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am335x power sequence.

SHU LIU
Intellectual 570 points
Other Parts Discussed in Thread: AM3358

Hi,

We use am3358 zcz processor with the TPS65217C PMIC on our board. My question is :

There is another regulator IC to generate 3.3V and the IC provides the 3.3V external pull up through a 10K resistor to some of the GPIOs of the am3358.

However, the 3.3V external pull up might be applied to the GPIOs of the am3358 before the am3358 gets powered from TPS65217C or they will be the same time.

Will this 3.3V external pull up voltage damage the am3358 chip? 

If anyone could answer my question, I would really appreciate it. Thanks.

Shu

  • 0
    TI__Guru**** 393215 points

    Hi Shu,

    The AM335X I/O pins are not fail-safe, except for USB0_VBUS and USB1_VBUS. Eternal voltages must never be applied to them before the processor supplies have fully ramped-up and stabilized. Please check the table in section 5.1 in the AM335X Datasheet Rev. G. Look for the parameter "Steady State Max. Voltage at all IO pins" and Note 8 below the table. The processor is very likely to get damaged if this requirement is not met.

  • 0 in reply to Biser Gatchev-XID
    Intellectual 570 points

    Biser,

    How do I know the IO supply voltage if the processor supplies are not fully ramped-up and stabilized? Is it 0V ?

    Steady State Max. Voltage at all IO pins  :                  -0.5V to IO supply voltage + 0.3 V

    Regards,

    Shu

  • 0 in reply to SHU LIU
    TI__Guru 62855 points

    The definition of 'IO supply voltage' is the voltage of the respective VDD power supply that powers the IO.

    The Ball Characteristics table in the AM335x data sheet has a column named 'ZCE POWER / ZCZ POWER' which defines the VDD supply for each IO.

    The best way to insure this parameter is not exceeded, is by designing your system such that every signal is only able to be driven from sources powered from the same power supply.  Let look at a couple of examples to help understand this requirement.  Signal 'A' is connected to an AM335x IO powered from VDDSHV1, two other devices that can source the signal, and a pull-up.  In this case, the IOs of these other two devices and the pull-up should be powered from the same power supply connected to VDDSHV1. Signal 'B' is connected to an AM335x IO powered from VDDSHV6 and one other device which cannot source the signal.  In this case, there is no issue for AM335x when the other device is powered from a different power supply than the one connected to VDDSHV6.  However, the other device may have a similar requirement which would require its IO supply to be connected to VDDSHV6.

    Regards,
    Paul

  • 0 in reply to peaves
    Intellectual 570 points

    Paul,

    Thanks for your detailed the reply.

    Why we use a different power supply is because the other devices/chips, which are connected to AM335x IOs/external pull-ups, require a high current draw. The power supply of AM335x (TPS65217C) does not have that much current output. 

    Can we connect the AM335x power supply (VDD1) to the "enable" pin of the other power supply(VDD2), therefore, the VDD1 will ramp up first and then VDD2 would be enabled by VDD1. By doing this, we can insure that AM335x gets powered by VDD1 before VDD2 is applied to the AM335x IOs or pull-ups of the IOs.

    Please let me know if my thought above is right or not and what is your suggestion? Thanks.

    Regards,

    Shu

  • 0 in reply to SHU LIU
    TI__Guru 62855 points

    This approach may resolve this issue for AM335x during power-up, but there still may be an issue during power-down if the AM335x power supply falls faster than the other power supply.  It may also violate the same requirement for devices attached to AM335x IOs if they have similar requirements.

    Your system must be designed to meet this requirement for all use cases/conditions.

    It has been several years since I designed circuit boards, but I was able to resolve this issue by using power supplies designed to track another supply.

    Regards,
    Paul