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3-State CMOS Driver

  • Thread starter Abstract Dissonance
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A

Abstract Dissonance

Jan 1, 1970
0
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
 
A

Anthony Fremont

Jan 1, 1970
0
"Abstract Dissonance"
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).

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.
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?

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

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

Abstract Dissonance

Jan 1, 1970
0
Anthony Fremont said:
"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.

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
 
A

Anthony Fremont

Jan 1, 1970
0
Abstract Dissonance said:
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)...

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

Sounds likely.
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?

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.
 
R

Rich Grise

Jan 1, 1970
0
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.

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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