I’m going to start this post off with the obligatory list of links to the previous parts in the series if you’re new here and are interested in seeing the full story: 1, 2, 3, 4, 5, 6, 7, 8, and 9. This is the tale of how I upgraded my Chumby 8 to run a modern Linux kernel.
I only had some minor things on my list to figure out before I could call my kernel upgrade good enough to be finished. The most important remaining thing was the real-time clock, or RTC. I noticed that whenever I rebooted the Chumby, it always started out with the date set to the Unix epoch in 1970:
Would it really be this simple? Did I just need to enable the RTC in my device tree file, make sure the kernel driver was enabled, and then be done with it? I had already experienced similar success with other PXA168 peripherals. So I tried it out.
I decided to check my Windows Update history, and sure enough, my computer was affected:
The truth is that I’m not using BitLocker, so the update doesn’t really matter for me. I hadn’t even noticed that it was failing repeatedly. Apparently it had also failed to install on January 11th. Knowing that my computer would continue over and over failing at installing this update bothered me though. It just felt wrong.
If you’re new to this series, I’ve been documenting the process I went through upgrading my old PXA166-based Chumby 8’s 2.6.28 Linux kernel to a modern 6.x version. Here are links to parts 1, 2, 3, 4, 5, 6, 7, and 8. At this point in the project, all of the main hardware peripherals were working great. I noticed something odd when running top though. The CPU usage was always really high, and it wasn’t obvious why.
After getting many of the PXA16x peripherals working in modern Linux kernels during my Chumby 8 kernel upgrade saga (here are links to parts 1, 2, 3, 4, 5, 6, and 7), I really felt like I was starting to reach the finish line. The display was working well enough to play low-resolution videos, I had basic 2D acceleration up and running, the touchscreen was operational, and Wi-Fi worked flawlessly. Audio was the only major component left to tackle. The Chumby 8 has built-in speakers, a headphone jack, and a microphone.
The Linux sound subsystem is something I had exactly zero experience with prior to this project, so it felt very intimidating. It took me a while to get familiar with it. The relevant project is Advanced Linux Sound Architecture (ALSA) and in particular, the ALSA System on Chip (ASoC) layer is where most of my work would be for this project.
Before digging in too deep, I needed to look at the Chumby 8 schematics and figure out what I would be dealing with. With no prior audio experience to lean on, I wasn’t sure what to expect. Let’s take a look at the relevant section of the schematic:
In my last post, I figured out how to use Apple’s leaked Flasher utility from the 1990s to reflash a ROM SIMM inside of my Performa 630. It’s basically the Mac equivalent of a BIOS update, but only for Apple’s developers. The research involved in that post was quite a journey of reverse engineering from both a software and hardware perspective. I had to disassemble the code to figure out which computers were compatible and what the software was expecting to find. I also had to create a replica of an Apple development ROM SIMM that was wired exactly the way Macs of the era expected it. Although I was very excited about my discoveries, one big question remained:
What was the purpose of the bottom right half of the main window labeled “PDS ROM Info”? And what would it take to enable it?
After I wrote about the possibility of programmable Mac ROM SIMMs in Quadras a couple of months ago, I suspected that there had been a way for developers at Apple in the 68k Mac era to reflash the ROM in their Macs during development, just like BIOS updates on PCs. The reason I believed this is because the ROM SIMM socket in the Quadras brought out pins for 12V (VPP) and write enable (/WE). I had verified that the write enable pin was going into the memory controller chip in several Mac models, so I was pretty confident that in-system programming was possible.
As luck would have it, multiple people pointed out to me that an Apple internal utility used for ROM flashing had been uploaded to the Macintosh Garden. It was recovered from a prototype PowerBook 520 purchased in 2020. Of course, I had to download this utility and figure out how it works.
I ran it on my LC 475 and this is what it looked like:
After getting the PWM backlight working in my last post (here are links to parts 1, 2, 3, 4, 5, and 6), there was only one piece remaining for having a fully functional display in my Chumby 8: the touchscreen controller. The display output worked perfectly fine but I couldn’t detect presses on it.
The Chumby 8 and Insignia Infocast 8 have a 4-wire resistive touchscreen:
These aren’t so common anymore — it seems like almost everything is capacitive touch nowadays. I wasn’t familiar with the theory of operation behind resistive touchscreens until I wrote this post. Basically there are two transparent resistive layers. One layer goes left and right and the other goes up and down. When you press the touchscreen they connect together. You can calculate the X and Y positions where the layers meet by driving a voltage across one layer and then measuring the voltage of the other layer. Here’s a nice document that does a great job of explaining it in detail.
TL;DR: The pinout of modern programmable Mac ROM SIMMs needs to be changed for correct operation in Quadras. If you’re interested in how I reached that conclusion, or at least want to see some cool pictures, read on below.
One thing that you usually discover when searching for info about these programmable ROM modules is that they’re compatible with the earliest 68030-based desktop Macs: the SE/30, IIx, IIcx, IIci, IIfx, and IIsi. The most common use of them is to set up a special custom bootable ROM disk using Rob Braun’s driver, Big Mess o’ Wires’ driver based on Rob’s, or Garrett’s Workshop’s driver. In general, the compatible ROM images out there are all using the IIsi ROM which is capable of booting any of the aforementioned Macs.
What you may not know is that most of the later 68k Macs also have provisions for a ROM SIMM socket, but aside from the very first Quadras (700/900/950) which always have the socket installed, it’s not usually populated. Some early production or prototype units have it, but most just have empty through-holes filled with solder where the socket would go.
Don’t worry, I already replaced those leaky capacitors before they had a chance to damage the board.
Several months ago, Will from CayMac Vintage reached out to me looking to resurrect my old Mac ROM SIMM programmer project. As a quick summary of that project, it provides a convenient way to program custom 64-pin ROM SIMM modules for vintage Macs from the late ’80s to early ’90s. There are several reasons you might want to do this, including: replacing an original ROM module that has gone bad, disabling the startup RAM test to decrease boot time in systems with a lot of RAM, bbraun’s amazing bootable ROM disk hack, or my startup chime hack. JDW recently made a cool YouTube video explaining custom ROM SIMMs if you’re curious about them. He even included some footage from 2003 of me playing basketball!
I used to make programmer boards and programmable ROM SIMMs and sell them to hobbyists, but it burnt me out. In particular, assembling the boards and the logistics of shipping were not fun to deal with. Thankfully, in 2016, Steve from Big Mess o’ Wires stepped in to take over. He made his own customizations to the programmer and made some really neat improvements to the bootable ROM disk driver. He still sells the Mac ROM-inator II SIMM to this day, but he stopped selling the programmer board. In the meantime, many other players have entered the market with custom ROM SIMMs, but nobody has been making the programmer available to the community, likely due to my non-commercial license on the PCB design.
Will was looking to fill that void. I helped him get going, but we discovered that the AT90USB646 microcontroller that I originally used was hard to find due to the chip shortage. At the time, it was easier to find the AT90USB1286 instead, which is essentially just the exact same chip, but with 128 KB of flash instead of 64 KB of flash.
I recently ran into an interesting warning on newer versions of ARM GCC, including the latest (as of this writing) Arm GNU Toolchain 12.3.Rel1. In particular I’m dealing with arm-none-linux-gnueabihf-g++. Here’s a very simple example program that demonstrates the warning:
