Maker Pro
Maker Pro

pullup AND pulldown on a chip input

S

samiam

Jan 1, 1970
0
I just looked at a schematic I pulled off the net and I noticed that the
Designer has a pullup resistor (to vcc) and a pulldown resistor (to
ground) on the chip input.

Exactly why is this done this way?

I am used to seeing either one or the other.
pullup for normally high
or
pulldown for normally low

Forgive me, I am a software guy who takes an interest in such things. Thanks
 
J

Jim Thompson

Jan 1, 1970
0
I just looked at a schematic I pulled off the net and I noticed that the
Designer has a pullup resistor (to vcc) and a pulldown resistor (to
ground) on the chip input.

Exactly why is this done this way?

I am used to seeing either one or the other.
pullup for normally high
or
pulldown for normally low

Forgive me, I am a software guy who takes an interest in such things. Thanks

It's called CYA... after the thing is built clip off the wrong one ;-)

...Jim Thompson
 
A

Anthony Fremont

Jan 1, 1970
0
"samiam"
I just looked at a schematic I pulled off the net and I noticed that the
Designer has a pullup resistor (to vcc) and a pulldown resistor (to
ground) on the chip input.

Exactly why is this done this way?

I am used to seeing either one or the other.
pullup for normally high
or
pulldown for normally low

Forgive me, I am a software guy who takes an interest in such things.
Thanks

What kind of chip? If it was an op-amp, then the resistors act as a voltage
divider to set the input bias. Not sure why everyone here is assuming it to
be a digital input. :-?
 
J

Jim Thompson

Jan 1, 1970
0
Oh I misread the request, sorry. I see on the same pin. Yes most likely
that is what it is. Although it could also be termination. Was this a
clock line. The two add up to make an impedance. If this is the case
the values are usually odd ball and pull up and down are different values.

Also - depending on what it drives the default state they may have
wanted was in the middle. Not many real world uses for this, but there
are some.

Hawker

Good point. I've devised several chips now where one pin gives a
select-one-of-three function...

Tie High = Mode A
Tie Low = Mode B
Float = Mode C

...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
I just looked at a schematic I pulled off the net and I noticed that the
Designer has a pullup resistor (to vcc) and a pulldown resistor (to
ground) on the chip input.

Exactly why is this done this way?

I am used to seeing either one or the other.
pullup for normally high
or
pulldown for normally low

Forgive me, I am a software guy who takes an interest in such things. Thanks

This is a "Thevenin termination". Using two resistors lets you nail
both the pullup voltage and the line termination impedance at any
values you like. The VME bus, among many others, uses this sort of
termination.

Typical practice might be to have the high level be twice the nominal
logic threshold or so, about 3 volts for VME, and the impedance
whatever terminates best.

John
 
S

samiam

Jan 1, 1970
0
What kind of chip?

a buffer/quickswitch

If it was an op-amp, then the resistors act as a voltage divider to set the input bias. Not sure why
everyone here is assuming it to be a digital input. :-?

:)
I think I am partly to blame for that. I mentioned that I am a software
guy by trade. My interest in this is purely with digital circuits.
 
R

Richard Henry

Jan 1, 1970
0
There are three main reasons why pull up and pull down resistors are
used that I can think of off the top of my head. They are

1) To put a pin in a default state while items power up. For example you
may want a processor to bring a pin high and another pin low on start up
but for a few ms while a chip initializes the outputs may be inputs or
high Z and not assert this. It prevents pin contention/latch up and
other issues. For example you may want an output enable pulled high
(off) and a external power supply pulled down (off).

2) Stiff (under 2.21k) Pull ups are used to increase speed transitions
at times. This is especially true with opto isolators and OC outputs
(see below)

3) Pull ups are used for open collector outputs for the VCC reference
since by nature open collector cutouts have no voltage on them.

Hawker






- Show quoted text -

2.2k used to be my favorite pullup resistor -- red-red-red.
 
H

Hawker

Jan 1, 1970
0
There are three main reasons why pull up and pull down resistors are
used that I can think of off the top of my head. They are

1) To put a pin in a default state while items power up. For example you
may want a processor to bring a pin high and another pin low on start up
but for a few ms while a chip initializes the outputs may be inputs or
high Z and not assert this. It prevents pin contention/latch up and
other issues. For example you may want an output enable pulled high
(off) and a external power supply pulled down (off).

2) Stiff (under 2.21k) Pull ups are used to increase speed transitions
at times. This is especially true with opto isolators and OC outputs
(see below)

3) Pull ups are used for open collector outputs for the VCC reference
since by nature open collector cutouts have no voltage on them.

Hawker
 
H

Hawker

Jan 1, 1970
0
It's called CYA... after the thing is built clip off the wrong one ;-)

...Jim Thompson

Oh I misread the request, sorry. I see on the same pin. Yes most likely
that is what it is. Although it could also be termination. Was this a
clock line. The two add up to make an impedance. If this is the case
the values are usually odd ball and pull up and down are different values.

Also - depending on what it drives the default state they may have
wanted was in the middle. Not many real world uses for this, but there
are some.

Hawker
 
R

Rich Grise

Jan 1, 1970
0
[Top-Post Repaired]
On Mar 27, 1:24 pm, Hawker
2.2k used to be my favorite pullup resistor -- red-red-red.

I used to use 2.4K routinely, since the time a bag of about 1,000 of them
fell out of the sky onto my desk. ;-)

Cheers!
Rich
 
R

Rich Grise

Jan 1, 1970
0
I just looked at a schematic I pulled off the net and I noticed that the
Designer has a pullup resistor (to vcc) and a pulldown resistor (to
ground) on the chip input.

Exactly why is this done this way?

I am used to seeing either one or the other.
pullup for normally high
or
pulldown for normally low

Forgive me, I am a software guy who takes an interest in such things. Thanks

What are the resistor values, and what chips are on either end of the
line?

Thanks,
Rich
 
B

Ban

Jan 1, 1970
0
John said:
This is a "Thevenin termination". Using two resistors lets you nail
both the pullup voltage and the line termination impedance at any
values you like. The VME bus, among many others, uses this sort of
termination.

Typical practice might be to have the high level be twice the nominal
logic threshold or so, about 3 volts for VME, and the impedance
whatever terminates best.

John

Yep, it is a termination for bus-lines used mostly on backplanes. The signal
sees the parallel resistor value at the end of the line and doesn't get
reflected. Whereas the driving current is only half of what it would be with
a single resistor to gnd or Vcc.
It makes only sense for CMOS, since these parts can source or sink the same
current.
A better way would be to create a middle voltage with a switching supply and
terminate all lines with a single resistor to this. Now the dissipation is
much lower, since we avoid the quiescent current in the dividers. There are
ICs designed for this purpose.
Some things have to be observed though. When no driver is active the middle
voltage might cause excessive current in an activated input being in the
linear region, so you have to stay above or below a bit. This will also give
a defined output in this condition, all lines are low (or high), easily
recognized by software.
 
J

jasen

Jan 1, 1970
0
Good point. I've devised several chips now where one pin gives a
select-one-of-three function...

Tie High = Mode A
Tie Low = Mode B
Float = Mode C

eg MC140526 / SC41342 ?

Bye.
Jasen
 
J

Jim Thompson

Jan 1, 1970
0
eg MC140526 / SC41342 ?

Bye.
Jasen

Nope. The only standard product I've designed since 1970 have been
ECL/PECL or LVDS. The parts with the three-way select were custom
ASIC's, WiFi Repeater, GPS, etc. I've also used the scheme for
production test... function not seen by ultimate user.

...Jim Thompson
 
Top