In the world of Arduino boards, the normal programming procedure is to fire up the Arduino IDE (or perhaps, Visual Studio Code), plug in your board via a "normal" USB connector, select the correct options, and hit the upload button. The Arduino Pro Mini, however, may seem to be missing something upon inspection: it has no USB port.
Arduino Pro Mini USB serial
This little board has a series of six pins on one side, into which you can plug an FTDI cable for serial data transfer. This does away with the onboard USB-to-Serial conversion, and offloads onto a device that can be plugged in and powered up only as needed.
This configuration also does away with the expense of the onboard chip itself, as well as the extra power requirements for something needed only intermittently. The downside is, you'll need a special cable for making the connection, which can pose a bit of a difficulty for beginners.
While it won't supplant the Uno as the de facto first maker board, if you are a bit more advanced and need something small, efficient, and cheap, the Arduino Pro Mini is a great choice. At 1.3" x .70" or .91 in2, its footprint is roughly 1/6th that of the Arduino Uno, with a thinner PCB. This should shave off a bit of additional weight -- if that's critical for your application.
These boards are available in the 3.3V version, and come in a 5V version as well. So, if you happen to, say, crack one mounted to your drone in a crash, at least that component is easily replaceable. These boards can also be quite power-efficient, which we'll discuss shortly.
Given the benefits of these little boards, let's first address how to program them.
Arduino Pro Mini programming
In the world of Arduino, it seems that once you can blink the LED as a "hello world," everything else is just details. For my experimentation here, I used an FTDI Serial TTL-232 USB Cable from Adafruit Industries. It features a 5V power supply, along with RX, TX, RTS, and CTS pins that operate at 3V logic levels, but are 5V tolerant. With the exception of the 5V pin, these cables are compatible with both the 3.3V or 5V dev boards. More on programming the 3.3V board below.
Programming pins conveniently line up with the Pro Mini’s header, so the order is simply a matter of plugging it in the correct way. For orientation, when using 90º headers, place the solid side of the header up, as I did, or remember that black is ground and green is RTS.
Plug the cable into your computer, and select the proper port, as well as the voltage and processor type. Hit the upload button, and a few seconds later, your board should be obediently blinking. Here, I’m using a Macintosh, but Linux or Windows should be similar.
Arduino Pro Mini 3.3V or 5V?
If you're using a 3.3V board, it's proper to use a device with a 3.3V source, such as this FT232RL UART Breakout Board that can switch between 3.3V and 5V. That being said, I've been able to plug the 3.3V board using the same 5V cable that I did for the 5V board and program it without issue. While it seems this may not be a major problem, do this at your own risk.
One potential solution would be to run the +5V pin through the RAW voltage regulator input to knock it down to 3.3V. I was able to do this successfully as well, though it does make programming slightly less convenient.
Finally, note that it's possible to use a programmer that doesn't have dedicated CTS and DTR pins, or to not hook them up. This requires you to hit the reset button on the Arduino at a specific time when uploading, so it isn't ideal.
Arduino Pro Mini advantage: low power
Besides the small cost and size, the other benefit to using one of these boards is that they use much less power than an Arduino Uno. As outlined in this Arduino power/sleep article, the Uno uses 47mA when running a blank program. The Arduino Pro Mini 5V consumes 16mA under those conditions, just over a third of that value.
In testing for this article, I used the “Blink” sketch for each, and found each oscillated between:
· 5V board @ 5V input Current: 18mA & 26mA Power: .09W & .013W
· 3.3V @ 3.3V input Current: 4mA & 8mA Power: .0132W & .0264W
So, when the LED is off, the 3.3V version uses under one quarter of the current of the 5V version. Consider that power = volts x current, meaning that the low value for the 5V board is .018A x 5V = .09W, while the low value for the 3.3V board is .004A x 3.3V = .0132W. This means during the low power cycle (presumably when the LED is off), the 5V version is using 6.8 times the power of its 3.3V equivalent.
While using 3.3V instead of 5V can present some complications, for battery-driven applications, the lower voltage may be well worth it. For that matter, it's easy to connect a 3.7V LiPo battery to the 3.3V version via the RAW input, while the 5V version is questionable, at best.
If you're using the voltage converter to drop inputs to a usable level, I did a few power tests on that as well, running the same blink sketch:
· 5V board @ 9V input Current: 16.75mA & 25.0mA Power: .15W & .225W
· 3.3V board @9V input Current: 3.9mA & 7.6mA Power: .035W & .068W
While current numbers are roughly equivalent to feeding the proper voltage directly into the 5V & 3.3V pins, keep in mind that the voltages have been raised, making the total power input less efficient. Of course, if you're really concerned about low power design, check out the power/sleep article linked above for a few more tricks and ideas.
Distinguishing Arduino Pro Mini 3.3V from 5V
If you don't know what board you have, the discussion of 5V vs. 3.3V is kind of moot. The simplest way to know is to look on the back and see what rating is checked, as shown below:
Unfortunately, this appears to be hand-marked, and could potentially be worn off. If you need further verification, you can check out the voltage regulator. While it's entirely possible that different boards could use different components, the 3.3V version here says, "KB33," while the 5V version says, "KB50." The oscillators are marked differently, as well, though the meaning is less obvious.
Arduino Pro Mini: great for quick development
If you want an advanced, low-power microcontroller board, the Pro Mini can be an excellent option. Small, inexpensive, and low-power, it provides more built-in possibilities than using a microcontroller on its own, allowing for quick project development.
For even lower prices and power requirements, though with fewer built-in capabilities and more programming hoops, you might instead check out the ATtiny85 tutorial or the more versatile ATtiny84 tutorial.
