Arduino pro micro 3v3

[artag]

I've purchased two lots of Arduino pro micro clones from ebay sellers that were described as 3v3. I believe these are more correctly called Sparkfun pro micro : they have a footprint similar to the Arduino pro mini but use the 32u4 and have onboard USB.

Both sets (different vendors, slightly different appearance) are what I would call 5v versions : they have 16MHz crystals and they regulate the raw input to 5v onboard for Vcc.

Am I missing something ? I know there's a link J1 on these devices that's supposed to bypass the regulator (it's not linked) but I was expecting to get 3v3 on Vcc when run off USB (5V will damage the attached nrf24L01 in my circuit). And I expected a lower frequency crystal - or will the 32u4 run at 16MHz on 3v3 ?


I've looked at the schematic and found various comments about these boards but I guess what I'm asking is : do all the chinese vendors lie about them ? Is there anywhere I can get one with a 3v3 regulator for less than the $20 sparkfun want ?
 

[ralphrmartin]

Not quite an answer to your question, but there are some Arduino boards out there which are switchable between 5v and 3.3v, which I find quite handy for trying things out. For example

http://wiki.epalsite.com/index.php?title=Infiduino_Uno_R3

and in nano size

http://aliexpress.com/item/32853817704.html

[ralphrmartin]

These guys have different versions - you can buy a 3.3v one which runs at 8MHz

https://www.aliexpress.com/item/32824418570.html

[artag]

Thanks ralph,

However, those are mini (atmega328) rather than micro (atmega32u4). I have been able to get 3v3 mini boards without any problem.

[ozcar]

I see plenty of sellers offering what they describe as 3.3V 8MHz ones, but as I've never tried to buy one, I don't know how unlucky you have to be to get 5V 16MHz ones instead. You'd think that if they offered a choice, your chances would be better, but then I've also seen some people report receiving 8MHz ones fitted with 5V regulators.

Seller may not know that they are lying though  - a lot of them seem to sell a huge range of tat and wouldn't have a clue.   

[ralphrmartin]

Sorry for the less than helpful replies  I'll try harder this time. 

This seller claims to have what you are after - 8MHz 3.3v 32U4 boards, and as on their page you explicitly choose a 5v 16Mhz or 3.3v 8Mhz one, I'd hope you would get shipped the right one (the 3.3v one is cheaper...). At least the seller knows there are 2 sorts.

https://www.aliexpress.com/item/32992577032.html

[Nominal Animal]

I've purchased two lots of Arduino pro micro clones from ebay sellers that were described as 3v3. I believe these are more correctly called Sparkfun pro micro : they have a footprint similar to the Arduino pro mini but use the 32u4 and have onboard USB.

Well, not exactly.  They use the SparkFun design files (Eagle), legally, under CC-SA 4.0 International (here at GitHub), but the Arduino Leonardo bootloader.  (Arduino Leonardo has the exact same microcontroller, ATmega32u4, and basically the same schematic, just a different layout and pinout.)

So, when you program one in the Arduino environment, you select Arduino Leonardo, not SparkFun Pro Micro.
(I have programmed these on bare metal in Linux also, using avr-gcc, avr-libc, and avrdude.)

Both sets (different vendors, slightly different appearance) are what I would call 5v versions : they have 16MHz crystals and they regulate the raw input to 5v onboard for Vcc.

They are then 5V/16MHz, not 3.3V/8MHz.  The difference is just one or the two components, the regulator and possibly the crystal/resonator.

ATmega32u4 has an internal oscillator calibrated to 8.0 MHz at 3.0V VCC.  So, technically, you could have an ATmega32u4 running at 3.0V/8MHz without an external crystal/oscillator/resonator, but I suspect that is exceedingly rare, as it is not precise enough for USB to work.  (So, without a crystal or very precise resonator, no USB.)

ATmega32u4 running off 3.3V is not stable at 16MHz; maximum clock frequency at 3.3V is 10 MHz or so.  It needs 4.5V or more to run at 16 MHz.

Is there anywhere I can get one with a 3v3 regulator for less than the $20 sparkfun want ?

I've only bought the 5V/16MHz ones, as there are much more "interesting" 3.3V ones (like Teensy LC), but let's see..

I believe this fleabay seller (Alice1101983 AKA TxHang) has the correct model.  It is not the cheapest, but the dozen or so various other modules I've bought from that seller have all been as described and pictured.  (Chivazhu was even better, with shipping time consistently less than two weeks to me, but has closed shop. )

[artag]

Thanks for hunting those down !

I've ordered from both and hopefully will have more luck this time (fortunately, the 5V ones are not useless. But it's annoying to get the wrong thing)
 
For the 5V version, I've generally programmed them as 'arduino/genuino micro' but there's no 8MHz/16MHz CPU option there. I seem to remember that the Leonardo is 3v3 only so presumably that would be appropriate for the 3v3 ones.

I agree there are more interesting 3v3 micros than the 32u4 but they're cheap, small and have real USB. Plenty of others that meet 2 out of 3 of those !  I don't particularly need crystal precision though - does USB require it  ? I know some implementations sync themselves to the host's USB clock but not sure if those do.

[Nominal Animal]

I don't particularly need crystal precision though - does USB require it  ? I know some implementations sync themselves to the host's USB clock but not sure if those do.

ATmega32u4 chips require a stable 8MHz or 16MHz crystal to be able to use the USB bus.  They don't have the capability to sync to an external clock via USB.
So, if you want to use the USB on these chips, even for just programming, the circuit has to have a "precise" 8MHz or 16MHz crystal.  (The requirement is not really that strict; all crystals should do, probably even many resonators.)

[ralphrmartin]

I believe this fleabay seller (Alice1101983 AKA TxHang) has the correct model.

Given that its the same name and number, I am sure this is the same seller as the one I linked, via fleabay rather than aliexpress.

[ozcar]

I seem to remember that the Leonardo is 3v3 only so presumably that would be appropriate for the 3v3 ones.

I'm not so sure about that. I programmed 5V 16MHz Pro Micros by selecting “Leonardo”. I think Sparkfun have some board definitions you could add though.

[Nominal Animal]

Given that its the same name and number, I am sure this is the same seller as the one I linked, via fleabay rather than aliexpress.

Could be, or could be a poser taking advantage of Alice1101983/TxHang having a very good reputation on eBay (since March 2012, 99.3% positive feedback from 514033 submitted feedbacks).

I seem to remember that the Leonardo is 3v3 only so presumably that would be appropriate for the 3v3 ones.

No, Arduino Leonardo is definitely 5V/16MHz.

(The "3.3V onboard regulator" refers to the dedicated on-chip 3.3V regulator for USB signaling -- although USB VBUS is 5V, the D+ and D- signals are 3.3V --, and cannot really be used to run the board at 3.3V, as it only provides 50 mA.)

[artag]

I've received some pro micro 3v3 boards from alice1101983 (via ebay) and they do indeed have an 8MHz crystal and 3v3 Vcc, which is a pleasant surprise . They arrived well within the delivery window (unlike another package from the same vendor that has just arrived a week late and with the wrong parts )

They identify themselves on USB as 'LilyPadUSB' and work (both initially and after reprogramming, which can be where problems appear) as expected if programmed as that. Haven't tried them with Sparkfun's board definition.

The aliexpress boards haven't arrived yet.