A couple of projects to hook a Pico microcontroller board onto a 6502 bus here. In the one case the 1MHz bus of an Apple II, in the other case a 2MHz bus on an Acorn BBC micro.

The Raspberry Pi Pico has some interesting advantages to serve as a peripheral on a 6502 bus, and the disadvantage of being a 3V device and pretty much needed level shifters to interface.

Among the advantages: the GPIOs are driven by small fast programmable state machines called PIOs, which can handle some handshaking, bit shuffling, marshalling. They are connected to the onchip bus by FIFOs, and there's a DMA engine to move data around. There are two moderately fast ARM cores, which are simple enough to be deterministic. So, if hard deadlines need to be met, this is a good platform to have a look at.

Here is another one:

https://github.com/tjboldt/Apple2-IO-RPi

That's a nice find Andy, but note that it's a Pi project, not a Pico project. In that category, see also PiTubeDirect, which has been used with Acorn and Amstrad micros.
https://github.com/hoglet67/PiTubeDirect/wiki
http://www.cpcwiki.eu/forum/amstrad-cpc ... strad-cpc/

(The Raspberry Pi is an ARM-based computer, credit card sized or less, with SD card, USB, HDMI and in some models Ethernet and Wifi interfaces. The Pi Pico is smaller, is an ARM-based microcontroller dev board, with USB interface. Both have GPIOs running at 3 volt levels. In the Pico case, the dual ARMs and the PIO engines which control the GPIOs are highly deterministic and allow for fast accurate responses and cycle-counted code. In the Pi case, the ARM is higher performance with caches and a shared memory bus and is not great for sub-microsecond responses. However, the videocore CPU can just about keep up with a 2MHz bus interface.)

Hi Ed,

After posting I did seem to notice they weren't exactly the same. My apologies for stepping on your thread. And thanks for the compare and contrast.

No problem!

The PI pico looks like a great product... but with the current Silicon crisis, its hard to get hold of them. Digikey has 2pcs in stock, but otherwise it looks like one has to wait.

Thanks, this is very interesting.

I've been thinking about using a Pico as the 'third chip' in a minimal '02 system. Picos have a separate IO state machine, which makes it possible to provide a fast and stable '02 clock without any load on the Pico CPU, for instance. I think it's possible to make the Pico respond extremely fast to incoming IO, and make it act as a fast peripheral for the '02.

Slightly OT: it feels a little odd to consider using a 2-core 133MHz 32-bit computer with 256K RAM and 2MB flash as a 'peripheral'... But at $4.00 (if you can get it for that -- eScalpers had bought out the inventory) it is one of the less expensive components in an '02 system.

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In theory, there is no difference between theory and practice. In practice, there is. ...Jan van de Snepscheut

Yes, I think Pico is a good match for a smart peripheral, or possibly as a boot-and-glue device. The I/Os are a lot more amenable than, for example, the I/Os of a Teensy, IIUC.

And yes, with PiTubeDirect ported to Pico, we have a proof of concept of interfacing with a 2MHz 6502 style bus.

And yes, as with Pi Zero, the simple fact of a very low price makes it worth consideration.

Interesting times already - and I don't doubt that there'll be a successor, and indeed that the bare RP2040 microcontrollers will be available.

US$4.00 is actually a fortune these days. Consider that a couple of years ago there were many micro-controllers selling for less than $0.10:
https://cpldcpu.wordpress.com/2019/08/12/the-terrible-3-cent-mcu/. I haven't checked lately; many things have doubled in price since two years ago...

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In theory, there is no difference between theory and practice. In practice, there is. ...Jan van de Snepscheut

We should distinguish chip prices from board prices - and also volume prices for as-delivered hobby prices. But I think mainly, for me at least, what matters is not the absolute cheapest bill of materials, but whether or not it falls under some psychological threshold: £10, £50, £100.

The Micro Center stores here in the US seem to have the Pico's back in stock for $3.99 (in store only). I picked up 5 at the Michigan store at an introductory $1.99 sale price (in store only) awhile back. As far as I know the Micro Center stores are found in 16 states.

Mike, K8LH