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detect ps/2 mouse attached to a microcontroller

Discussion in 'Electronic Basics' started by newssvr, Nov 21, 2003.

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  1. newssvr

    newssvr Guest


    I am making a device that uses a (st9) microcontroller and i have the clock
    and the data lines attached to some i/0 lines on the microcontroller. The
    other two lines of a ps/2 mouse are thepower and ground lines and they are
    hooked to the same 5v power source as my microcontroller.
    The clock and data lines have weak pullups on them as per the way ps/2 mice
    and keyboards should be interfaced, so these lines are normally high unless
    me or the mouse decides to bring them low in the course of generating

    My question is this: how can detect if the mouse is attached to my
    microcontroller since the user could unplug or plug it in at any time?

    Since the clock and data lines have pull ups they wont go low when the user
    unplugs the mouse.

    Should i do it by detecting the current the mouse draws when it plugged in?
    I basically want to make this thing plug and play. But i am guess that
    since personal computers dont do this after all these years, that is simply
    is not doable.

    I am a bit of a newbie when it comes to elecronics to i might be missing
    something obvious,

    Thanks! ~ Mark
  2. Luhan Monat

    Luhan Monat Guest


    One way to detect power draw is by using a PNP transistor as follows:

    E - connect to +5 power
    B - connect to mouse power feed
    C - 10k to ground.

    When the mouse is plugged in, the transistor is turned on. The voltage
    at the collector goes to near 5 volts.

    If there is any leakage in the transistor, you may get some voltage at
    the collector without the mouse plugged in. In that case, try a
    resistor from the Base to Emitter (1k - 100k) that keeps the transistor
    turned off unless the mouse is plugged in.
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