Programmed Bank Switching

[RalphSommerer]

I am looking for a way to switch between (for now two) different ROMs at runtime, e.g. by reading a certain (possibly unused) address. Is OneROM's internal processing fast enough to accommodate an additional address check? Judging from a cursory reading of the techical details, it appears that the timing is rather tight. The target system is a 8085-based synthesizer.

[artgodwin]

Do you have control over the synthesizer code or must you use existing code ? If you can craft the instructions used during the changeover you can provide extra time for processing : for instance, at the trigger address you could arrange a RET to be fetched. The processor will then do two RAM read cycles to pop the stack before fetching more code (from the address that was stacked, which can be arranged to be a safe restart).

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On Sat, Mar 14, 2026 at 4:49 PM Ralph Sommerer ***@***.***> wrote: I am looking for a way to switch between (for now two) different ROMs at runtime, e.g. by reading a certain (possibly unused) address. Is OneROM's internal processing fast enough to accommodate an additional address check? Judging from a cursory reading of the techical details, it appears that the timing is rather tight. The target system is a 8085-based synthesizer. — Reply to this email directly, view it on GitHub <#176>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAFFOGPM4GH27DPKQBCZBDT4QWESFAVCNFSM6AAAAACWR7JYCCVHI2DSMVQWIX3LMV43ERDJONRXK43TNFXW4OZZGY2DKNBTGM> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[piersfinlayson]

All the One ROM Fires (the red, RP2350 based ones) now use a completely autonomous PIO&DMA chain to serve the ROM. This leaves both cores free for other things.

I am in the process of adding plugin support so that users can write their own code that will run on one or both of these unused cores during runtime, to perform custom operations.

I depends a little on the specific ROM type you're emulating here, but the easiest way to switch between two ROMs programmatically like this is to

• have both ROMs loaded into the RP2350's SRAM
• have a "spare" GPIO as one of the group of address GPIOs that are read and used as an index into RAM
• toggle the read state of that GPIO using the RP2350 hardware registers (there is a register for this specific purpose - to make it read a 1 or a 0 no matter what signal the GPIO actually has on it.

Actually, there's another approach I recently thought of. Have the two ROM images at different locations in RAM, and then have the PIOs change the base address they are giving DMA to look up bytes from.

The most tricky part of what you want to do is monitor the address lines for a trigger sequence. I've been thinking about this off an on for over 6 months now but haven't got around to prototyping it. It might require some additional base firmware feature, or a modification of the PIO/DMA algorithms.

There's lots of other uses for this type of function (it could essentially become a general communications transport layer between host CPU and One ROM's MCU), so I do plan to get to this.

[electricwool]

suggestion: rom switching triggered by a cli command? or if the latency is acceptable, simply use the CLI to write the desired rom using the control command?

[piersfinlayson]

Yes had something like this in mind - hold multiple ROMs in RAM, and then switch between them by updating the base RAM address in the PIO. This has the advantage of being atomic - the entire new RAM is available at once, as opposed to the updating the live image where only every byte is updated atomically.

[ewanuno]

this is very relevant to what i am planning on doing with oneroms in the future (my first jlcpcb order of 28pin oneroms is on its way.)

the first thing i want to try is swapping whole whole rom images, using two buttons to change between a range of eprom images, or maybe even an encoder.

but further down the road i would also be interested in swapping only certain ranges of a rom image.
i have some drum machines which use a 32pin eprom like a 27c010 or 27c040 to store a collection of drum sounds, it would be great if i could swap certain sections of the ROM individually.

[jamiegrier7-alt]

I've been doing this with a Casio RZ-1, swapping sections of the rom image live using the usb plugin , works well

[bricox54]

Hello,
My brother and I developed a web browser-based Bank changer using an ESP32-C3 super mini and 1 super-capacitor, 0.1F.
See that, in the french Forum "https://forum.system-cfg.com" : (with schematic and block diagram)
https://forum.system-cfg.com/viewtopic.php?p=284431&sid=90cfac6168eb4fd0ca3d1905f1e065c3#p284431
All sizes and all hosts One Rom compatible.

[piersfinlayson]

That's very cool - it's great to have wifi access to switching ROM images!

In response to one of your recent questions in that thread, last time I measured it, One ROM was ready to read addresses and serve data in around 17ms from power being applied. I haven't measured it for a while, and it's probably got worse, but I can probably improve it if necessary. I'd be interested to know what your measurements indicate, if you do test.

The host-based ROM switching should work for most ROM types, and for at least 24 and 28 pin One ROMs, and hopefully for all types of CPUs. I looked at the code today and realised that it's not limited to just 2364 ROMs like I thought 😳 - that was an older implementation I was remembering.