# ATmega 328 pull up resistor current

Discussion in 'The Projects Forum' started by sirch2, Aug 28, 2013.

1. ### sirch2 Thread Starter Well-Known Member

Jan 21, 2013
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Hi,

Just want to check my understanding. I am putting together a low power battery powered project and want to wake the ATmega from sleep when an interrupt goes low. So there will be a pull up resistor to hold the pin high and it will be pulled low by a reed switch.

Am I correct in thinking that the current draw through the ATmega is very low on a pin that is held high and that the pull-up resistor (commonly 10k) is there to limit the current when the pin is pulled low?

Oct 15, 2009
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3. ### #12 Expert

Nov 30, 2010
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9,616
I think that means, "yes".

4. ### sirch2 Thread Starter Well-Known Member

Jan 21, 2013
1,019
352
Thanks, I'll take that as a yes then.

I know what a pull up resistor does, what I want to achieve is absolute minium current through a pull-up resistor and Atmega328 pin combintation. I'll atempt to measure it, although I doubt my multimeter is accurate at low currents.

5. ### #12 Expert

Nov 30, 2010
18,078
9,616
If you set your volt meter to its lowest DC voltage scale and put it in series with the load, you can multiply the voltage reading by the resistance of the meter to get current.

4 instance: If the voltmeter is specified as having 1 million ohms of resistance and the reading is 1.2 volts, the current is 1.2Volts/1Million ohms.
1.2/1e6 = 1.2 microamps.

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6. ### sirch2 Thread Starter Well-Known Member

Jan 21, 2013
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Thanks for that, a really useful tip and one that seems so obvious when someone points it out

7. ### DickCappels Moderator

Aug 21, 2008
4,863
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There are internal high resistance pull-up devices on the chip. I suspect you have to set some bits somewhere in some register so that the input pins you plan to use have the pull-ups active. At least that's how it works on other AVR controllers. I have not studied that *datasheet, but you should since you are using it.

*With the current crop of controllers, we should probably refer to "data books" since the datasheets can run into several hundred pages.

8. ### sirch2 Thread Starter Well-Known Member

Jan 21, 2013
1,019
352
Yes, it certainly is a book, I tried to wade through it but didn't seem to give the expect current flow through a high input

9. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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You meant divide, correct?