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PUDC_B pin

Former Member

Hello everyone,

 

According to Zynq TRM, the PUDC_B pin must not be left floating. I agree with that but I am understanding that it is for a design in which only the Zynq chip is used. Am I rigth?

In case of a custom carrier card for inserting a Microzed board, is there any problem to drive this specific pin in the microheader as an input signal? I have read that one must be careful but, in what way? I mean, all the commercial carriers have a specific circuit with a jumper to select the PUDC_B status, however, the Microzed already has its own circuit to pull this signal up or down, so what is the need to drive this signal on both boards instead of using this pin as an additional external signal as I have in mind?

 

Thank you

 

Víctor

  • 0

    Victor,

    You will find this signal on every Xilinx chip that has PL (programmable logic).  All of the series 7 have that pin.  That pin is used to set the default boot settings of the PL pins.

    So to answer your question, no, you are not right, you will find that pin on any chip with PL.
    Please see this AR for more information:
    http://www.xilinx.com/support/answers/50802.html

    You can use this pin as an input, however there are complexities involved in this.  You need to take care to not drive the pin prior to the bank being powered, you also will need to ensure that the circuit driving the pin does not change the pull-up or down the pin is set to.
    Please see UG470 for more information regarding this pin:
    http://www.xilinx.com/support/documentation/user_guides/ug470_7Series_Config.pdf
    Take a look at Table 2-4 (document page 27).  You will see warnings and the complexities to use this pin listed in the description.

    There is no need to drive this pin on both boards.  The pin only needs to be set at boot.

    --Dan

  • 0
    Hello Dan,
     
    Thank you for your reply.  
     
    Now I understand that this pin requires a stable state in any point of the net at boot, so definitely it is better not to 
    use it as an input in my design.
     
    But now the doubt is: by default, on MicroZed, PUDC_B is pulled high via a resistor (JT4) disabling the pull-ups during configuration and according to what is commented in the following post about the PUDC_B signal:
     
     
    It is indicated "If pulled high, then all pins will effectively be floating and external loading will dictate their state." then I would need an external signal to drive the I/O pins so that if I want them to be always low at boot, would I have to connect the PUDC_B pin in the Microheader to GND? Is it what is pretended for example, with the PUDC selection circuit in the IO carrier card?
     
    Regards
     
    Víctor
  • 0

    Victor,

    Per the AR listed above:
    "When PUDC_B is High, internal pull-up resistors are disabled on each SelectIO pin. The state of this pin effects the state of the I/O from power-on until configuration completes. Therefore, the I/Os will be 3-stated after power-on when PUDC is High."

    This means they are tri-stated.  They can be "set" easily by using external signaling.  Once configuration is complete, whatever you set the pins to in your bin file will dictate their state.  As such, tri-stated is the safer of the two options provided by the fabric for MOST cases.  In this case, it will be DRIVEN neither HIGH nor LOW.  It will be HIGH impedance with a VERY weak pull-up.  Once configuration is complete, the pin will be defined as whatever you set it to.

    If your circuit cannot accept the pins being tri-stated for a small period of time, and you need the signaling to be LOW, you will need to have something external to the signals pulling them low.  Only if you need the signals to be pulled high, would I suggest you modify the PUD_B signal.  Note that modifications to the board are done at your own risk, and I would suggest using a reputable Board House or In-House talent.

    For this, typically your circuit has a reset or similar signal, pulling that ONE signal to the proper level is typically all that is needed as data lines / etc. will not matter when the entire circuit is being held in reset.

    Typically you want signaling to be high impedance or pulled HIGH, as are the two choices provided.  Most reset signals are active low due to the use of open-collectors as well as the signal integrity provided through DRIVING a reset signal.  You gain noise immunity in both the hard DRIVE and the "short" to ground using an open-collector.  You also gain a resistance to capacitive effects due to the drive and shorting.

    --Dan

  • 0 in reply to drozwood90

    Hello Dan,

     

    Thank you for your reply and your detailed description.

     

    Definitely, I am going to dedicate this pin for its specific function. Would the best solution be to leave this pin floating in the Microheader, making use of the by-default very weak pull-up in the MicroZed, avoiding in this way any additional external circuit?

     

    Víctor

  • 0

    Hi there,

    If you check on page 6 of the schematic, it is noted to be a 1K pull up to the bank voltage. 

    If you do not need to change the Power Up Default State, or utilize that pin, the MicroZed is setup to boot properly with or without a board.  As such, you can just NO connect that pin on your carrier, that is if you do not need to use that IO.

    --Dan