Arduino pro micro atmega32uf no aref pin?

Title says it all. I don't see an aref pin on here and I really need it. Is there a workaround to connect to pin 42(aref)?

Is it a Arduino Pro Micro from Sparkfun ? https://www.sparkfun.com/products/12640 Why would you buy something like that ?

The Aref, pin 42, is connected to a capacitor. You can check which one and maybe solder a wire to the capacitor.

Yes, it's one just like the sparkfun one. I need one as small as possible, as it's going in a tiny box. I'm going to check that out, thanks. I have a nano 3.0 as well that I may use. I appreciate the help.

Have a look at the Arduino Pro Mini on Ebay. Very small and cheap, I'm looking for one with Aref pin, but not found yet. Some have extra connecting points, but without label.

Why do you need Aref ? It is almost never needed.

I looked at pin 42 and it does go to a little cap. can't really see what's after that. I'm using an lm4040 voltage reference diode for an analog reference so I can get precise voltage from batteries. Everything else the pro micro can do is great for me. This, so far, is the only problem.

I've read that the internal 2.56v voltage reference only has accuracy to +/-5%. I need it to be more accurate. I'm googling info on soldering to the cap, and can't find any info. Tried different searches.

would you just solder a wire to the cap like it was a pin and set analogreference(external)?

Hi

One option is to use a sketch to enable the internal reference voltage and measure it from the aref pin then use that in your conversion. This should get you something much more accurate. Of course this may not be practical if you are making loads of your project but for one or a few it is a good way of working around the 5% tolerance.

Regards

Phil

Thanks for the replies. :) I do need something more accurate than 5%, hence the lm4040-4.1. What do you think? Looks like the little cap on aref pin in just a filter. I can solder to it just fine on the correct side and use it, I believe.

Okay, making a precision volt meter is one of the situations that require a voltage reference to Aref. As far as I can tell, the Aref is going to the capacitor and that's all. You can solder a wire to the capacitor and maybe put some glue over it. For example hot melt glue, or flux spray. Don't use just a glue, because that will harm more in a few years.

The datasheet writes: Tolerates Capacitive Loads That's nice, the 100nF will remove noise and make the reference work better for the Arduino.

They cost 5 dollars on Ebay =(

What if you solder the reference on top or next to that capacitor ? and add a resistor to 5V ? So it would be like a modification to the board.

You mean the lm4040 is $5? I was planning on using the filter cap that's already there like you were saying. I have some other 100nf caps as well. I think I can solder a wire without glue or anything.

I'm reading the lm4040 datasheet, and how they come up with the resistor is a little over my head atm. Page 25-26. They have an example there with 909ohm resistor. Page 25 has the calculation for it.

Here's more info. I really want to understand all this..

http://www.skillbank.co.uk/arduino/measure.htm

Yes, inclusive shipping the LM4040 is 5 dollars on Ebay.

Page 25 includes a current for the load, IL. However the Arduino input current is so small, just forget it. I would have to look into the datasheet, to be sure how much it is.

That link to that website explains it nicely. The LM4040 needs a minimal current. They assume that the minimal voltage (when powered with the usb connector) is 4.4V. They also assume that with 4V at Aref, there is 0.128 mA leaking into the Arduino (I'm not sure about that). The voltage over the resistor is 4.4-4.096 Volt, that is 0.304V. The current through that resistor is the minium of 0.4mA through the reference plus 0.128mA leaking into the Arduino. That results into 0.304 / 0.528E-3 is a resistor of 560 ohm.

Would that 560 ohm cause to much current in some conditions ? No, 2.16 mA is not too much.

When you assume that the minimal voltage is 4.5V, you get a slightly different value.

I think I just needed to take a break. I get it now. Thanks so much for the explanation. I scored a couple of these. Hopefully will be here tomorrow. :)

http://www.ebay.com/itm/310943494516?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

My voltage will be between 6.4-8.4v

So if I have this right to find R

R =6.4-4.096/0.528=4.3k

Max current will be

8.4-4.096/4.3=1ma

Look correct?

That's cheap, but they don't ship to Europe. There are many other voltage references on Ebay, but I'm not impressed with 1% initial accuracy. I bought a LT-1019 2.5V for 7 dollars. I like those numbers in the datasheet a lot more.

Yes, calculation is correct. I would use 3k9 or 3k3 or even 2k7 resistor. Why is that voltage 6.4 to 8.4 V ? I would use the 5V from the Arduino board.

Honestly, I'm not 100% on how things will end up working out with this project... If I use 3s lipo @ 11.1v, I can use a lm7805 voltage regulator or to power the arduino. That would work, I believe. Right now I'm using two 18650 lion 3.7v batteries in series and with the load I'm putting on them they will definitely get under the 7v required for the 7805(I believe it's 7v). Fully charged = 8.4v or so and I don't want to discharge them lower than 6.4v.

If I used the regulator with the 3s lipo(or 18650lion), I may be able to get away with using 5v lm7805 as a reference. I'm going to sleep on this tonight. This is a project for fun so trying to keep the cost down. Already have the 18650's. I have enough to put 3 in series, so maybe I can test with that. I already have the 7805 regulators. I see the datasheet says 4.8-5.2v, so that may not work... I'll have to test it with my setup.

1% accuracy would do me just fine with this project.

Thanks again for all you help. I pressed + karma, but it only let me do it once....

After reading more, I see this just won’t work for the accuracy I need… Will wait for lm4040’s to arrive.

The LM7805 isn't a good choice for a voltage reference for two main reasons, One is that Accurate ones are scarce. Production starts IIRC @ +/- 5%. The second is the quiescent current or no load current is about 10 mA. Good accurate devices such as the Analog Devices part or an equivalentcan be found in several places with prices comparable to Fleabay and one to 3 day shipping.. Microchip sells several devices that should work well and at relatively low cost in 5 piece quantities, I think singles are available but I can't remember for sure and TI will sample as well as will National Semiconductor in 1 to 3 quantities for trivial parts. As I remember I paid about $0.55 ea for 5 TO-89 MCP1702's and about the same for 5 TO-92 MCP 2703's in addition they are available in 50 mV output voltage increments from ~1V to 5V and at current levels to 250 mA (TO-89) and with voltage inputs from Vout + ~.3V to 16V max. Although this is not a reference device, Per Se, it is an example of what can be found easily and inexpensively including really accurate parts too, Should you require them. Finally several of the companies offering liberal samples also offer free or relatively inexpensive shipping...

Doc