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Topic: mixing 3.3V and 5V in a very tiny space (Read 1 time) previous topic - next topic

davidb

Seems to be my week for asking questions about power!
After reading this thread about running the 5V/16Mhz Pro Mini @ 3.3V:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1236970695
I've tried it myself and it seems to be working OK...but as those in the thread who have tried to essentially "overclock" their 16Mhz mini (this is the 328 chip, FYI)...all is good at room temperature. Well, we've had some hot days here in the valleys of Los Angeles and my data isn't looking so good. Could be other issues, but I'm thinking it's time to revisit the overclocking issue. The only reason I need to have 3.3V is to power an xbee in the same system. There is also a compass in the system that can tolerate 3-6V.

So my two questions:
1. would there be an issue powering the minis and compass with 5V and the xbee with 3.3V in the same system (and assuming a common/shared ground)?

2. is there a simple, uber-compact method for converting 5V to 3.3V, or a way to have the mini provide 3.3V given a 5V source?

I know the easy answer would be to use a 3.3V/8Mhz Pro Mini, but I'm sitting on 20 of the 16Mhz for this project (in order to work with an earlier version of newsoftserial), and would rather not purchase any more for while!

Thanks for any advice!
Best,
David

knuckles904

#1
Jun 28, 2009, 10:09 pm Last Edit: Jun 28, 2009, 10:16 pm by knuckles904 Reason: 1
smallest thing ive seen is the sparkfun logic level converter. Not a bad price either at $2. Of course you'd need a power source too so add in a 3.3v reg and some decoupling caps, but that all should still fit in a relatively small space. Small can mean a lot of things tho.  ;)

(edit) link posted here: http://www.sparkfun.com/commerce/product_info.php?products_id=8745

EmilyJane

#2
Jun 28, 2009, 10:32 pm Last Edit: Jun 28, 2009, 10:36 pm by EmilyJane Reason: 1
Take a look at the schematic for the XBee shield that Sparkfun sells. It already addresses the issue.

http://www.arduino.cc/en/uploads/Main/XbeeShieldSchematic.pdf

Ran Talbott

There's also a compact adapter from Adafruit that includes both 3.3V regulator and level shifting.

Ran

EmilyJane

Even slicker. Just make sure that your 5V logic can accept 2.7V as a logic 1 input as neither circuit addresses that issue.

davidb

Hi all,
Thank you all so much for the replies! What it's highlighted is a deficiency in my electronics knowledge, however (and I must say I've had quite a whirlwind education over the past couple months!)! If I understand correctly now, simply having a 3.3V power source for the xbee and a 5V supply for the others will not do...I must also convert the serial (TTL) messages between the two!

The adafruit version seems ideal, but it's a bit too large some of my applications. Sparkfun also has one:
http://www.sparkfun.com/commerce/product_info.php?products_id=9132
however there seems to be quite a few answered questions wrt level shifting (whether it does it) and 150mA vs 250mA current threshold. Actually the other sparkfun BOB mentioned above seems lovely, if only it didn't need both power sources (the size is certainly right). @EmilyJane: slick is ideal! But the concern is also what the max the xbee can receive as logic 1 without creating smoke, which is what I am trying to figure out now.  Actually does anybody know what the minimum voltage the pro mini can accept as logic 1 input (r the max the xbee series 1 can handle)?

Anyway, thank you all for the advice. So much more to learn! And here I thought 12 years of college was enough :)

Cheers,
David

EmilyJane

#6
Jun 29, 2009, 08:49 am Last Edit: Jun 29, 2009, 09:02 am by EmilyJane Reason: 1
It looks like both protect the XBee from being over-driven from 5V logic. Depending on what is on the 5V side, it looks like neither meet minimum drive levels. The minimum guaranteed logic 1 output from the XBee is 0.82*Vcc or 2.7V.

The minimum logic 1 input for the Atmega168/328 is 0.6*Vcc.

If you operate the Arduino at 4.3 V, it will run at 16 MHz and the XBee will meet its Vih specs. :)

davidb

Hi all,
To sort of conclude the conversation here, I received a handful of the sparkfun devices:
http://www.sparkfun.com/commerce/product_info.php?products_id=9132

...and am a very happy camper. Now everything can exist in 5V land...and there was much rejoicing.

Best,
David

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