Disclaimer, warnings, caution

Don't do this, don't read this, close this page, because what is following will break your Arduino board, it will disintegrate it, it won't work anymore, you'll just have to throw it away. Unusable. This is extreme Arduino, this is Forbidden Arduino. Do it at your own risk, if you completely and undoubtedly know what you're doing, because I'm not responsible in anyway for anything that would occur before, while and after, for decades and decades. And much more.

This is not for the faint of heart.

You've been warned.

Twice.

Jaluino Cuckoo PCB design

After several trials, Jaluino Cuckoo PCB has been designed. This wasn't easy, due to the inherent physical constraints of this design: Cuckoo must be plugged flawlessly onto Arduino while still allowing shields connections. There are some spaces between Atmega connector, crystal, push-button, and shield connector, all were used. This is a double-side PCB, here are some pictures of the final result.

Cuckoo PCB, top view

Cuckoo PCB, bottom view

Most components are on the top side, as usual,only a LED is visible on the bottom side, but this PCB will be plugged upside down, so the bottom side will be the visible one (so the LED will be visible too...).

PCB for real

Thanks to Vasile Surducan, I've been able to get two Cuckoo PCB units, made on a CNC router.

There are four vias in Cuckoo PCB, linking top and bottom sides. Because used CNC doesn't provide through-hole technology, I had to solder a small wire in vias to connect sides together. This really challenged my (poor) soldering skills (and reminds me, if ever needed, to stop trying surface soldering)...

Soldering other components is straight forward. Jaluino Cuckoo is almost finished, only PIC 18F25K22 is missing. For now, it's not needed as I just want to make sure the concept is viable in terms of physical constraints. Now, let me introduce: Cuckoo !

Cuckoo top side. PIC 18F25K22 is missing.. Male connectors will plugged onto Arduino board.

Cuckoo bottom side. This is the visible side when plugged, that's why a LED is on this face.

Arduino/Cuckoo, step by step surgery guide

Cuckoo's ready, now let's "take care" of Arduino, our parasitic host (victim). Look at your Arduino this is the last time you'll see it like this...
First, decerebrate Arduino by removing Atmega chip. Use a small screwdriver, act on both sides and, be careful, try not bending pins too much. If surgery fails, you may need to go back (if you can).
Let's try to plug Cuckoo! Well... not good news. Pin headers seem to be too big to fit in Atmega socket. It"s very hard to plug it. ICSP connector will also be problem as it can be in contact with Cuckoo PCB.
More, crystal caps touch Arduino's crystal.
Not a problem, caps can "safely" be bended.
Now they aren't in contact with crystal anymore. One problem solved.
Still, when Cuckoo's plugged, shields can't be connected anymore, first because of the LED. I initially wanted to use a squared LED and bend it to preserve space above it, but it seems it's not even possible. LED has to be moved on the other side.   Even without this LED on the bottom side (remeber, the visible one), shields can't be connected. Cuckoo has to be plugged some more.
I decided to plug pin headers directly on Atmega socket, without PCB, then plug the PCB and solder headers.
Result seems promising, pins are shorter than before. Cutting remaining wire on the side will save some more space for shields.
About ICSP connector, I can't cut Cuckoo's PCB as some tracks are present. But... I can unsolder it! Dirty... This will have to be fixed in another version. On Cuckoo, I've also removed jumpers, I'll have to find smaller ones...
So, how is it now ? Much better...
Shields can now be connected...
... though very little space is available
Removing plastic rows helps Cuckoo to be plugged some more.
More space above, but less space under! PIC18F25K22 will probably need to be directly soldered, without any socket...

Conclusions

Main goal was to verify if Cuckoo concept, unplug Atmega and plug a small daughter board instead, is a viable one. That's not easy, that's tricky, but it seems possible. Main problem comes from pin headers. They are too big, making Cuckoo very hard to plug. And I think some more space could be saved if using another type of pin headers (any idea ?). Last photo from surgery steps shows Cuckoo when plugged at maximum. LED is blocking as it's in contact with Arduino's PCB. If moved somewhere else, when it could be bend, and/or if using a square shaped LED, Cuckoo might be plugged some more. On the other side, PIC18F25K22 won't any socket, but I think this is an expensive luxury in this kind of constraint design...

Epilogue

Before: Arduino, fresh, innocent, naive.

After: Jaluino Cuckoo parasite slowly eating Arduino host. That's Forbidden Arduino!