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TMS570LS3137: Data sheet does not list pull strengths for all pulled I/O

Tom McTaggart
Intellectual 345 points
Part Number: TMS570LS3137

Re the TMS570LS3137 data sheet: some I/O pins say that they are pulled but don't provide a value. For example MDCLK (an ouput). What does "Pull Up" mean for this pin. Another example is MII_TX_CLK (an I/O).

  • 0
    TI__Guru**** 196176 points

    Hello Tom,

    Those signals are pulled up or pulled down at reset.

    Pull-up and Pull-downs are normally used to ensure a signal has a defined state while not actively driven. They are used on inputs to prevent floating lines, rapidly switching between high and low and a middle "undefined" region. Outputs normally do not need them. Most GIO pins are defined as inputs instead of outputs at startup.

    Another normal output setup that uses pull-ups is open-drain or open collector connections. They only drive a connection low, or release the line. The pull-up is used to bring the line to a high logic state.

    Regards,

    QJ

  • 0 in reply to QJ Wang
    TI__Guru 64420 points
    Tom,
    To add to QJ's comments, the TMS570LS3137 datasheet, in Table 4-1 provides an indication if a pull up or pull down function is available for a specific pin. Furthermore, it details the default configuration upon assertion of reset and immediately after the deassertion of reset with the "RESET PULL STATE" column. The "PULL TYPE" column indicates if this is configurable (ie. pull up or pull down) through configuration registers and also the pull strength.
    As an example for the signal you mentioned, MDCLK, the default state is a pull up with strength of 20uA.
    For the other example you mentioned, the MII_TX_CLK has a default state of a pull down with strength of 20uA.