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Arduino Pro Mini 328 - 3.3V/8MHz
project on project

Mapping Household Temperature Flow with Cheap Sensors
by Andrew Jawitz

$ 9.95

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Description: It’s blue! It’s thin! It’s the Arduino Pro Mini! SparkFun’s minimal design approach to Arduino. This is a 3.3V Arduino running the 8MHz bootloader. Arduino Pro Mini does not come with connectors populated so that you can solder in any connector or wire with any orientation you need. We recommend first time Arduino users start with the Uno R3. It’s a great board that will get you up and running quickly. The Arduino Pro series is meant for users that understand the limitations of system voltage (3.3V), lack of connectors, and USB off board.

We really wanted to minimize the cost of an Arduino. In order to accomplish this we used all SMD components, made it two layer, etc. This board connects directly to the FTDI Basic Breakout board and supports auto-reset. The Arduino Pro Mini also works with the FTDI cable but the FTDI cable does not bring out the DTR pin so the auto-reset feature will not work. There is a voltage regulator on board so it can accept voltage up to 12VDC. If you’re supplying unregulated power to the board, be sure to connect to the “RAW” pin on not VCC.

The latest and greatest version of this board breaks out the ADC6 and ADC7 pins as well as adds footprints for optional I2C pull-up resistors! We also took the opportunity to slap it with the OSHW logo.

Can’t decide which Arduino is right for you? Arduino buying guide!

Note: A portion of this sale is given back to Arduino LLC to help fund continued development of new tools and new IDE features.

Dimensions: 0.7x1.3" (18x33mm)


  • ATmega328 running at 8MHz with external resonator (0.5% tolerance)
  • Low-voltage board needs no interfacing circuitry to popular 3.3V devices and modules (GPS, accelerometers, sensors, etc)
  • 0.8mm Thin PCB
  • USB connection off board
  • Weighs less than 2 grams!
  • Supports auto-reset
  • 3.3V regulator
  • Max 150mA output
  • Over current protected
  • DC input 3.3V up to 12V
  • On board Power and Status LEDs
  • Analog Pins: 8
  • Digital I/Os: 14


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Customer Comments

  • A thought that might help some others if they run into the same situation as I did on this board. This board lacks the 1K resistors on the RX and TX lines. This seems to cause some programming issues when something else is attached to the broken out RX and TX lines. I noticed that programming was hit or miss without them, and for now, it seems that adding in the 1K resistors on the RX and TX lines between the board and the secondary UART device helps greatly. Anyone else had that issue? Those resistors are pretty standard in most of the other “Arduino” designs.

  • Two warnings: First, if you’re going to solder this into another circuit, I’d recommend programming it at least once (even with just a “blink” demo) before doing so. Although I’ve used several ProMinis (both 3.3V and 5V) in projects, I just had one that after soldering into a circuit board (using male headers extending through the ProMini) it would not program. It’s not worth the effort of trying to take it off the board (OSH Park has a minimum 3 boards for custom boards, so I have a spare). My “standard operating procedure” is now to install the programming header (which has all “N/C” pads under it anyway) into the ProMini and program it once BEFORE soldering it into the board. (It at least gives me “warm fuzzies” that the ProMini is working, though it isn’t a full-blown test of all the pins.)

    Second, once I had a board built up that would program, I had trouble getting the Arduino “Serial Monitor” to “sync up”. I’d done it a few weeks ago using a “prototype” (point-to-point on a “PermaProto” board), so grabbed the “prototype” and had trouble with it, too. I finally discovered that the “Processor” type was set to “5V 16MHz”, and when I corrected it to “3.3V 8MHz” (and reprogrammed the most recent board), I could get the Serial Monitor to work for both boards. (I also tried the “won’t program” board again, but still with no luck there.)

    • This is getting really weird: Today, the aforementioned PermaProto version would not “sync up” to the serial monitor, but the (second) PCB version would. (Fortunately, I had in my notes what to adjust the settings to, since without the serial monitor, I couldn’t get them out of the PermaProto.) Weird… very weird…

  • I got Arduino Pro Mini 328 3.3V/8MHz with S20G label on voltage regulator. I can’t find any datasheet or vendor, but I measured that it consumes 1.6mA in sleep with power LED on (and 230uA with LED removed) and less than 7mA awake. Surprisingly good result. I would just like to know maximum current rating and quiescent current.

  • ben_r / last year / 1

    Is a 3.3V FTDI Basic required, or will a 5V connection safely drive the 3.3V Mini over the programming headers without risking damage to the board or any 3.3V devices connected to its I/O?

    • M-Short / last year / 1

      The power pin on the FTDI connection is VCC and not RAW so it will not go through the voltage regulator. 5V is safe for the Pro Mini, but you will also have 5V on anything connected to VCC (possibly including 3.3V sensors). This will also cause the Pro Mini to operate at 5V meaning its I/O will also be running at 5V. Whether that will risk damaging another board depends on the specs of the sensor.

  • Any chance of getting the board into the Sparkfun Library with the off-grid headers? I’m designing a PCB to mate with the arduino and it would be nice to have those available from the get-go.

    • agreed! Would be nice to have in the Sparkfun Library.

      • If anyone is interested I made an updated footprint to put in my library that includes all holes. It has NOT yet been tested as I’m waiting on the PCBs but the dimensions are taken directly from the eagle file given on this page.

        • Hi, I am looking for something similar. I am looking for a part which might consist of correctly spaced out female headers, to mate with the Arduino Pro Mini. If you could give me a link, that would be awesome.

  • Can I upload to the 3.3v mini using an uno R3 board? Would this work - take out the ATmega328 chip on the uno and connect BLK, GND, VCC, RXI, TXO, GRN (on Mini) to Ground, Ground, 3.3V, RX, TX, reset (on the uno) and then upload a sketch selecting the 3.3v mini board from tools in the Arduino IDE?

    • I meant to say when you plug in all those things respectively. Anyway, I tried it with the blink sketch and it seems to work.

      • What did you select as the board / processor in the Arduino Dev Env?

        • I found out from an Instructable and from trying it that you pick the board you’re trying to program, not the board (i.e., UNO) that you are programming with. Just FYI for anyone reading this.

  • Has anyone ever used an Arduino Pro Mini as a SMD on another board? Any advice?

  • My mistake … didn’t account for it being 8mhz instead of 16 mhz as I usually use!

  • Could one hook up one of SF’s small lipo batteries directly to this? Or would something else be needed or recommended?


  • Can I run this “Arduino Pro Mini 328 - 3.3V/8MHz” with CR2032 coin cell battery ? Since nominal voltage of this battery is 3v , and this will be reduced with time , this arduino pro mini will work till which minimum voltage ?

  • I just got one of these to work with a 3.3v interface chip. However, I cannot get it to program with any of the USB-TTL breakouts I have. i don’t have FTDI - only the PL2303HX-based versions. I can do loopbacks on these boards but they will not program the pro mini. Is there something else that I have to do when using one of these alternatives? They don’t bring the DTR out - just RX, TX, GND, and VCC (which I don’t connect because it’s powered off the breadboard).

    • The DTR pin on the FTDI boards is used to do a reset of the board to put it into bootloader mode. If you don’t have DTR broken out you will need to manually reset the board. Basically when the code is finished compiling and you see the white text giving you the file size etc., hit the reset button and it should work.

      • Tried that a few times, but maybe didn’t hit the right time slot. I WAS able to use an UNO board with the processor removed, jumpered over to it. Just picked Pro Mini with the correct processor and speed and it worked first time! At least I know it’s not a bad board!

  • In the description of this product, it says that the ADC6 and ADC7 pins are broken out. Does the Arduino software natively support A6 and A7?

    • Yes. The Pro Mini is actually an official Arduino board so everything is fully supported in the IDE. I believe this is the only official Arduino board to use these pins and the ATMega328 PTH version does not have them, so very old versions of the IDE might not support these pins, but any IDE from the past 3 years (since this board was released) definitely will.

  • Just to clarify: If we connect the 3.3 Pro Mini to the FTDI Basic Breakout via the broken out pins, it will fry the board? ie) USB is 5V and the ProMini is 3.3V?

    So we would want to leave VCC disconnected from the broken out pins and instead connect the FTDI power to RAW on the Pro Mini?

    • If you use a 5V FTDI it will put 5V on the VCC line. If you use a 3.3V FTDI than 3.3V will be on the VCC line. I believe the maximum voltage the ATMega328 can handle is 6V (absolute max, not recommended) so the IC is fine. Basically the 3.3V designation of the board is the value of the voltage regulator and when the board is powered through the regulator the I/O voltage of the controller. In other words if you put 5V on VCC than you will just have a Pro Mini running at 5V. As long as you don’t have components that will fry at 5V connected to the board everything should be fine.

  • I like the 5v version of this enough that I wrote KiCad .lib and .mod files for those who want to put an Arduino Pro Mini on a PCB of their own design.

    Free.. Should work just as well with 3v3 version?

    The small size of the Arduino Pro Mini reduces the expense of the PBB you are making, as there is little board area under the Arduino.

  • Hello, If I want to connect a 1 cell Lipo Battery (3,7->4v) can I connect it directly to VCC or do I have to use the RAW pin ? Thanks.

    • The ATMega328 typically runs at 5V and its rated maximum is about 6V, so you should be able to put anything up to 6V on the VCC pin. The RAW pin will run the voltage through a 3.3V regulator meaning that VCC will now be 3.3V. Either of these will work, one may be preferable over the other based on the peripherals you are using and their voltage requirements.

  • I originally bought this for my project because of the cost. I found the location of the I2C SDA & SCL pins (A4 & A5) to be so annoying that I just bought a Pro Micro instead. I am much happier with it and I don’t have to worry about keeping up with [the now rare] USB Mini cable for my FTDI Basic.

    I find it pretty ridiculous that this thing has 2 on-grid GND pins and 2 on-grid RST pins, yet the I2C is off-grid. Could have been a nice little board otherwise.

    • The off-grid decision is actually very intentional. Enough people solder headers to the board and stick them in a breadboard that we didn’t want to run the risk of you shorting those pins to something else. The original was based off of the Arduino Mini and uses the same footprint, I believe the I2C was actually added later on. In general its a great little board, but so is the Pro micro. Thanks for the feedback.

  • How would this best be connected to a raspberry pi? i2c, SPI, UART of some sort?

    i do not want to apply a full FTDI and USB connection between the two.

  • So…I was told if I had an LED set up requiring 12V, and an XBee that I was controlling off this version of the pro-mini, I would need a step down regulator in order to only use one power source for all three things. Is this not correct since it has an on-board voltage regulator? Couldn’t I hook the 12V battery to the pro-mini’s raw pin, hook the LEDs directly to the battery, and the XBee to the VCC? I’m still learning all this electronics stuff so I want to be sure I don’t fry my Arduino.

    • Carefully check what type of “12V battery” you’re using. If it’s a lead-acid (including gel cells, car batteries, etc.) then the actual nominal fully charged voltage is actually around 13.8V. Other chemistries have other voltages. The rating for the input of the proMini is, according to the “Features” section above, is 3.3V to 12V. You might be able to get away with running it at 13.8V, though there’s also a good chance of “letting the smoke out”. The conservative (safe) way would be to use an off-board regulator to drop the battery voltage to, say, 9V, and feed that into the proMini. Also note that if you’re running this on a car’s electrical system, you can get spikes of up to 70V or more, thanks to that ginormous inductor otherwise known as a “starter” (this is why most cars turn off radios, etc., when you move the key to the “start” position).

      • It’s an alkaline A23 12 volt

        • If you look at the A23 data sheet which is at you’ll note that the initial (fresh battery) voltage is about 13V, though it does quickly drop. Although you might be able to get away with connecting it directly to the “raw” input, but it wouldn’t be considered “good engineering practice”. I’d be inclined to put a (high efficiency) switching regulator to drop the voltage to something like 7 or 8V, though that’s beyond the beginner. If you’re willing to put up with some power loss, you might consider an LM-7809 or LM-7805 voltage regulator (9V or 5V, respectively), though (a) you’ll probably have to order the LM-7809 (if you use it) from DigiKey and (b) with either one, be sure to read the spec sheet carefully and use the bypass caps (I found this out the hard way nearly 40 years ago). There is a third, possibly less expensive option, though: get a 12V Zener diode and put it in parallel with the Arduino power input, and BE SURE to provide a resistor sized to drop 1V at the rated current of the diode. This may introduce unwanted noise (as well as wasting some battery power), but it will protect your proMini from the over-voltage from a fresh battery.

          • Hm. OK, sounds like I should just use the regulator. I was just trying to minimize required wiring. Pololu has one that steps down 12-14v down to 3.3v.

  • The Arduino buying guide suggests that this has: 14 Digital I/O 6 Analog 6 PWM 1 UART

    Here it says 8 analog and no mention of PWM.

    Here is what I want to know: Does this thing offer PWM? On what pins? Are the analog pins pulling double duty?

    • The ATMega328 has 14 digital I/Os, 6 of which do PWM, and 2 of which are the UART port. It also has 6/8 analog pins. The original Arduinos use DIP chips and those have 6 analog pins which were all broken out. The SMD chips have the 2 extra pins which were never really broken out partly because no one wanted to change the footprint. We did add those extra two pins on the current version of the Pro Mini. Otherwise it has the exact same pins as any other ATMega328 based Arduino including PWM. If you take a close look at the picture of the Pro Mini you will see an extra ring around pins 3,5,6,9,10, and 11, these are the PWM pins.

  • Hard as I look, I can’t find that “power isolation jumper” that’s on the schematic, nor the footprints for the I2C resistors (I just see the I2C pins broken out to the off-grid headers.) Am I missing something here?

    • The “power isolation jumper” is bridged - it’s the blob of solder by GND and RST.
      The two resistors are optional, and you can find the footprints on the bottom of the board by the A4 and A5 off-grid pads.

      • Ah, I see the resistor pads, and the jumper in the pictures above. However, my board is different – there is no space between the crystal and the 10uF cap on mine! Out of 6 Pro Minis that I have, 2 of them have the jumper and 4 don’t (but all have the resistor pads). I guess the jumper was added after the resistor pads.

        • Yep - looks like the resistors were added in v1.3, while the solder jumper was added in v1.4. As the ‘previous versions’ list only points to a product matching the v1.2 iteration: should you need the schematics/board for v1.3, just change the ‘v14’ in the links to ‘v13’ :)

          • Aha, thanks. I’d never seen the “previous versions” link before… :-)

            So the question then becomes: will the voltage regulator get unhappy if I connect my board to the FTDI breakout and “back-power” it with 5V? I looked at the datasheet but couldn’t find any info about what happen if Vout is higher than Vin.

  • Can I use this just to drive things? Like interchangeable tools sort of.

  • Could I use this just to drive a servo motor?

  • Does this support I2C? I’m not really familiar with I2C yet, but I want to learn and I am wondering if I can use this with an accelerometer to build a self-balancing robot. I’ve got a few months experience with an UNO, so don’t spare me the technical details.

    • Yes, A4 and A5 are the I2C lines or the Arduino. Basically the Pro Mini is the same circuit as the Uno just a lot smaller and without a USB port, bu the actual microcontroller is the same, so anything the Uno can do the Pro Mini can do. This Pro Mini also only runs at 3.3V/8MHz.

  • Are the rx/tx lines 5V tolerant? I want to know if it is possible to program this with a 5V FTDI board, without needing to get an additional 3.3V FTDI just for this board.

    • Yes, the only difference in the 3.3V and 5V Pro Minis is the crystal and the voltage regulator. Since the FTDI bypasses the regulator the only difference is the speed the run at. The ATMega328 is fine at 5V. Keep in mind if you have 3.3V on VCC and 5V on the I/O pins that is technically out of spec (you shouldn’t put more than VCC on the I/O pins), but will probably work as well. Keep in mind that the 5V FTDI will put 5V on the VCC line, so make sure you don’t have any 3.3V only devices connected when you do this.

  • Does the VCC pin output 3v3 when I put 12V to RAW pin ?

    [self-answer : yes it does.]

  • Has there been any talk of an Arduino Mega like this? A megamicro? I love the pro mini but my project uses a relay shield and an sd shield and a few buttons and switches so now I am out of pins for a LCD. 1st world problems.

    • We do have a version of it here but because of the number of pins, the layout is a bit bigger.

      • Oh! I saw that and it didn’t click in my tiny duke head. Layout size isn’t a problem, I just ran out of pins. Thanks!

  • Hello! I it possible to run 3.3V version with 1.8V on 1MHz ?

  • I was thinking: what if a board like the Pro Mini had a pinout that was compatible with one of the four headers of the Arduino shield form factor? For instance if one side of the board had digital I/O 8-13, GND, AREF, and SDA/SCL, in that order? Then if you were building a project around a standard Arduino shield (but didn’t want to use a standard Arduino) you could put a socket header on those 10 pins and plug the Arduino right into one corner of the shield (or just solder it to the shield, pin-to-pin), and then wire up the remaining pins by hand, or something…
    Something like that could be nice as a way to make Arduino projects more compact, while still using the same shield design.

  • I’m having trouble with floating inputs. No matter what I do I can’t get the analog inputs to stop floating. I’ve tried pull up/down resistors, that just resulted in 0 or 3.3v readings. I have 2 of these and they are both doing the same thing. I’ve tried battery, USB, and regulated wall worth power.

    • This is the sort of thing you see if nothing is connected to an input. What are you connecting, and is the connection solid?

  • I am wanting to use this to drive 14 LEDs. If I use an external 3.3v supply (COM-00526) could I do this? I know in the Arduino website it says max 40ma per I/O, this is fine, but I need to supply about 300ma in total. If i use the external supply connected to VCC would I fry the chip by pulling 20ma through 14 of the I/O pins? This is going inside an r/c car so I would prefer to not have an external transistor board. Also, all 14 LEDs need separate control.

    • would I fry the chip by pulling 20ma through 14 of the I/O pins

      That slightly depends. If you’re trying to source the current, yes. If you’re trying to sink it, then your pin selection matters. See pages 303 and 304 (the notes on page 304 detail how to calculate the maximum ratings based on pin selections) of the datasheet for ATmega328 for details.

      Your safest bet without a driver board would be to simply limit the current below 20mA. This does mean your LEDs will light up less brightly (source nice, bright LEDs if this is a concern). Another option is to only have a few LEDs on at the same time, turning them on/off as quickly as you can to make them appear to be on all the time, while reducing the current required. In this case you could also source the current. Note that if your program has an error and turns all LEDs on, you would have a problem :)

      Note that a driver board doesn’t have to take much extra space. Two Shift Register 8-Bit - 74HC595 will decrease current requirements from the Arduino as less pins are used, two Shift Register 8-Bit High-Power - TPIC6B595s could easily drive all 14 at the same time, and a single STP16C596 (suggested at the arduino site) would be practically an all-in-1 solution. There’s many other options, try googling for ‘16 channel led driver’. But, it’s not required as long as you can work within the confines mentioned above.

      • The link for the ATmega 328 datasheet is broken.

      • Thanks for the reply! I’m very much a noob with these, and the part about the shift registers has gone right over my head…. Is there a way you could explain that a little more? My plan is to have the Arduino turn LEDs on/off based on the signals it gets from the cars RX. I guess I’m failing to see how this works with the shift registers? The applications I am seeing with them is to turn a bank of lights on/off or cause them to blink. Not to have individual control of each LED.

        Also, looking at the ATmega328 data sheet I’m failing to see anything other than the already stated limit of 150ma. Perhaps this is my ignorance showing through..

        • My plan is to have the Arduino turn LEDs on/off based on the signals it gets from the cars RX

          Let’s say your signals dictate that you need LEDs 5, 6, 7 and 10 to be on. You would then push essentially serial data into the shift register to match that pattern (00001110010000: LEDs 1, 2, 3, 4 off, 5, 6, 7 on, 8, 9 off, 10 on, 11, 12, 13, 14 off).

          Now let’s say that, theoretically, you, can only have 2 LEDs on at the same time due to current restrictions in the shift register itself. No problem, you would create two patterns, and switch between them really fast so that it still appears as the desired pattern:

          Shift registers can generally, thankfully, handle more than just 2 LEDs,but either way your Arduino would only have to deal with a minute amount of current required to drive the shift register, rather than 2, 4 or even the full 14 LEDs.

          As far as the ATmega goes, check e.g. page 303 the first table (absolute maximum ratings): “DC Current VCC and GND Pins…………………………… 200.0mA” Let’s say you wanted all 14 LEDs on at 20mA at the same time, giving a total of 280mA. That’s more than the VCC pin is supposed to handle (in fact, while the maximum per pin current is 40mA, you’ll find further down that they prefer 10mA when VCC is 3.0V), so you can’t source that much current. But it’s less than the combined 400mA of the GND pins, so you could sink it. Page 304 then explains in the notes the specific calculations you should perform to see if you exceed the maximum current you can sink per section (e.g. “1] The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100mA” - note that this is different from the 150mA for sourcing).

          But, again, as long as you’re okay with trusting the programming / the components not to fail, then you could easily drive the LEDs directly from this board by just keeping 1 or 2 on at a time :)

  • The getting started tutorial link is broken.

  • figured out my own problem.. it was that I used a cheap china made PCB.. i guess for me to make a real prototype i will have to spend some more money :(

  • Why exactly is this running at 8MHz instead of 16MHz? I would like to use this at 3.3V but want the extra speed. Is it because you’re worried about turning on too much I/O and dropping voltage too fast and throwing errors? I’m using this as a prototype, and am planning on building my own board, but wondering why you went with a 8MHz resonator. Thanks!

    • This is a limitation of the processor itself. If you read the very large datasheet for the ATmega 328, you’ll find an interesting graph in the back that shows the maximum safe clock speeds for various VCC voltages. At 5V, you can reliably run it at up to 20MHz. But at 3.3V, you can only reliably run it at up to ~10MHz. This is why we use different resonators for the different voltages. (And note that the slower you run a chip, the less power it uses, which is better for battery-powered circuits. You can even clock these chips down to 32KHz, which runs programs very very slowly, but uses miniscule amounts of power).

      • It’s not as bad as that I think, you should be able to get 12MHz comfortably.

        Section 29.3 (“Speed Grades”) of the ATmel datasheet “Atmel 8-bit Microcontroller with 4/8/16/32KBytes In-System Programmable Flash” states that the Maximum Frequency is linear across 2 intervals: between (1.8v=4MHz, 2.7v=10MHz) and between (2.7v=10MHz, 4.5v=20MHz).

        This means that 3.3v implies a Maximum Frequency of 13.33MHz.

        So I don’t see why we are not running at (say) 12MHz instead of 16MHz - apart from one having to do a relatively-straightforward patch to the NewSoftSerial library, as detailed here: I run at 3.3v and 12MHz, using optiboot recompiled for that speed as the bootloader and everything works very well.

        Food for thought if you are looking for a new version? I know intrinsic support in Arduino IDE is a little smoother for 8MHz but getting 50% more cycles is not to be sniffed at.

  • I’ve seen it stated a bunch of places that using the wire library for Arduino enables internal pull-ups to 5V, and that you therefore shouldn’t use it with 3.3V components. See, e.g.: and

    But if I’m using a 3.3V arduino pro mini, I would imagine that the internal pull ups would connect to 3.3V, and this should be a problem. I can’t find specifics about this anywhere though. Is this correct?

    • Correct - the internal pull-ups just pull the pin up to whatever is on the ATmega328’s power input line. If it pulled them up to 5V, that’d be quite interesting as it would mean it would have an internal boost circuit ;)

  • I’m not OPPOSED to buying an FTDI breakout board, but I am trying to figure out whether I can program this puppy with the usbtinyisp I already have.

    I’m able to use the usbtinyisp it to program my UNOs and Duemilanoves, but they both have headers that are clearly marked ICSP. This doesn’t, but I am hoping that I can rig up a simple cable or jumper set. I’m sure the schematics would tell me, but I’m not sure what I am looking for.

    Thoughts Anyone?

    • Check the schematic for the SPI pins. You’ll need to connect to those and will require some jumper wires, but you should be able to.

      • Thanks. Sounds like a relatively trivial problem to solve, so these boards will be ideal.

  • According to the schematic, there is an LED on pin 17. Yet in the example blinky, it uses pin 13, and yes the LED blinks. So is there a mapping or translation, or is the interpretation of 13 different/unclear/wrong/strange ?

    • The interpretation of 13 is different/unclear/strange. Welcome to Arduino :)

      What is referred to on the Arduino as ‘Pin 13’ actually maps to ‘Port B, Pin 5’ on the AVR, regardless of the chip’s pinout which can vary. For example, in the DIL/DIP version of the ATmega328 you can see that it maps to physical pin 19. On the TQFP version of the ATmega328, however, it maps to pin 17, which his what the schematic is referring to. ( And to yet another, pin 15, on the 28-pin MLF/QFN package. )

      • Thanks very much for the response. I surmised there was a mapping, what I couldnt find and what you kindly provided was the TQFP diagram detailing it. Much appreciated.

  • With the different USB hardware on this board, I am wondering if it can use the Keyboard library like the Leonardo? It’d be perfect as a low power - low volt, low clockrate - board for an IR detector.

  • I was looking at the schematic and wondering, why do we need both C3 and C10? Would C3 or C10 work? can anyone explain that to me?

    • The larger cap takes a bit of time to let go of its electrons, the lower value cap lets them go more quickly. More precisely: the capacitors have different resistance and inductance, the 0.1 uF is used to smooth out high frequency ripples, the 10 uF handles lower frequency ripples. On board layouts you will find the 0.1 very close to the IC that is causing the ripples, the 10uF is more likely close to the incoming power.

      • C3 and C10 are both 0.1uF. C13 is the 10uF cap. However, I think I can apply what you explained to C3. It is there to smooth out the high frequency ripples caused by the ATMEGA and would be positioned physically close to that IC. Is that correct? If you were designing a circuit would you typically use one of these capacitors per IC?

  • So maybe I need some help, for someone with a bigger brain or more experience with these. I’ve built a clock using both a Arduino Pro Mini 328 5v/16 MHz & Arduino Pro Mini 328 3.3v/8MHz. I’m using the “Time Library” on the 328 5v/16 it is very consistent, I loos about 10 seconds a day and can compensate for this in code or eventually at a RTC. I bought 328 3.3v/8 thinking less power needed, lower speed, more energy efficient. This will eventually be a watch. I just hooked up the 328 3.3v Within 15 minutes it has lost 7 minutes. I’m running the same code on both processors. What did I miss? Please help if you know what is going on.

    • Self Edit - Helps to select the right board from the tools menu or you take 2 seconds to move the internal time clock 1 second froward. Divide by 2 - Kicks self and moves on - Doh!

  • it weighs 1,53g no-headers and with 3.67g with headers

  • i got one of these today, very nice. ya know if you reverse the raw and gnd pads you wouldnt have to solder the jst connector on the bottom of the board upside down :)

  • Does the RX/tx operate at 3.3v? I.E. I wouldn’t need a logic level to connect to something like a raspberry pi.

    • Yes, if you power the board through the RAW pin it goes through a 3.3V regulator and so the board runs at 3.3V with 3.3V I/Os. If you power the board through a VCC pin though the board will run at whatever you put into that pin, so be careful not to put 5V on that pin.

      • If I put 5v in RAW, the TX/RX lines aren’t voltage regulated? I want to avoid buying more boards (ie logic levels) or having a separate 3v battery source.

        • If you put 5V on the RAW pin the entire board runs on the regulated voltage. The ATMega328 is running at 3.3V, so all its I/Os (including TX/RX) run at 3.3V. The only place you will see 5V is on the RAW pin.

  • I’m not sure what I’m doing wrong. (n00b warning).

    I’ve got the following project working with my uno ( but when connecting the wall wart dc jack to the pro mini I keep blowing it up. I connect the positive from the jack to “raw”, then the negative to the ground on the mini board. I get a red light on the pro mini blinking fast and also the wall wart has a red light that begins to blink.

    I’m not sure what I"m doing wrong. but the board powers up fine with a FTDI friend.


    • wild guess, is the wall wart unfiltered dc? a single diode turns the power output off 60 times a second. if your volt meter reads less dc than the wall wart label and or your volt meter displays a significant voltage on the ac scale, that maybe the problem. try a capacitor on the wall wart output.

  • i am totally new to arduino, i need to connect a rf receiver and a transmitter to one of this boards, in the tutorial i got they use a duemilanova and on that board there is a 5v pin identified but i can’t find it in nano, any help?

  • Hello! Could anybody tell me what brown out level this board has? I was expecting 2.7V (extended_fuses=0x05 in boards.txt). But it seems something like 2 Volts (which could be 1.8V=>0x06 …) The pro mini 5v 16Mhz I got at the same time also fades out at 2 volts! Thanks

  • Can you power this with a usb connection?

  • One of my problems with this board is that the FTDI header is VCC instead of VRAW. Since VCC is 3.3v and FTDI is usually 5v, it leaves me always having to stick a wire in my FTDI cable’s VCC to connect it to VRAW instead of the ftdi header vcc on the board.

    Also, for some odd reason, twice my board has gone into infinite blink mode. I got it out once by following some instructions I found elsewhere that say to unplug the usb (ftdi cable), hold reset, and plug it back in, then upload and let go of reset when the TX starts. This worked once. Now a couple days later, my board is stuck in blink mode again and even this doesn’t work. Not only that but shortly after this happened, the ftdi cable I was using with it now no longer works. Anyone have any ideas how I can reset this thing?

  • Does this board have a resonator or crystal? The schematics suggest a crystal, but some sources say it has a ceramic resonator (but the original mini’s had a crystal resonator). There is a part on the board that looks like a very small crystal (probably a ceramic resonator), but is only marked “A4”. Many sources seem out of date.

    • it uses an smd crystal. “A4” is an analog input. Q1 is the crystal and is located adjacent to input 2 and GND. The small silver rectangle with the 08 on it.

      I’m trying to find this part, any reason sparkfun doesn’t carry it?

      • But it looks like 3 pin non standart layout. I couldn’t find these type of 8mhz smd crystal on digi or mouser.

        I just found a resonator ( with build in capacitor ) in digikey. Could anyone confirm that.

        Or any link for these smd part ?

        • this is the part:

  • I tested SD file read/write with an UNO R3 and microSD. I want Final product to be this 3.3v mini and a standard SD card. The UNO is 5V/16Mhz and this is 3.3V/8MHz. Will the read/write times to the SD card be significantly slower at 8Mhz? (Is this the obvious answer: “The 8Mhz board will take twice as long as the 16Mhz to write to the SD card.”)?

    • Jut a follow up. I got the pro mini 3.3V 8Mhz connected to a standard SD card and it is still lightning fast on the SD Fat read/write. I have to write a lot of bytes to a binary file and use Seek a lot. I do see some hiccups when I write several hundreds of bytes and the cpu resets. Am I overflowing some pointer? Should I limit my writes to so many byte, close and reopen the file??

      • I suggest looking through the source code for the sdcard library. There are a huge amount of useful comments in there, and you may find constants for buffer sizes that you can either change (as long as you have the RAM available) or use the knowledge of those limits in your own code. Another tip, from experience, is pay attention to return values, which may be trying to warn you about such errors. Good luck!

  • Can i power this with a 3.7v battery connected on Vc directly,without regulator?

  • The description of the pro mini on the arduino website –– states that an ATmega168 is used for the 3.3v version. Is this information incorrect? The image shown above indicates an ATmega328 is used, which matches the description of the product, but my concern is, if I order this pro mini, can I be guarnteed to recieve them with the 328? The application I intend to use them for requires the SRAM capacity of the 328. Thank you.

    • Yes you can. These boards are designed and built here at Sparkfun so our page is always going to be the most accurate. Arduino is just really bad at updating their page.

  • Great board!

    Can anyone confirm what the lowest current consumption should be on this board? The 328P datasheet suggests that I should be seeing under 1 uA with maximum power reduction in place, but the lowest I’ve been able to see is about 102 uA. And that’s with the LED1 removed from the PCB.

    The datasheet for the MIC5205 regulator shows a typical ground pin current of 80 uA at a load of 100 uA. With the 328P in full shut-down, it should be presenting a load of < 1 uA. There is no data in the sheet for loads that low, but I imagine the ground current would be at or below 80 uA. The enable pin has a typical draw of 5 uA.

    That’s a total of 85 uA typically for loads of 100 uA, so the total current for a < 1 uA load should be somewhat lower. Why am I seeing consistently 102 uA? Is this the best that I can expect?

  • Hello everyone! I have this chip, but i haven’t UART TTL decoder. Can you answer: Can I use this UART TTL converter that comes with APC-220 ( for download sketches in this Arduino??? And if it possible, how i must connect it???

    • OK! Problem is missing! ) But now i have problem with 3.3v & 5V, because this UART TTL get 5v on Arduino, but my MPU-6050 must have 3,3v on board. How i can repair that problem???

  • @sparkfun can you attach a mini usb to this? Thanks, Nate

  • I saw a reverence elsewhere on your site that you have the programmed microcontroller available standalone I couldn’t find it. Do you still sell it?

  • I can program this board via a 3.3v FTDI without problem. However, I can’t get the serial monitor in the Arduino to work with this board! Don’t know why … just get junk like the serial rate, parity or something is incorrect. Tried multiple settings.

    • Ran into this problem, tried another ProMini that I knew I’d done a few weeks ago. Finally tracked down that the “Processor” (under the Arduino “Tools” menu) was set to the 5V processor.

  • If this is used in an automotive setting where the voltage could potentially reach up to 14.5v, is this too far above tolerance levels? The Duemilanoves could handle up to 20v, but I’m hoping I can get away with the mini version, and I’m not seeing any data sheet for this design other than for the microcontroller itself, which is only tolerant of something like 5v.

    • Take a look at the thermal characteristic for that SOT23-5 package. You would need to drop from 14.5V to 3.3V and depending on your current, it is not advisable to do that.

  • a Ethernet shield for this would do me wonders. i mean i want to go small with the pro mini but then have to use the full sized shield to get internet use?

    two words: COME ON!

  • Sweet! The extra ADC pins and I2C pull-up pads are both welcome improvements to my favorite prototyping board. Thanks guys!

    • Also, here’s a picture of a neat little single-sided Pro Mini derivative I knocked out recently :-D

      • That’s awesome, and btw thanks for all your work that ultimately ended up on i2cdevlib in figuring out the DMP on the MPU6050! I actually want to make something similar (with some other stuff on it). Would you mind sharing a schematic/board? Also, is that PCB homemade?

Customer Reviews

4.6 out of 5

Based on 36 ratings:

5 star
4 star
3 star
2 star
1 star

3 of 3 found this helpful:

The low voltage feature is awesome!

So removing the solder blob allows you to run the board off a 3.7V lipo. from full charge(4.2V) to discharge (~3.2V) the microcontroller runs its 8MHz quite happily with no complaints. Does really well in deep sleep modes as well.

2 of 2 found this helpful:

Does the job but a 9V battery let the smoke out of the voltage regulator

I’m using this as an ADC to convert an analog pot into a digital value which is then input into a Raspberry Pi. I had a 9V battery in my project case, so I tried powering this board up using the 9V line on the RAW pin. The product description for this board says the max voltage on RAW is 12V (and the schematic says 16V), but as soon as I connected 9V to RAW, there was a loud “POP” and the bitter taste of disappointment (and melted plastic) in the air. Happily, it was only the regulator IC which blew, and I’m able to power and use the rest of the board using the 3.3V pin on the Raspberry Pi driving the Vcc line on the FTDI header.

Another great SparkFun product, but I still wonder what happened with the voltage regulator. Anyone had a similar experience?

1 of 1 found this helpful:

Pro Mini

Really great product. Reasonable cost, documentation sufficient, and great compatibility with Arduino family. Low power consumption, too less than 10ma - that along with the small size and easy hook-up (flexible use of header pins if desired) makes this my go-to Arduino platform.

2 of 2 found this helpful:

My favorite Arduino board (maybe)

With the possible exception of it’s 5V sibling, this is my favorite Arduino board. It’s small, it’s inexpensive, and it does everything one would expect of an Arduino.

1 of 1 found this helpful:


I really like this and the 5V version. For many projects I prefer them to the UNO. My only beef and the reason for the loss of a star is the fact that pins A4 and A5 are not on the outside of the board and worse, they are offset. So for I2C you have to run wire or build your own board. Not the worst thing ever, but for a tiny bit bigger of a footprint, I personally would have preferred those pins on the outside.

1 of 1 found this helpful:

Great little Arduino

This is perfect if you want to embed an Arduino into a small project. The 3.3v version is awesome as you can directly connect to things like the small WiFi and nRF24 radio boards that are native 3.3v chips - no need for the Sparkfun 5-3.3v converter board.

Make sure you pick up a FTDI 3.3v logic board or cable. The 5v one you probably have lying around won’t do you any good with this board.

2 of 3 found this helpful:

Good match for balloon project

Some of our local amateur radio folks (aka Hams) are supporting a climate change class taught at Williams College. Last year the class launched a balloon that reached 30K feet and we received telemetry data from the balloon. This year we plan to design a more capable payload for the balloon. The light weight, low cost, low power requirements, and availability of software make this product a great choice for generating telemetry data.

Great board, great price

these are awesome little development boards. Highly recommend the FTDI breakout programmer, makes life very easy. This board is very thin, compact, and has plenty of IO options. Easy to solder.

Great for such a tiny thing

If you need to integrate a small ready-to-use arduino/ATmega in your project this is a pretty great little board. this is made to be as small as portable as possible. Even the PCB itself is thinner then usual.

It's from Sparkfun

Of course it’s awesome, it’s from Sparkfun.

Tiny and power efficient

If you need to reduce the size or power of your project, graduate from the Arduino Pro to this board. Possible to power the MCU directly bypassing the on-board regulator if low power is the goal. I wish the LED wasn’t on an SPI pin, but that’s the same with the larger board too.

I want AREF:

The biggest problem with Sparkfun boards is, the AREF missing!!! So don’t buy it if you want to scale the sensor readings or to improve the resolution of your measurement.

small and perfect!

is there anything better than this tiny, easy to use computer?

Love these things

I’ve basically abandoned all Arduino models except this and the Moteino (which is basically a Pro Mini with a radio). I haven’t done a project that needed a larger processor or more IO, and for $10, you can make a lot of really cool stuff!

Excellent low power board

I still could not believe the low current it takes in power down mode. (I hope I use the board correctly to measure the ultra low power down mode current) It is ~200 nA with out the power LED. I have seen others do many changes to the board to get 4.5uA, but just removing only the LED I was able to get it down to ~200NA. It is probably due to MIC5205 Vreg the board. Great Product, love it.

Almost amazing

I seem to be collecting microcontroller boards and these are my newest acquisition.

I’ve used one of these very successfully in a micro/macrophotography stacking rig.

I power mine with 12V or 9V to the RAW pin. No smoke. If there is smoke, something is wrong with your unit or wiring. I always use these with external reverse polarity protection diodes (Schottky) or even MOSFETs. It’s easier than you might think to wire things up backwards.

I am a bit perplexed, however, by the location of those four analogue input pins. I guess it makes the board a bit shorter by putting them there, but I’d rather a board that’s 5mm longer with those pins placed in-line with the others, two per side.

Another thing to note is that the silk-screening for the TX and RX pins is quite confusing unless you consult the ATMega328 pinout. Arduino UNO, for example, labels their pins TX->1 and RX<-0. Letting you know that TX (out) is pin 1, and RX (in) is pin 0. On this board, however, the pin numbers are missing - but to a casual observer, it may not appear that way. Instead, the letters I (in) and O (out) appear, which are easy to confuse with pin numbers. RXI is in fact pin 0, and TXO is in fact pin 1. So beware of that, if you plan to actually use those for something other than serial.

Small, tiny and sips power

The Arduino Pro Mini is a very powerful Arduino with lots of features and I/O in a very small package. If you need an Arduino that is physically small or has to run on batteries/low power this is a great choice. This comes in a 5v and 3.3v version, as well as an 8 or 16 MHz option.

One typical usage might be a remote sensor that needs to report data (say temperature or weight) occasionally. The Pro Mini may wake occasionally from h/w powerdown, power up a radio, report the data and then powerdown again. Depending on the factors you might be able to run a year or more on AA batteries.

The Pro Mini comes without headers (you need to add your own) and without a USB connector, so to program it you will need Sparkfun’s serial adapter for that. Although it’s small it does come with a voltage regulator that can feed peripherals as well as a handy reset button.

Note for ultra low power applications there are many tricks you can find online such as removing the power LED as well as the voltage regulator (assuming you don’t need these). Removing one or both of these can have substantial impact on battery life.

Overall it’s a great Arduino in a tiny package!

The perfect Arduino board for highly miniturized projects

I’m currently hacking a PS2 remote controller, in order to use it to drive a RC DIY quadcopter. I emptied the controller, because I found the electronic inside very ugly, and wanted to make all the logic of the controller on my own. I like using Atmel microcontrolers, but I don’t have the skills to solder CMS ones: indeed, any DIP-24 package don’t fit in the controller. Then, the Pro Mini was the perfect board: it’s tiny enough to fit in, and there’s no need to CMS solder.

Great very little Arduino

The Pro Mini either in 3V or 5V is my goto Arduino when I don’t need the traditional form factor for shields. They are just the thing for building into zombie costumes or tiny robots. These Sparkfun Pro Minis and well made and well supported.

Wonderful little board for projects!

I’ve been using the Pro Mini boards since they were first introduced and they’re just the ticket if you need a super small Arduino board for your project. I like to keep a few of both 3.3V and 5V versions on my bench as they’re my go to board for quickly prototyping circuits.

Excellent little power house

Easy to get up and running with these guys, sufficient memory, compatible with a ton of 3.3V devices without the need for level shifting…same reasons everyone else loves them…and they aren’t expensive at all…

0 of 1 found this helpful:

I didn't realize it has only one UART port.

I was spending a lot of time trying to figure out why I couldn’t get the Serial1 port to work as it kept screwing up and hanging the Serial data also. I finally looked at the specs and noticed that it has only one UART which is the same as the FTDI port. Actually it doesn’t make sense to me. Why would there be wasted RX and TX when they are the same as the RX and TX on the FTDI?

Really useful in prototyping

I really like it. It is the best Arduino board for the small size projects. I recommend this product.

Nice but quirky

I bought 3/3V and 5 V versions of this and got an immediate download and run. Then they stopped taking downloads and it took 2 days to figure out what it was. In my circuit I was using one of the BT boards (Bluetooth) hooked up to the RX and TX pins. With that connected it will not program. Pull one of the two lines and it programs fine. It’s a little irritating but it works and the mini pro is the exact right size and does a great job. The only other thing I can say is that you can’t easily hook A4 and A5 to your breadboard if you want to use I2C. You have to solder wires in to A4 and A5 and plug the rest into your breadboard then plug the wires from A4 and A5 into your breadboard. A little irritating but if they were outside then the board would be bigger and it would not fit what I need. Overall I am happy with the mini pros I bought.

Hi, You’ll commonly want to disconnect any devices from your hardware serial when programming. The trick is to run software serial for your Bluetooth device. That will move the communication lines over to non hardware UART lines and will allow you to leave the BT hooked up when programming.

Works like it should

Works exactly like it should so 5 stars.

Love this form factor

Yes, there’s some additional educational overhead because it has less onboard, so this shouldn’t be your first arduino. But! It’s tiny and uses less power (good for batteries), and fits into wee little projects. This is my go-to device for my embedded systems projects, though I tend toward the simple, so I’m not driving huge light arrays or anything requiring a lot of computational horsepower. I mostly do small art projects or simple detectors. For those, this thing is way more than enough functionality.

Pro Mini 328

Useful as low-cost and already-wired processor in testing data communication (RS232) programming with a PC serial port by assembly-language and by the Arduino IDE

Perfect size and Perfect price..

I built my wife a birthday card using this board and some red LEDs. The Pro Mini is the perfect size to hide in a piece of cardboard along with all the wiring and a coin battery. She loved the card.. I constantly look for opportunities to use one of these little guys.. Great product!

Amazing capability at an incredible price point and small footprint

The Pro Mini is a powerful tool in an incredibly small footprint. It took me a bit to get going with it but there really are no great mysteries to solve. Just carefully follow the “Getting Started Tutorial” and you’ll be fine, even if you’re a newbie like me. For the price this little device can’t be beat!

Reliable chip

This low-voltage arduino can fit every tiny, cableless proyect you can imagine!

Homebrew drone made easy

This is the perfect little device, great for miniaturizing your projects. It did take me a while to figure out how to program (if programming with an Uno, for instance, you have to remove the ATmega).

Nice solution

This is a great way to get an MCU into a tiny project based on the easy-to-use Arduino platform. It’s very powerful and has the same numnber of i/o pads as the big boards. Getting the program on is a bit tricky, but not too terrible once you have the right set-up.

Great little computer

Versatile, I have used both the 3.3V and 5V versions for projects ranging from replacing a Dan Foss fan controller in diesel motorhomes to solar control to smart on/off switch functions in motorhomes.

Easy to program and very reliable, and cheap, what more could anyone ask?

Versatile and Easy to Use Device

Perfect for my various embedded projects, and at amazingly low cost.

i find that the “Sketch” programming approach used by the Arduino folks is great for most applications, but that the ability to embed lower-level code in the applications is also helpful for specialized applications, and, if necessary, to reduce application footprint and save flash space.

The flash capacity of this device (32 KB) is pretty good and having purchased a few of these from Sparkfun, the quality is excellent. I have never received a DOA device.

My current project is using this device and Sparkfun’s microSD level-shifting breakout to add an insane amount of storage (like 64GB) to an early 1980s industrial control Z80-based system that has just 16K RAM on board. Weird but fun. I have the hardware working and am developing the communication protocol and device driver routines.

I have also used this device with environmental sensors in a musical training application, where the device and sensors are embedded -within- a medieval instrument to provide wireless, real time feedback to the musician to help him/her improve their playing. Another “weird but fun” thing.

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