1. Home
  2. Hobby Electronics Basics
  3. 3-State CMOS Driver

3-State CMOS Driver

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

Reply to
Abstract Dissonance
Loading thread data ...

"Abstract Dissonance"

enough

or the

will

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.

"Steal"

on the

I

does.

flashing but

parallel

to the

means the

to some

though ;/

IC's.

Will I

by

If you feel a shock or hear a snap, it's probably all over but the crying.

and

didn't

time).

What part of the country do you live in? How humid is your environment?

another IC

that I

but I

easy

Reply to
Anthony Fremont

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

Reply to
Abstract Dissonance

ic

pic

will

positive

PIC to

was

line.

here to

running

I take it that the LED is connected between a PIC pin and ground.

only

the

into

into

Sounds likely.

the

the

it

parallel

drops

the IC's

is that

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.

Reply to
Anthony Fremont

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

Reply to
Rich Grise

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.