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Active pull-up circuit? MCP42010 — Parallax Forums

Active pull-up circuit? MCP42010

W9GFOW9GFO Posts: 4,010
edited 2010-05-12 20:27 in General Discussion
What is an "active pull-up circuit" and how could it be floating if it is actively pulled up?

This is for the Reset and Shutdown pins on a MCP42010 digital pot. Since I'm not supposed to leave them floating can I just connect them directly to VDD or is a resistor necessary? The reset or shutdown occurs when they are pulled low, and I do not need that to happen.
MicroChip said...
In order to minimize power consumption, this pin
has an active pull-up circuit. The performance of this
circuit is shown in Figure 2-12. This pin will draw negligible
current at logic level ‘0’ and logic level ‘1’. Do not
leave this pin floating.

Rich H

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
The Simple Servo Tester, a kit from Gadget Gangster.

Comments

  • kwinnkwinn Posts: 8,697
    edited 2010-05-12 16:30
    An active pull-up is a transistor or other component that pulls the pin up to Vcc when turned on. When it is not turned on the pin is floating.

    The external signal that controls the chip needs to hold these pins either high or low to minimize power consumption. If that external circuit is always active when power is on then no pullup or other connection is required.

    Post Edited (kwinn) : 5/12/2010 4:42:20 PM GMT
  • W9GFOW9GFO Posts: 4,010
    edited 2010-05-12 18:50
    I almost understand, so is the pin actually the base of a transistor?

    Would I need a resistor on these pins or is it okay to connect them directly to 5V?

    Rich H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    The Simple Servo Tester, a kit from Gadget Gangster.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-05-12 19:35
    The input is voltage sensitive, not current sensitive, so a direct connection to Vdd should be fine.

    -Phil
  • W9GFOW9GFO Posts: 4,010
    edited 2010-05-12 19:46
    Phil Pilgrim (PhiPi) said...
    The input is voltage sensitive, not current sensitive, so a direct connection to Vdd should be fine.

    -Phil

    Okay good. I wanted to make sure because I understand that current limiting resistors are mandatory for the base of a transistor.

    Thanks,

    Rich H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    The Simple Servo Tester, a kit from Gadget Gangster.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-05-12 20:27
    It's a CMOS device, which means its transistors are all MOSFETs. So "base" current is not an issue.

    -Phil
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