Pico Micro Mac (pico-umac)

v0.1 15 June 2024

This project embeds the umac Mac 128K emulator project into a Raspberry Pi Pico microcontroller. At long last, the worst Macintosh in a cheap, portable form factor!

It has features, many features, the best features:

• Outputs VGA 640x480@60Hz, monochrome, using three resistors
• USB HID keyboard and mouse
• Read-only disc image in flash (your creations are ephemeral, like life itself)

Great features. It even doesn't hang at random! (Anymore.)

So anyway, you can build this project yourself for less than the cost of a beer! You'll need at least a RPi Pico board, a VGA monitor (or VGA-HDMI adapter), a USB mouse (and maybe a USB keyboard/hub), plus a couple of cheap components.

Software prerequisites

Install and build umac first. It'll give you a preview of the fun to come, plus is required to generate a patched ROM image.

Build essentials

• git submodules * Clone the repo with --recursive, or git submodule update --init --recursive
• Install/set up the Pico/RP2040 SDK

Do the initial Pico SDK cmake setup into an out-of-tree build dir:

mkdir build
(cd build ; PICO_SDK_PATH=/path/to/sdk cmake ..)

ROM image

The flow is to use umac installed on your workstation (e.g. Linux, but WSL may work too) to prepare a patched ROM image.

umac is passed the 4D1F8172 MacPlusv3 ROM, and -W to write the post-patching binary out:

~/code/umac$ ./main -r '4D1F8172 - MacPlus v3.ROM' -W rom.bin

Disc image

Grab a Macintosh system disc from somewhere. A 400K or 800K floppy image works just fine, up to System 3.2 (the last version to support Mac128Ks). I've used images from https://winworldpc.com/product/mac-os-0-6/system-3x but also check the various forums and MacintoshRepository. See the umac README for info on formats (it needs to be raw data without header).

Let's call this disc.bin.

Putting it together, and building

Given the rom.bin and disc.bin prepared above, you can now generate includes from them and perform the build:

mkdir incbin
xxd -i < rom.bin > incbin/umac-rom.h
xxd -i < disc.bin > incbin/umac-disc.h
make -C build

You'll get a build/firmware.uf2 out the other end. Flash this to your Pico: e.g. plug it in with button held/drag/drop. (When iterating/testing during development, unplugging the OTG cable each time is a pain – I ended up moving to SWD probe programming.)

The LED should flash at about 2Hz once powered up.

Hardware contruction

It's a simple circuit in terms of having few components: just the Pico, with three series resistors and a VGA connection, and DC power. However, if you're not comfortable soldering then don't choose this as your first project: I don't want you to zap your mouse

Disclaimer: This is a hardware project with zero warranty. All due care has been taken in design/docs, but if you choose to build it then I disclaim any responsibility for your hardware or personal safety.

With that out of the way...

Theory of operation

Three 3.3V GPIO pins are driven by PIO to give VSYNC, HSYNC, and video out signals.

The syncs are in many similar projects driven directly from GPIO, but here I suggest a 66Ω series resistor on each in order to keep the voltages at the VGA end (presumably into 75Ω termination?) in the correct range.

For the video output, one GPIO drives R,G,B channels for mono/white output. A 100Ω resistor gives roughly 0.7V (max intensity) into 3*75Ω signals.

That's it... power in, USB adapter.

Pinout and circuit

Parts needed:

• Pico/RP2040 board
• USB OTG micro-B to A adapter
• USB keyboard, mouse (and hub, if not integrated)
• 5V DC supply (600mA+), and maybe a DC jack
• 100Ω resistor
• 2x 66Ω resistors
• VGA DB15 connector, or janky chopped VGA cable

Pins are given for a RPi Pico board, but this will work on any RP2040 board with 2MB+ flash as long as all required GPIOs are pinned out:

GPIO/pin	Pico pin	Usage
GP0	1	UART0 TX
GP1	2	UART0 RX
GP18	24	Video output
GP19	25	VSYNC
GP21	27	HSYNC
Gnd	23, 28	Video ground
VBUS (5V)	40	+5V supply
Gnd	38	Supply ground

Method:

• Wire 5V supply to VBUS/Gnd
• Video output --> 100Ω --> VGA RGB (pins 1,2,3) all connected together
• HSYNC --> 66Ω --> VGA pin 13
• VSYNC --> 66Ω --> VGA pin 14
• Video ground --> VGA grounds (pins 5-8, 10)

If you don't have exactly 100Ω, using slightly more is OK but display will be dimmer. If you don't have 66Ω for the syncs, connecting them directly is "probably OK", but YMMV.

Test your connections: the key part is not getting over 0.7V into your VGA connector's signals.

Connect USB mouse, and keyboard if you like, and power up.

Software

Both CPU cores are used, and are overclocked (blush) to 250MHz so that Missile Command is enjoyable to play.

The umac emulator and video output runs on core 1, and core 0 deals with USB HID input. Video DMA is initialised pointing to the framebuffer in the Mac's RAM.

Other than that, it's just a main loop in main.c shuffling things into umac.

Quite a lot of optimisation has been done in umac and Musashi to get performance up on Cortex-M0+ and the RP2040, like careful location of certain routines in RAM, ensuring inlining/constants can be foldeed, etc. It's 5x faster than it was at the beginning.

The top-level project might be a useful framework for other emulators, or other projects that need USB HID input and a framebuffer (e.g. a VT220 emulator!).

The USB HID code is largely stolen from the TinyUSB example, but shows how in practice you might capture keypresses/deal with mouse events.

Video

The video system is pretty good and IMHO worth stealing for other projects: It uses one PIO state machine and 3 DMA channels to provide a rock-solid bitmapped 1BPP 640x480 video output. The Mac 512x342 framebuffer is centred inside this by using horizontal blanking regions (programmed into the line scan-out) and vertical blanking areas from a dummy "always black" mini-framebuffer.

It supports (at build time) flexible resolutions/timings. The one caveat (or advantage?) is that it uses an HSYNC IRQ routine to recalculate the next DMA buffer pointer; doing this at scan-time costs about 1% of the CPU time (on core 1). However, it could be used to generate video on-the-fly from characters/tiles without a true framebuffer.

I'm considering improvements to the video system:

• Supporting multiple BPP/colour output
• Implement the rest of DE/display valid strobe support, making driving LCDs possible.
• Using a video DMA address list and another DMA channel to reduce the IRQ frequency (CPU overhead) to per-frame, at the cost of a couple of KB of RAM.

Licence

hid.c and tusb_config.h are based on code from the TinyUSB project, which is Copyright (c) 2019, 2021 Ha Thach (tinyusb.org) and released under the MIT licence.

The remainder of the code is released under the MIT licence:

Copyright (c) 2024 Matt Evans:

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.