Making a FRAM (Batteryless Save) MBC3 Flash Cart with RTC
Channel: makho
Date: 2019-09-22

Functionally, this cart is identical to the Crystal based flash carts I've made previously: https://youtu.be/on1YissRnM4 except that when/if the battery dies in this cart, the save data is still retained. These carts are a bit more difficult to make as they require transferring components from one PCB to another and they are a bit more expensive since the use FRAM instead of the original SRAM. 

See this page for more info on what the cart is: https://www.retroreboot.net/HTML/projectpage.html

This cart requires making custom 0.8MM custom PCBs. I don't recommend getting white. Whatever you do get though, make sure you get ENIG finish (gold plating). You don't want HASL (solder finish) on the pins as those will wear over time and corrode and ruin your Game Boy. You can get the specific board I used and see the BOM here: https://www.retroreboot.net/HTML/downloadslist.html but I'd actually recommend using MrHDR's version here instead: https://github.com/MrHDR/MBC3-Flashcart

The retroreboot cart DOES work just fine, but it does have some compromises that are largely fixed in HDR's version. The biggest issue is that the RTC crystal is too close to the top of the board but HDR moved it down and made a cutout so that the case closes with no issues. The next is that 1206 components are a lot more expensive than their 0603 counterparts. This board is sized for 0603 components instead. Finally, HDR's version has all the data lines hooked up to the MBC3 chip instead of just the required lines. In theory, this means up to 4MB support but only MBC30 mappers support that and they have a different pinout. These carts only work with 2MB chips. 

I mistakenly mentioned a few times that HDR's carts use 0805 components. They use 0603. My opinion is unchanged though, I like working with the smaller ones better and they are still a lot cheaper in small quantities.