This is Gwenesis on Pico Held - A Sega Genesis/Mega Drive emulator

by bhzxx ported to the Pico Held Handheld (a RP2040 handheld).

Summary

This is a Sega Genesis/Mega Drive emulator running on the Pico Held - a RP2040 powered DIY opensource handheld. It can playback quite some Genesis games. Performance (especially sound output) majorly depends on the game played - YMMV.

Building

Since this is intended to be built with the Arduino IDE building is as simple as clicking on the "upload" button. However, you will need to install Earle Philhower's Raspberry Pi Pico Arduino core (no further dependencies are involved). Tested working with Arduino IDE 2.1.0

You can find a prebuilt binary in the artifacts folder.

Compiler switches

OVERCLOCK

Enable overclocking.

GWENESIS_AUDIO_SAMPLING_DIVISOR

This compiler switch (found in gwenesis_bus.h) determines the sound quality (to be more specific: the sound samples generated per second). Ranging from 1 (best quality) to 10 (very poor quality). The default ist 6. This has a huge impact on performance.

Playing games

!! UPDATE !!

There has been a transistion to the SD library by carlk3 which works around a previous bug where some data of the ROM failed to be read.

There now is an emulation manager which can be used to manage some options (like ROM selection, saving/loading games states, brightness settings and a couple more things).

The ROM manager currently supports English and German (select appropriate language header file in emumgr.cpp).

Since the Pico Held only features three buttons. The start button may be evoked by short pressing all three buttons together.

Once a ROM is loaded (i.e. flashed into the internal memory) a SD card is only necessary to load/save game states. ROMs go into /md/roms and game states will be saved to/loaded from /md/save.

!! ATTENTION !!

Due to a bug (in the SD/FAT library?) creating the save game directory randomly fails. So either create it yourself or try until the creation successfully completes. Once the directory has been created there should be no further issues.

Controlling emulation parameters

When in game press and hold all three buttons to reach the emulator menu.

You can then save/load game states as well as load new ROMs.

Note: ROMs will be loaded (byte swapped) into flash (for performance reasons). Therefore make sure, your flash size is at least the ROM size plus an additional 900K for the emulator code.

Sound

You can choose volume and the sound accuracy (i.e. either frame or cycle accurate sound generation). Changes to the sound accuracy should only be made before launching the game. Changing these settings while a game is running might crash the emulator or lead to unpredictable behavior. Choosing sound Off will disable sound generation and hence likely give you the most fps.

Brightness

Self-explainatory.

Settings

You can enable FPS display, change the button layout (TODO) and launch the bootloader.

Notes

I ported the emulator at the very beginning when I started designing the Pico Held and so the pplib (hardware layer) that comes with this emulator is to be considered as premature. Best would be if you did not look at it at all. I just have not had the time to update the emulator to the current pplib library. If you're interested in the pplib software library for the Pico Held you might want to take a look at the current version - which is also at a very early stage of development :)

If you do not want to use a SD card but rather flash (a) game(s) directly into flash, you may use one of the rom converters (in the romconverter subdir) to do the byte swapping for you.

Bugs and limitations

I found some bugs but did not have the time to fix them. There are random issues with the underlying Gwenesis emulator which usually result in a hard fault and are hard to track down.

Moreover, the RP2040 is not powerful enough to emulate all the Mega Drive hardware with sufficient performance. Therefore, I had to do some "tweaking" in order to improve the execution performance:

• overclocking the RP2040 approx. by a factor of 2 - Pimoroni does this by default with their Picosystem so this can't be too bad, right?!

• reducing the sampling rate in both the SN76489 as well as the YM2612 sound chip by a factor of 6 (see GWENESIS_AUDIO_SAMPLING_DIVISOR).

• making the sound generation frame accurate (instead of cycle accurate). This causes the Z80 and the sound chips to get out of sync - and in many games this leads to flaws in the sound output (e.g. Sonic I: No "Sega" voice at the beginning and many more). However, this increases the performance by as much as 25 fps and I do not see a way around this other than having no sound at all. However, you are free to choose cycle accurate audio generation from the emulator settings menu.

• distributing the load to both cores. However, RAM/ROM bandwidth becomes an issues and cores often stall waiting for the other core to finish access to the memory.

• A bug in the SD subsystem causes the creation of the save directory on a "fresh" SD card to occasionally fail. (see "Playing games")

Feel free to report bugs/open issues and I will try to come up with a fix. Contributors welcome :)

As the framerate is usually below 60 fps and varies over time, the sound output has to adapt to the current framerate. The pitch will be leveled up at high framerate and leveled down at lower ones so sound output might get audibly distorted (especially music). The sound output system will try to keep up with the video pace.

Running without the Pico Held

Of course, you can run this on basically any RP2040. If you have a ILI9341 LCD with parallel connection attached you might not even need to alter the code at all.

Just be sure to get the pin assignment right (see src/pplib/setup.h)

Credits

All credits go to bhzxx for his amazing work on the Gwenesis emulator! Credits to carlk3 for his SD/FatFS implementation on RP2040. As well as khoih-prog

Thanks to all!