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Author Topic: Pull-up or direct connect?  (Read 2928 times)

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Offline GizmoTopic starter

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Pull-up or direct connect?
« on: October 25, 2014, 08:19:22 am »
I've got a chip and I want to just connect logic high to a pin, is there any issue with just connecting directly to Vcc, or is a current limiting resistor a good idea?

In this particular case I have some 245 bus transceivers and I'm setting the direction of each permanently by either connecting DIR to GND or Vcc. This all works fine, but I was wondering if this will increase current draw or potentially damage the IC by not having a resistor in-line?

Thanks!
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Offline con-f-use

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Re: Pull-up or direct connect?
« Reply #1 on: October 25, 2014, 08:48:36 am »
The datasheet should tell you the input impedance and maximal current rating of the input pins. Based on that you can calculate whether you need an input resistor. Also ESD is somethimes a consideration if the pins are exposed and someone might touch them.
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Offline TonyStewart

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Re: Pull-up or direct connect?
« Reply #2 on: October 25, 2014, 08:53:56 am »
Test Engineers frown on direct connects to Gnd or Vcc because they lose the ability to test if that input is faulty which may be an indication other parts in use on that chip are almost faulty.  So a single pullup for all unused inputs or connected to an unused output, which can be be back-driven by ATE.


As I recall it was only TTL that demanded a shared pullup resistor for current limiting on overvoltage on the supply to the input.
« Last Edit: October 28, 2014, 10:02:38 am by TonyStewart »
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Offline GizmoTopic starter

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Re: Pull-up or direct connect?
« Reply #3 on: October 25, 2014, 09:02:59 am »
Hmm, I've had a look at the datasheet but don't really know what I'm looking for. Would it be "Control inputs" listed as 1uA max? In which case that's all good.

I'm less bothered about testing as I know it's all setup correct and it's a dead simple board (only 7 IC's, most of which 245's). I'm more interested in unnecessary current draw as this is battery powered.  :D
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Offline Tandy

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Re: Pull-up or direct connect?
« Reply #4 on: October 25, 2014, 12:04:17 pm »
Using a resistor or directly connecting will not make any difference to the current used. Holding an input high or low essentially uses no current, a pull-up or pull-down resistor only consumes current while they are being overridden to change state.
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Offline GizmoTopic starter

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Re: Pull-up or direct connect?
« Reply #5 on: October 26, 2014, 06:25:55 pm »
Excellent, thank you for the response!
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Offline dannyf

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Re: Pull-up or direct connect?
« Reply #6 on: October 26, 2014, 11:13:45 pm »
Quote
not having a resistor in-line?

No need to have serial resistors if you think the mcu will never encounter unexpected conditions.
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