BMC built with another Raspberry Pi Pico instead of STM32?

[CHiPs44]

Hello,

I find the whole idea of the Neotron computer, especially the Pico one, as something that would quite fit many of the ideas I have (cf https://github.com/CHiPs44/YetAnotherPicoComputer).

Since the Pico is so good and cheap, why not replace the STM32 with another Pico for the BMC?

PS: my skills aren't too high for now, so pick that with a grain of salt, I am rediscovering micro-electronics (my initial degree) after 30+ years being mostly a web developer, and I only read parts of the tutorial on Rust, which let me a little skeptical for now...

[thejpster]

This is something I have considered in the past, and it's definitely an option for the next spin. However:

• A Pico is $4 and a bare STM32F030K6T6 is $1
• A bare RP2040 is $1 but you need a crystal and SPI flash, and it's a QFN so it's much harder to hand-solder than the 0.8mm pitch LQFP-32 STM32 (that's why it's not an LQFP-48 because they are 0.65mm pitch)
• The RP2040 is a strict 3.3V device, and so the PS/2 interface will need extra transistors or level shifters - possibly using more pins if you need to split input and output.
• It's kind of weird to have one $4 board running the whole operating system, including video and sound and file system and another $4 board doing keyboard and UART and PC-speaker beeps.
• On the plus side, a second Pico might be able to replace the MCP23S17 and 74LS138 3:8 decoder used for the chip-selects.

Should some future Pico 2 appear which had more RAM and more performance, you could imagine using a Pico 2 as the main CPU and a Pico 1 as the board management controller.

[CHiPs44]

Hello,

Thanks for the answer!

As I said before: "pick that with a grain of salt", cost and this 5V thing with PS/2 are sufficient to go for this STM32.

I know the Pico a bit, but nothing on STM32, so I consider to build everything with what I know.

And that would make the project very different to have 2 Picos cooperating at another level, for example having video on the second Pico, like https://github.com/picocomputer (2 Picos + 1 6502) or some high level ideas I had and wrote in https://github.com/CHiPs44/YetAnotherPicoComputer.

[thejpster]

There are an almost infinite number of computers you could make with a Pico. And I encourage everyone to take what exists and learn from it and make something new.

My requirements included having a memory mapped text buffer and framebuffer, like an IBM PC, which helped me rule out a bunch of potential designs - like doing the CPU and the video on two different chips. The RP2040 has no external bus and no memory mapped SPI RAM support, so I was basically limited to a single chip for CPU + Video. But other projects have different requirements. And other chips are available now and will be available in the future.