Spresense as ISP

I am one of the winners of Sony's Make It Better contest as announced a month and a half ago. While waiting for my prize to be shipped, I decided to do another project. I'm not going to repeat someone's work, I just wanted to clearly document what worked for me and what to watch out for. Here is what I have and your steps may vary if you are using a different board or not using Linux.

Here are the specs.

Make sure you have the correct voltage on Spresense.

Build a harness to connect the boards together. Male side will go to Spresense and Female side will go to Arduino Pro Mini. You can also use ready to use jumper cables if you are not cheap like me.

Note that RST is not pin 10 on Arduino Pro Mini so we will have to offset that Female.

Hardware

Start by plugging in the Male side of the harness to pins 10, 11, 12, and 13.

Next, plug the Female side of the harness to Arduino Pro Mini. Make sure you offset the Red wire as described before.

Connect the offset Red wire to RST pin.

Connect Grounds of both board. I used Black Alligator Clips.

Finally, connect Vout pin of Spresense to Vin in Arduino Pro Mini. I used White Alligator Clips.

Software

Once we are done with Hardware setup and connected to computer with Arduino IDE we can start on Software loading. As soon as you connect the boards to your computer you will notice there is a slow flashing LED on Arduino Pro Mini. That is because the board is delivered with Blink sketch already loaded as you can see in the animation at the beginning of this tutorial.

I'm using Out Of The Box Blink sketch to increase the flashing speed of on-board LED. Establish a baseline by loading the sketch to Spresense first.

Here are the changes I made to the sketch.

Note that changes will not be saved since we are using OOTB file, but we still can load the sketch to Spresense.

Everything worked and green LED on Spresense started blinking fast.

Keep this one open, we will come back to it later.

The next OOTB sketch we need to load is ArduinoISP

Needed to uncomment one line to use pins 11, 12, and 13.

Loading the sketch was successful. Close the file without saving since we no longer need it.

From this point on you can load any sketch to Arduino Pro Mini via Spresense if you remember to press Shift while clicking on round Upload button in the top left corner. Going back to Blink sketch we need to change the board to Arduino Mini.

Yes, I realize that there is an option for "Arduino Pro or Pro Mini". I just wanted to save you some time and frustration since it does not work.

Note that the following step is necessary even though it looks like an error. Performing this step will save you more time and frustration. Click on Upload button with out Shift. You can also press Ctrl+U.

Now you can upload correctly using Ctrl+Shift+U or from the menu below.

If you get the following screenshot while expecting one of the previous two that mean you have wiring issue.