Pull-down resistor for negated pin...

[Franco]

Hi to everyone:

I was showing an electronic design to one friend and he told me I
should always use a "pull-down" resistor for permanently grounding a
negated pin in a chip (previously I had it connected to GND - 0V).

Can someone explain the reason of that? or some source of information?

Best Regards...

Franco.

 

[Joel Kolstad]

I was showing an electronic design to one friend and he told me I
should always use a "pull-down" resistor for permanently grounding a
negated pin in a chip (previously I had it connected to GND - 0V).

Can someone explain the reason of that? or some source of information?

The idea is usually just that it's a lot easier to undo if you change your
mind and need to use the pin for something after all.

For engineering prototypes, it's one of those things where it "can't hurt,
might help." For production boards, at least for high volumes, most people
would choose to save the money by tying to ground as you've done.

 

[Franco]

The idea is usually just that it's a lot easier to undo if you change your
mind and need to use the pin for something after all.

For engineering prototypes, it's one of those things where it "can't hurt,
might help." For production boards, at least for high volumes, most people
would choose to save the money by tying to ground as you've done.

Great and honest answer.

This pin is normally driven by a uC, so if I want to change the design
I will connect the uC which has an internal pull-up resistor in this
case.

I have a big problem with the space in this board (it is a 4 layer
25mm diameter circle!!!). So, even an 0603 SMD component is painful.

Best Regards...

Franco.

 

[Spehro Pefhany]

Great and honest answer.

This pin is normally driven by a uC, so if I want to change the design
I will connect the uC which has an internal pull-up resistor in this
case.

You may also be able to program the pin as an output and leave it
floating. If it cannot be programmed as an output, then there is
little reason to use a resistor.

I have a big problem with the space in this board (it is a 4 layer
25mm diameter circle!!!). So, even an 0603 SMD component is painful.

Best Regards...

Franco.

Some kind of RF or RFID gadget?

Best regards,
Spehro Pefhany

 

[Franco]

You may also be able to program the pin as an output and leave it
floating. If it cannot be programmed as an output, then there is
little reason to use a resistor.




Some kind of RF or RFID gadget?

Best regards,
Spehro Pefhany

Mmmm, no. It is a little controller board for a robotic application.

Thanks for the answers...

Franco.

 

[Tam/WB2TT]

Hi to everyone:

I was showing an electronic design to one friend and he told me I
should always use a "pull-down" resistor for permanently grounding a
negated pin in a chip (previously I had it connected to GND - 0V).

Can someone explain the reason of that? or some source of information?

Best Regards...

Franco.

An automated test machine will want to drive any pin either high or low. It
can't drive a pin that is connected to either gnd or vcc. 220 Ohms is a good
value for a pulldown. Same goes for unused functions; you will want these to
work.

Tam

 

[JosephKK]

Tam/WB2TT t-tammaru@c0mca$t.net posted to sci.electronics.design:

An automated test machine will want to drive any pin either high or
low. It can't drive a pin that is connected to either gnd or vcc.
220 Ohms is a good value for a pulldown. Same goes for unused
functions; you will want these to work.

Tam

Not really. I do not know of any that were not "fully programmable"
even 25+ years ago. They tested what they were told to. The
expensive complex ones, even then, had a modest computer (high range
pdp-11's running rsx-11) running the controls of the hardware and
logging the test results. And "bed of nails" tester configurations
were being abandoned in favor of early JTAG; the speeds were starting
to really climb and pin driver electronics and fixturing was getting
really difficult.

 

[Tam/WB2TT]

Tam/WB2TT t-tammaru@c0mca$t.net posted to sci.electronics.design:


Not really. I do not know of any that were not "fully programmable"
even 25+ years ago. They tested what they were told to. The
expensive complex ones, even then, had a modest computer (high range
pdp-11's running rsx-11) running the controls of the hardware and
logging the test results. And "bed of nails" tester configurations
were being abandoned in favor of early JTAG; the speeds were starting
to really climb and pin driver electronics and fixturing was getting
really difficult.

I agreee with you, but pin testing is where the rules concerning pullups and
pulldowns came from.

Tam