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Idiot guide for 3.3v conversion?

Posted: Sat Jan 09, 2010 8:38 pm
by devlinse
Hi there

There are a couple of threads on here about running a RBBB at 3.3v but they diverge a little from a straightforward howto. Is it simply the case that you can change the voltage regulator for one that outputs 3.3v and be done with it, or is there more to it than that? I'm revisiting my heartrate monitor thing and the Nordic radio chipsets are all 3.3v only (It all works ok at +5v, but I'm sure it can't be doing the nrf24 chip any good in the medium/long term..)

I've got some sparkfun logic level convertors kicking around, but it just seems simpler to run the whole lot at 3.3v if it's a simple VR swapout.


Re: Idiot guide for 3.3v conversion?

Posted: Sun Jan 10, 2010 11:44 am
by floresta
... is there more to it than that?
There's more to it than that! The oscillator frequency is rated at up to 20 MHz only when operating at 4.5 - 5V. When operating at 3.35 it is limited to 10 MHz. I understand that there are some tricks you can play by operating your chip at 8 MHz and telling the Arduino IDE that it is a LilyPad.


Re: Idiot guide for 3.3v conversion?

Posted: Mon Jan 11, 2010 8:33 am
by devlinse
That's kind of why I asked. In one of the other threads, Paul (I think) said
Atmel's spec on the speed is also really conservative, because the 16 mhz resonator also works fine at 3.3V. Early on I had a small batch of 3.3V regulators that I apparently misordered and the boards ran fine. I discovered the mistake, to my dismay, only when doing some programming and checking the status of a pin with a meter.
An idiot list of what to change to get 3.3v (with caveats if any) would be quite helpful.

As I have a bunch of Sparkfun logic level convertors, perhaps the easiest thing for *me* to do is leave the RBBB at +5v and do the SPI conversions to 3.3 via the logic level board.

Alternatively, perhaps there's a market for a device with the RBBB form factor but that maintains both +5v & +3.3v outputs.


Re: Idiot guide for 3.3v conversion?

Posted: Tue Jan 12, 2010 8:53 am
by paul
The short answer is, just change the regulator to 3.3V and forget about it.

Are you outside rated performance? Yes. Far outside? No.
However we've been running the JeeNodes at 16 Mhz w no problems so far. We've got our eyes open for them, but so far, so good.

Nevertheless I've been remiss in not writing to Atmel to see where their ratings curve came from. I did poke around on the web for over an hour with no useful tidbits coming to light.

Here is some speculation:
Startup issues - apparently crystals start harder than resonators, also the full swing mode, used by Arduino (and us) may start easier than the low-power mode where the swing on the crystal isn't as great.

Thermal issues: There may be problems at one of the thermal range - and one would suspect the low end first, since everything moves slower in the cold.

Anyway- I'm off to Atmel to see if I can finally get an answer to this question. I'll post any response here when it comes in.


Re: Idiot guide for 3.3v conversion?

Posted: Tue Jan 12, 2010 9:16 am
by devlinse
Lovely. That's just what I was after.

Thanks Paul