PicoCart64 VS UltimateCart

There's an open source effort to build one with a Pi Pico: https://github.com/kbeckmann/PicoCart64/tree/develop

I have no clue how tested it is, and README mentions only up to 2MB ROMs, but it is a start.

The main idea is to make the cartridge as simple and cheap as possible. Contrary to Konrad's idea of multiplexed PSRAM chips and two RP2040, I decided to use one SPI flash memory chip and one RP2040. Modern flash chips allow to erase and flash data more than 100,000 times, which is more than enough for home use for many years. Since the RP2040 does not support SPI flash chips larger than 16MB, it was decided to use page mode with page switching through the Extended Address register (EA register).

PicoCart64: Nintendo 64 flash cart using a Raspberry Pi RP2040\ (30 comments)

The UnoCart is the Atari 800 version of this:

https://github.com/robinhedwards/UnoCart

Uses an STM32, a tight loop responds to changes on the Atari's address bus to emulate a ROM, all through STM32 GPIO. Atari 800 has a 1.8 MHz bus, but it's still cool that microcontrollers are fast enough for this to work.

Alternative is to use an FPGA, the UltimateCart:

https://github.com/robinhedwards/UltimateCart