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Pull-ups and pull-downs for disabled peripheral

khaled saab
Genius 3155 points
Other Parts Discussed in Thread: TMS320C6472

In section 6 (page 26) of the TMS320C6472 Hardware Design Guide SPRAAQ4B states:

 

Each peripheral section includes recommendations on how to handle pins on interfaces that are disabled or for unused pins on interfaces that are enabled. Generally, if internal pull-ups or pull-downs are included, the pins can be left floating. Any pin that is output only can always be floated. If internal pull-ups and pull-downs are not included or disabled, pins can normally still be floated with no functional issues for the device. However, this may allow additional leakage current that can be eliminated if internal or external pull-ups or pull-downs are used. When pull-ups or pull-downs are used the leakage current is  approximately 100 μA per pin. When the pins are floating the leakage current can be several milliamps. Although the recommendations normally indicate using external pull-up resistors, pull-down resistors can also be used. The leakage is the same whether pull-ups or pull-downs are used. Connections directly to power or ground can be used only if the pins can be guaranteed to never be configured as outputs.

 

Four questions on the TMS320C6472:

  1. If a peripheral section is disabled and not used, and the pins have pull-ups or pull-downs, do the pull-ups or pull-downs still function?
  2. If a peripheral section is disabled, and the pins do not have pull-ups or pull-downs, is an external pull-up or pull-down still needed to minimize leakage current?
  3. It mentions pins can normally be floated. For a disabled peripheral section, are there any peripheral sections and pins that cannot be floated for reliable operation of the device?
  4. It also mentions connections directly to power or ground can be used only if the pins can be guaranteed to never be configured as outputs. Does this apply to pins of a disabled peripheral section?

Thanks

Khaled.

  • 0
    TI__Mastermind 43660 points
    Khaled,

    I do not have enough information on this. I will check with the team and update you.

    Regards,
    Senthil
  • 0
    TI__Mastermind 42545 points

    H Khaled,

    The IO cells in the C6472 are associated with IPs but not part of that IP. Generally, disabling the IP will not change the configuration associated with the pins.

    1) If a peripheral section is disabled and not used, and the pins have pull-ups or pull-downs, do the pull-ups or pull-downs still function?

    The pull-ups/pull-downs are part of the IO cell and not controlled by the peripheral section so they are still active.

    2) If a peripheral section is disabled, and the pins do not have pull-ups or pull-downs, is an external pull-up or pull-down still needed to minimize leakage current?

    The pulls are still active so no external pull is needed. Note that the use of internal pulls instead of external pulls is only recommended if the pin is unused and has no external trace connected to the pad. 

    3) It mentions pins can normally be floated. For a disabled peripheral section, are there any peripheral sections and pins that cannot be floated for reliable operation of the device?

    This is a broad question. Can you be more specific about the pins you are concerned about?

    4) It also mentions connections directly to power or ground can be used only if the pins can be guaranteed to never be configured as outputs. Does this apply to pins of a disabled peripheral section?

    Yes! Since the configuration of the IO cells are not controlled by the peripheral, any pin that can possibly be set as an output should not be connected directly to power or ground. Note that many of the IO cells in the C6472 are IO buffers, even when they are used exclusively as inputs. 

    Regards,

    Bill

  • 0 in reply to Bill Taboada
    Genius 3155 points

    Hi Bill,

    I'm not sure that I understood your reply to the question 1:

    Khaled Question: If a peripheral section is disabled and not used, and the pins have pull-ups or pull-downs, do the pull-ups or pull-downs still function?

    Bill reply: The pull-ups/pull-downs are part of the IO cell and not controlled by the peripheral section so they are still active.

    Khaled Question: Do you mean that the pins are pulled-up or down by default at power up?

    As for the 3th question:

    The interfaces that are of interest are: the Ethernet, SRIO, and DDR interfaces. All of those interfaces don't have pull-ups or pull-down.

    1) If the peripherals are disabled and not used, can we leave those pin floating?

    2) If the peripherals are disabled and not used, and we leave them floating, will they draw excess current?

    Khaled.

  • 0 in reply to khaled saab
    TI__Mastermind 42545 points

    Hi Khaled,

    Do you mean that the pins are pulled-up or down by default at power up?

    In the terminal functions table, the pulling resistor for each pin is defined as either IPU (pull-up), IPD (pull-down) or blank (no pull). Once the part has been released from reset, those pulls will be active regardless of whether the associated peripheral section is active.

    The interfaces that are of interest are: the Ethernet, SRIO, and DDR interfaces. All of those interfaces don't have pull-ups or pull-down.

    1) If the peripherals are disabled and not used, can we leave those pin floating?

    2) If the peripherals are disabled and not used, and we leave them floating, will they draw excess current?

    Each of these interfaces uses a different supply voltage and the data manual includes information on how to connect the supply voltages for unused peripherals. Check the description of the supply rail in table 2-5, Terminal Functions. For example the DDR EMIF supply rail is CVDD1. The description on page 45 explains how to handle this rail if the DDR is not used. Additional information on how to configure these interfaces when they are not used can be found in the appropriate section of the Hardware Design Guide. 

    Regards,

    Bill

  • 0 in reply to Bill Taboada
    Genius 3155 points

    Hi Bill,

    I'm starting the get the feel for it :-). For DDR2 the documentation is clear. For SRIO and Ethernet it is less clear. Could you please point out the specific documentation page or please advise as to whether the SRIO and Ethernet pins (additional pins, not supply voltage pins) can be left floating when these two peripherals are disabled.


    Thanks.

    Khaled.

  • 0 in reply to khaled saab
    TI__Mastermind 42545 points

    Hi Khaled,

    If you follow the instructions for the supply voltage pins, you will remove the power from the IO cells associated with the Ethernet and SRIO. Since the IO cells have no power, they can be left floating. I'm sorry that wasn't clear from the documentation. 

    Regards,

    Bill