Has A2VGA been tried on higher bus speeds?

[caiannello]

Hi, I think this is a brilliant project!

I have a couple questions. (Apologies if these have already been asked and answered, I did search a while but didn't see them.)

1. Will the A2VGA work with higher system bus frequencies such as 4 MHz?
2. I read that a Pico 2 can be a drop-in replacement in a lot of Pico 1 projects. Might that be the case with A2VGA?

Geek on!
Craig I.

[markadev]

Hey there, thanks!

The simple answer to your first question about the higher bus speeds is: no, but it might be possible by overclocking the Pico. To be fair, I don't know of any Apple II peripheral card that could operate at those speeds either. Add-in accelerator cards normally slow down when they access anything over the Apple bus because all the logic and ROMs/RAMs on the mainboard are expecting a 1 MHz bus speed (it's why the good accelerator cards have their own local RAM). The IIGS has a similar slowed down cycle when accessing the peripheral slots or the IIe compatibility address space. Is your question in reference to the IIGS, or another Apple clone or homebrew system? (Disclaimer: The VGA card doesn't work on IIGS systems, but for a slightly different reason)

I have not tested a Pico 2 but it would probably work, assuming using the ARM cores. The RP2350 chip has one errata related to the GPIO pull resistors that I'd be worried about having a problem with but it's just a matter of trying it out. I'm grabbing a Pico 2 in my next parts order to try out. I'd say if you had a choice then use the original Pico because it's less expensive, will just work, and the extra features of the Pico 2 will just go to waste. If all you have is Pico 2's on hand then definitely socket it in case it doesn't work 😅

[caiannello]

Thanks for taking the time to answer my questions!

The reason for the bus speed question is that I was wondering about the feasibility of adapting your project to do video for other, non-apple 8-bit hobby systems. (In particular, I have a HD63C09 system, clocked at 3.58MHz, which is currently using a Yamaha V9958 for graphics, but those are getting more rare and expensive.) But the question might also apply to other 6502 systems such as the Atari 8-bits at 1.79MHz.

The reason I asked about the Pico 2 is that since it has double the SRAM, there might be an iterative path to enlarging the framebuffer to try to support modes offering more colors and/or features.

Cheers,
Craig I.

[markadev]

Gotcha. I think it will depend somewhat on the exact use case. The constraints imposed by the VGA card's current design means that one Pico core is dedicated to snooping and processing all of the Apple bus activity. If that one core can't keep up with the bus speed then it's basically unusable.

I'm not too familiar with the systems or ICs you've mentioned but I think you're hinting at two or three different use cases:

1. The Atari system, where you would want to work similar to the Apple VGA card, snooping bus activity and emulating the video output subsystem to construct a "perfect" RGB signal. This could probably be done even at 1.79 MHz bus speed. I imagine the Atari video subsystem doesn't have as many modes and soft switches as the Apple II so there's less to keep track of and Pico 1's are now qualified to run stably up to 200MHz so there's lots of headroom.

2. A replacement Yamaha chip for your homebrew system. This would be a bit more complicated because the Pico is no longer a passive observer but must handle a bus read within 280ns, which could be challenging even when running at 200MHz. But luckily that Yamaha chip has a WAIT pin to insert wait states so that could be what makes emulating this specific chip possible.

3. I'm not sure if your second paragraph is alluding to a custom GPU-like subsystem but that would be another use case, functioning similarly to case 2 but with the added flexibility that you can make the entire design asynchronous (thinking: CPU writes a register setting and then polls a BUSY bit for it to be applied). Again, this is similar to use case 2 in that it's a fully participating device on the bus but a clever design could reduce or remove the need for wait states, improving performance.

[Hubidrei]

Is there a plan to use ther RP2350B ? It should make things easier because it has more IOs and statemachines also more RAM. AFAIK the PIO bug does only occur in the A2 revision.

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________________________________ From: Craig Iannello ***@***.***> Sent: Wednesday, March 26, 2025 2:22 PM To: markadev/AppleII-VGA ***@***.***> Cc: Subscribed ***@***.***> Subject: [markadev/AppleII-VGA] Has A2VGA been tried on higher bus speeds? (Discussion #59) Hi, I think this is a brilliant project! I have a couple questions. (Apologies if these have already been asked and answered, I did search a while but didn't see them.) 1. Will the A2VGA work with higher system bus frequencies such as 4 MHz? 2. I read that a Pico 2 can be a drop-in replacement in a lot of Pico 1 projects. Might that be the case with A2VGA? Geek on! Craig I. — Reply to this email directly, view it on GitHub<#59>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ASAJXW6URXOZ4SAVL67O27D2WMLBVAVCNFSM6AAAAABZ3MLF2CVHI2DSMVQWIX3LMV43ERDJONRXK43TNFXW4OZYGEZTGNJVGM>. You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[markadev]

Hey there. Nope, I don't have any plans to make a new board design right now. It would be nice to have the extra GPIOs for access to a few more Apple bus signals but using the RP chips directly without the carrier board is a pretty big design change.