Bridging J1 on a Pro Micro

Although the Pro Micro is one of the most popular development boards, with the plethora of documentation and examples, the solder pads labelled as J1 or SJ1 (solder jumper) remain a bit of a mistery for many builders. Let's explain its use!

In this write-up I embark on a mission to bust some myths and explain the effects of soldering the J1 bridge.

Pro Micro voltage regulation

The Pro Micro was designed to run on batteries and it has a built-in voltage regulator for this very reason.

Feeding it with the regulated +5.0V from USB (UVCC) there is a penalty: the voltage regulator will output only +4.9V (VCC).

The microcontroller should run well on 4.9V, but bypassing the voltage regulator by bridging J1 with solder is an option too.

Voltage drop with 5V boards

This slight voltage drop is only present on 5V variants, usually with the board connected to your PC/laptop throught USB. That's the price of using a regulator.

With a different power source, like 6V to 12V the VCC is 5V. (Make sure the J1 jumper is disconnected in this case).

(With a 3.3V Pro Micro the voltage regulator can work with as little as 0.5V between input and output. This means that as long as the input is above 3.3+0.5 = 3.8V, you will get 3.3V output. And that's exactly the case with an USB power source this time.)

3.3V vs. 5V Pro Micro differences

There are different variants of the Pro Micro, the most common being the 5V, but there's a 3.3V version (and various types with slightly different form factors) too.

There are two main differences between these two variants:

• 5V or 3.3V regulator,
• 16 or 8MHz crystal.

So the difference is in the type of voltage regulator and crystal. That's it. Other than that, the two variants are identic.

(The original Arduino Micro and the Arduino UNO have two regulators for both 5V and 3.3V work. But with the Pro Micro it's one or the other.)

On a 3.3V the AVR is limited to 8 MHz (the firmware needs to be made with this in mind), thus, you will see a 8 MHz crystal on the board, instead of the 16 MHz one.

(Checking the crystal is a good way to identify a 3.3V Pro micro.)

What does the voltage regulator part do?

Let's check the voltage regulator part along with some accompanying components - cut from the Pro Micro schematics:

The UVCC (USB power/VBUS) goes through a 500 mA fuse (F1) and diode (D2) before it meets with the RAW pin and into the regulator. (This might be the case only on some products, e.g. the Sparkfun version. On cheap Chinese boards this fuse may not be present.)

Without this protection there were certain cases (RAW voltage fed into the USB) potentially blowing up the PC's USB interface circuit.

The D2 diode protects both the RAW power from being fed into the USB device it's connected to and into the MCU/board if the SJ1 jumper is closed.

J1 myths

J1 indicates 3.3V vs. 5V

Not entirely true. There are two main variants of the Pro Micro, the most common 5V and a 3.3V version.

Even the Pro Micro schematics claims about the SJ1: "Closed for 5V Board" and "Open for 3.3V Board". This is misleading: the jumper is not for chosing the working voltage.

It is true for the 3.3V one connected to regulated USB: this jumper should be open to chose the regulator path. Otherwise (closed) it gets 5V and acts like a 5V variant with 8 MHz crystal.

But as we have seen above, the 5V variant can both have an open or closed J1.

You should close J1 on a 5V Pro Micro

You don't have to, but you got this option if:

• If you have a board working on 5V, and
• you want the VCC pin have the proper 5V (without voltage drop),
• or you want more juice to drive external modules,
• and your input voltage is already regulated to 5V (USB).

In this case you have the option to bypass the voltage regulator. However, in the overwhelming majority of situations you won't need this.

Conclusion

You may consider bridging the J1 solder jumper of a Pro Micro if you feed it regulated 5V (e.g. USB). In this case you can circumvent the built-in voltage regulator and prevent the voltage drop resulting in 4.9V VCC.

Ready to building? Check the Pro Micro pinout or give a chance to a Teensy when more pins are needed (Pro Micro vs. Teensy comparison). In addition to these two popular choices, there's an ever growing list of common controllers with similar form factor (not just for keyboard projects).