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3-State CMOS Driver

Discussion in 'Electronic Basics' started by Abstract Dissonance, Mar 8, 2006.

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  1. I have a74HCT125N and I'm using it to program a pic.... strangely enough
    though when there is no external power(VCC from PS) applied to the ic or the
    pic(yet the pic happens to be "hooked" up on the breadboard) the pic will
    still run(when used to blink an LED).

    I was reading AOE and it said that output ports on CMOS IC's could "Steal"
    power from the inputs... is this what is happening(remember, no VCC is
    applied to the PIC or or the IC from the external power supply).

    Basicaly, the way its setup is that the output of one of the channels on the
    125 drives a transistor that is hooked up to the VCC of the pic... but I
    disconnected the collector VCC so the pic shouldn't run... but it does.
    When I remove the ground to the inputs on the 125 the LED stops flashing but
    the parallel port is the only source of power but there is no direct
    connection from the parallel port to the pic(all the outputs of the parallel
    port go into the 125 except, ofcourse, the ground which is connected to the
    ground of everything else).

    Now, I assume what AOE mentions is true but it doesn't say if that means the
    IC is bad or not.... Because the IC does seem to work like it should to some
    degree(I can program a 16F688 fine but I'm having problems getting the
    18F4431 to work.... could be from the IC or just something else).

    Do I need to replace the IC? (It happens to be the only one I have though ;/
    and it seems to work fine for programming the 16F688).

    Another question I'm curious about is static electricity when handling IC's.
    How much is to much and how likely is one to screw up a component? Will I
    know by the "shock"(like when you touch some metal and you get shocked by
    it) that it will have screwed up the component?

    The reason I ask if I've been sorting a lot of IC's from a grab bag and
    didn't really care to much about protecting them from ESD since I didn't
    feel I had any problem with it(I haven't been shocked in a very long time).

    Also, if, say, I do ruin an IC by ESD then if I immediately touch another IC
    what are, about, the chances of ruining it too? I'm slightly worried that I
    might have ruined some components since strange things are happening but I
    have no idea if its from something like that or something else. Any easy
    way to test an IC with a DMM to see if its ok?

    Thanks,
    Jon
     
  2. "Abstract Dissonance"
    You probably have one of the PIC i/o pins connected (directly or
    indirectly such as thru an LED) to a source of power. This power will
    flow thru the internal protection diode of the PIC and onto the positive
    supply rail. This is usually more than enough power to cause the PIC to
    start.
    If you feel a shock or hear a snap, it's probably all over but the
    crying.
    time).

    What part of the country do you live in? How humid is your environment?
     
  3. I only had 2 pins on the PIC connected. One was VSS and the other was
    driving the LED connected through a 10k resistor... But since the LED was
    flashing and acts as a diode then the power could not come from this line.
    The VCC was also connected to the VCC of the IC but this VCC was not
    powered(i.e., there was no other connection... I would hook up a 5V here to
    run both but was not connected unless I was programming the PIC or running
    it as the LED flasher)...

    But since the only power source was coming from the parallel port and only
    the parallel port was connect to the pins of the IC(but not the VCC of the
    IC) somehow the power was getting through the input pins of the buffer into
    the PIC.... which I guess it was going through the VCC of the IC then into
    the PIC. Similar to what you said but not with the PIC but the IC.

    I just measured the voltage on the VCC of hte IC and its about 2.6V
    When it is not connected to anything and 2.1V when it is connected to the
    power line(the line I use as common 5V ) with the PS 5V connected(but the
    PSU is "off")... with the PS disconnected completely from the circuit it
    reads about 3.8V. (I assume the PS is sinking some current from the parallel
    port through its reverse current protection diodes and thats why it drops
    some)

    In any case it seems the parallel port is supplying current through the IC's
    VCC for some reason. My question is, Does this mean the IC is bad or is that
    normally how CMOS IC's work?

    Jon
     
  4. I take it that the LED is connected between a PIC pin and ground.
    Sounds likely.
    I think that it's fairly common for CMOS parts to have "protection"
    diodes on i/o pins. So any time that the voltage on a pin rises above
    Vcc (like when juice is applied to the pin, but not the IC's Vcc pin)
    the diode turns on and conducts to the Vcc rail and sometimes powering
    on other devices. The same thing happens when you bring an input below
    ground, another diode turns on and allows current to flow out the pin.
    The PIC datasheets show these diodes fairly clearly. Running current
    thru the protection diodes is not considered good "care and feeding" for
    your parts, but that doesn't necessarily mean that your parts are
    ruined.
     
  5. Rich Grise

    Rich Grise Guest

    The output of the 125 is high; this pulls up Vcc of the PIC through the
    B-E junction. - i.e., your PIC is being powered by the 125 output.

    Cheers!
    Rich
     
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