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Arduino Pro-Mini ISP Programmer UNO

How to use an Arduino UNO to program a Pro-Mini

Glowing LED

The Arduino Pro-Mini is a tiny microcontroller board with capabilities similar to that of the UNO. It will work with a Vcc as little as 1.8v and with a little surgery it is possible to reduce the current drawn in power down sleep mode to just 5µA making it possible to run some devices for more than a year on a single 3v button cell. The best part about the Pro-Mini is the price. Boards can be purchased for less than 2 GBP or 2 USD making them useful for all kinds of things.

To keep the size down the USB socket has been omitted from the Pro-Mini board. The Pro-Mini can still be programmed through it's serial port using the onboard bootloader but you need a USB to serial adapter before you can connect it to your PC.

There is another annoying difference between the Pro-Mini and the UNO. The bootloaders are not the same and they behave slightly differently. As a result of this I got into trouble when I loaded my low power sleep program. It went to sleep ok but the bootloader forced it into a perpetual reset loop. While it was in that state I could not program it with the serial port. Time for a change of plan...

Programming with an Arduino ISP

The Pro-Mini can be programmed using an In System Programmer or ISP which bypasses the bootloader altogether. The ISP connects to a USB port on your PC and the ISP connector on the Pro-Mini. But hold on the Pro-Mini doesn't have an ISP connector so you have to connect the wires on a breadboard or something. Still not an ideal solution.

A purpose built Arduino ISP module isn't that easy to find and it costs about twice the price of a UNO. It turns out that you can use an Arduino UNO as an ISP so why not use one for programming the Pro-Mini.

Using an Arduino UNO as an ISP

There is a handy sketch in the examples section of the IDE that turns the UNO into a programmer. Once loaded and running on the UNO all you have to do is make the appropriate connections between the UNO and the Pro-Mini and you have the ability to program not only the application code but the bootloader and configuration fuses of the Pro-Mini too. It can also be used to program most other Arduinos that have ISP connectors.

Arduino UNO Pro-Mini ISP shield

I decided to build a shield rather than just lash up a few wires. That way I can easily use it over and over. I used Veroboard or copper stripboard to make the connections and mount terminal strips into which you can plug in a Pro-Mini. I've also included a 6 pin ISP connector on the shield so that I can program devices that have ISP connectors. You will find the layout for the shield later in this article.

Pro-Mini programming shield mounted on UNO

Click to the circuit diagram page for the circuit diagram for the Arduino UNO ISP shield.

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