wakeup from sleep."

    the reason is we are using a special gpio and we are not using an external serial resistor, the nonstandard voltage domians, power-up sequences will let the gpio in float state during the start-up state? is my understand correct? 

    Using an external erial resistor as TRM mentioned can fix this problem?

    thanks a lot.

  • 0 in reply to leon zhang
    TI__Guru**** 393215 points
    Yes, it seems to me that this is what happens. I don't think an external serial resistor will fix this. Rather you should check what happens to the power supplies that power this I/O cell during the transition.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 70 points

    Biser, thanks a lot.