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[SOLVED] pull up resistor is necessary or not ?

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Haier

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pull up.jpg
Normally we pull the ENABLE to VCC through a resistor, instead of directly connecting to VCC as the pic shows.
I wonder the benifis of this resistor. Anyone could help me?

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Hi,

The same discussion was recently about pulldown resistors. Maybe one week ago.

Klaus
 
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    Haier

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Pull up and pull down resistors are to avoid high impedance reading on the pin and force it to be either high or low all the time. Some chips comes with built in pull up/down resistor hence why no external resistors required. You have to read the datasheet of the component to confirm if it has internal pull up/down resistors.
 
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Pull-up or pull-down resistors also act as voltage dividers and reduce noise by the same ratio. If the pin is internally pulled down you need an external pull up etc etc. If the input is high impedance, you must connect a pull-up or pull down...
 

Many devices can tolerate a limited pin current that would
send the pin beyond the rails, before instigating latchup.
This is specified on most modern ICs (see Abs Max ratings).
A resistor can provide authoritative current limiting and
still have a solid H/L on high impedance (when between
the rails) pins. When you connect directly to a "VDD"
global net, you are trusting the layout person to connect
right and tight to the chip pin and not some more remote
but routing-convenient branch, which in real time may be
jacked apart from the ideal VDD by switching impulses,
current loops and so on.

Do you need it in a perfect world? No. Do you live in a
perfect world? No. Would you like to be sure about the
absence of pin electrical induced latchup? Yes. So you
either spend the effort to check, and make sure any
board re-spin is checked in this detail again, or you
spend the pennies and square millimeters to make it
idiot proof (until somebody decides to be a cost hero
after your eye is off the ball, and undoes your work to
claw back that penny and little patch of FR-4).

Back in the day, there were whole families of 74xx logic
parts that would blow up if you tied their inputs directly
to VCC. Not necessarily immediately, but often and not
at convenient times. The 74LS series was free of this
and people where I worked at the time moved over to
it right quick.
 
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Agree with the above and remember that even if you think you've pulled the pin to the same rail as the chip power rail, momentary current surges on-chip can cause current to be pulled from an input pin. This can cause latching or over time degradation.
 
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dick_freebird said:
Many devices can tolerate a limited pin current that would
send the pin beyond the rails, before instigating latchup.
This is specified on most modern ICs (see Abs Max ratings).
A resistor can provide authoritative current limiting and
still have a solid H/L on high impedance (when between
the rails) pins. When you connect directly to a "VDD"
global net, you are trusting the layout person to connect
right and tight to the chip pin and not some more remote
but routing-convenient branch, which in real time may be
jacked apart from the ideal VDD by switching impulses,
current loops and so on.

Do you need it in a perfect world? No. Do you live in a
perfect world? No. Would you like to be sure about the
absence of pin electrical induced latchup? Yes. So you
either spend the effort to check, and make sure any
board re-spin is checked in this detail again, or you
spend the pennies and square millimeters to make it
idiot proof (until somebody decides to be a cost hero
after your eye is off the ball, and undoes your work to
claw back that penny and little patch of FR-4).

Back in the day, there were whole families of 74xx logic
parts that would blow up if you tied their inputs directly
to VCC. Not necessarily immediately, but often and not
at convenient times. The 74LS series was free of this
and people where I worked at the time moved over to
it right quick.
Click to expand...

Wow, wonderfull explanation! really appreciate that.
 

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