ESP32 Game Boy Printer

This project lets you rescue all those precious childhood pictures you snapped with the Game Boy Camera. Simply use an ESP32 module, a level shifter and ~~a WiFi enabled device with a web browser~~ your smartphone to save those snaps before the camera's on-board battery fails.
Alternatively, you could go and do some 2-bit street photography and earn some serious Hipster Cred[tm] on Instagram.

Install and set up the Espressif IoT Development Framework.
Study the documentation!
Clone this project and initialise it by calling submodule update --init --recursive.
Run make defconfig.
Run make erase_flash.
Run make flash monitor.
If you run into problems during steps 4 - 6, refer to step 2.
Connect the Game Boy to the ESP using a 3.3V <-> 5V logic level shifter.
Print away.
Connect your phone to the WiFi network GB-Printer.
Point your web browser to http://192.168.4.1.
Revel in the 160x144x4 retro glory.
???
Profit!

How to Connect

Consult the Game Boy Link pin-out and connect SO to GPIO 13, SI to GPIO 4 and SC to GPIO 14. If you are not using bi-directional level shifters, make sure that for SO and SC the signal direction is from the Game Boy (GB) to the ESP and for SI it is from ESP to the GB. You will also have to connect GND on both ends and both VDDs to the respective reference voltage inputs.
N.B. Do not try to power the ESP from the Game Boy. Most ESP32 modules out there use inefficient linear regulators and can pull more than 500mA of current.

Usage

Once the ESP has received a complete image from the GB, it will encode it as a PNG and store it in flash. Stored images can be viewed at and downloaded from the HTTP server at address 192.168.4.1. Images are named imgxxxxx.png where xxxxx is a sequence number counting up from 0. New images are stored using the first free number found, starting at the last index used.

On the web page there are two links above the displayed images. Delete all pictures will remove all stored images without resetting the sequence counter. So, if you hit this link after printing images 0 to 22, the next image printed will still be saved as img00023.png. This should help preventing accidentally overwriting older images when downloading newer ones.

Reset sequence numbering resets the sequence counter to 0, so new images are again saved at the lowest index available. So, for example, let us say you printed images 0 to 22, then hit Delete all pictures. Now you continue printing images from 23 to 41. If you hit Reset sequence numbering, the next image will be saved as img00000.png. If you keep printing, image numbers will go up until they reach 22. Since number 23 to 41 are already taken, the next image will have number 42.

Limitations and Quirks

The code has only been tested against the Game Boy Pocket with the Camera module and might not work with other versions of the Game Boy or different game modules. The compressed image transfer format is not supported at all. Without a way of testing there was no point in implementing it.

Since the whole protocol is bit-banged, it is susceptible to delays caused by other higher priority tasks running on the ESP. This means mostly the WiFi task, so avoid connecting devices or accessing the web server while printing to the ESP.

Error detection has been designed to err on the side of caution, so the ESP might signal a transfer error to the GB, even though the image has been transferred and saved correctly. This was deemed preferable to silently losing data.

Due to the way filesystem access and caching directives are handled in libesphttpd, browsers might cache images that have been deleted and re-written with different data. Do not be surprised to see unexpected images after deleting all images and resetting the image sequence numbering. Clearing the browser cache should fix this problem.

Security Considerations

There is no security, only Zuul. Well, not even Zuul. Everything is accessible on an open WiFi network and HTTP server. Do not use this to pull your d*ck pics from your Game Boy Camera.

Acknowledgments

This Project was originally based on applefreak's esp8266-gameboy-printer, but the only part that survived was the style.css, so thanks for that.
PNG encoding is done using Lode Vandevenne's LodePNG.
Web server functionality is provided by Chris Morgan's ESP32 fork of Jeroen Domburg's libesphttpd.