LVCMOS totem-pole output needs pull-up/down resistor for what?

Discussion in 'General Electronics Chat' started by erasmooth, Aug 5, 2013.

  1. erasmooth

    Thread Starter New Member

    Aug 5, 2013
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    In which case LVCMOS totem-pole output needs (internal or external) pull-up/down resistor?

    Usually, totem-pole type I/O has (internal or external) pull-up/down resistor only when it's an input pin. But, I know for some time it has pull-up/down resistor even when it's an output pin.

    Why is that?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
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    What part number?
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    If you look closely at the schematic for a CMOS output you will see that actual resistors are uncommon. What you will see is a FET configured as a resistor. Is that what you are talking about? Give us a part number or a chip schematic.
     
  4. erasmooth

    Thread Starter New Member

    Aug 5, 2013
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    TI TMS320TCI6608 device. EMIF16 output pins. For example, EMIFOE_L (output enable) is Hi-Z output LVCMOS type. It has internal pull-up of 1uA. I think this LVCMOS output has the "active" pull-up but at default it's Hi-Z state, so internal pull-up is required. At run-time, the output has two pull-up at the same time, one is "active", the other is "passive." What kind of effect does the internal "passive" pull-up have?

    http://www.ti.com/lit/ds/sprs623d/sprs623d.pdf
     
    Last edited: Aug 6, 2013
  5. erasmooth

    Thread Starter New Member

    Aug 5, 2013
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  6. WBahn

    Moderator

    Mar 31, 2012
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    If it has the option to produce a Hi-Z output, then it is not a true totem pole output. The totem pole legs can be disabled (usually by means of series FETs in both the upper and lower legs) and so then you would need a pullup/pulldown in order to establish the state (or have the design such that exactly one output connected to the node is actively driving it at all times).
     
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