Designing a 68K Single Board Computer

http://www.bigmessowires.com/2014/10/27/designing-a-68k-single-board-computer/

by omnibrain • 12 years ago

65 49 12 years ago

rpledge2• 12 years ago

I'm afraid the lack of an MMU will make running Linux impossible unless things in the kernel have changed dramatically since last looked into this.

chrisdew• 12 years ago

"Luis Alves’ machine used a regular 68000 at 20 MHz, and got decent performance running ucLinux."

ucLinux doesn't need an MMU - that's its reason for existing.

rpledge2• 12 years ago

Missed that - very cool, I hadn't heard of that project.

vertex-four• 12 years ago

μClinux[0] is still a thing, although is still a fork of 2.6.

[0] http://en.wikipedia.org/wiki/%CE%9CClinux

wglb5• 12 years ago

MMU is not manditory. Coherent http://en.wikipedia.org/wiki/Coherent_(operating_system) ran quite nicely both on the original IBM PC/XT and on the Atari ST (No mention in the wikipedia article--I can't remember if it ever escaped the walls of MWC). Coherent on the PC was shown at the National Computer Conference in Houston in 1983. The boot was packed 8 deep the entire show.

jacquesm9• 12 years ago

Interesting tidbit: Coherent was produced by Aaron Swartz' father.

icantthinkofone• 12 years ago

As the article said, I loved the 68K. Far more than anything Intel had at the time and I wasn't alone. For work, I designed a complete 68K system from the component level. For me it was easy. Everything made sense. It was a great time.

davelnewton• 12 years ago

I loved the 68K and built several boards with it, although my board skills were maxed out after the 68010. I had a PT68K2 (http://bitsavers.trailing-edge.com/pdf/peripheralTechnology/...) and several other commercial 68k machines including an Atari 1040ST and wanted a Falcon.

I still have all my original reference material, and this book, http://www.amazon.com/MC68000-Assembly-Language-Systems-Prog... was one of my favorite computer books ever--I devoured that thing.

I still actually reference that book, and a couple others of that era, regarding microcontroller interfacing, even though I've moved on to Arduinos, RPis, and the bazillion other incredible SoCs I embed into stuff all the time now.

adestefan• 12 years ago

Anyone interested in this might want to look into the N8VEM Project [0]. It started out as a single board Z80 system, but people have now designed expansion cards. Other people have also added other SBCs with 6x0x, 68k, and 808x CPUs

[0] http://n8vem-sbc.pbworks.com/w/page/4200908/FrontPage

unwind2• 12 years ago

Cool! I love the 68k, I learned assembly on it (on the Amiga because Europe) and it ruled.

It sounds like a nice project, although I personally almost can't consider a 68K-based hobby computer without graphics. :) Of course graphics isn't trivial, so I totally understand the design.

This part:

A CPLD is strongly preferred over an FPGA, because the CPLD’s configuration is held in its internal non-volatile memory. An FPGA’s configuration is RAM-based, so it would require something else to configure it every time the board boots up.

Actually isn't true anymore, there's at least one FPGA family with built-in flash: Lattice's MachXO2 family (http://www.latticesemi.com/Products/FPGAandCPLD/MachXO2.aspx).

jacquesm9• 12 years ago

How Motorola managed to squander its lead is still a mystery to me. The 68K ran rings around anything available at the time and the 68030 had an on-chip MMU removing one of the last barriers between micro and mini computing.

Easy to code for, very nice instruction set, linear memory model able to address lots of RAM, memory mapped IO, what else was there to wish for. And yet, IBM picked X86. Beats me. The 6809 would have been a better choice for a micro than the 8086 was.

cp512• 12 years ago

Totally agree. Some things do not make sense

kabdib• 12 years ago

There was a project at IBM that used the 68K. If memory serves, IBM thought it would be "too powerful" so they chose the 16-bit 8086.

Writing boot ROM code for the 68K [at Atari, not IBM...] was a lot of fun. For instance, the first thing we needed to do, before we could use any RAM, was to size the available RAM; this code ran a little memory test using just the 16 registers (and no stack, of course). Didn't take long to write, but it was a neat little puzzle.

Writing embedded software is neat. I still get a rush when a new design wakes up and prints out "Hello world!" on a serial port, and on the ST it was way cool when the floppy drive did its first seek.

"Hey guys, watch this!"

Grknnnnkknknkg

Little things. :-)

Don't know if you could do CPLD initialization with the same constraints, but it doesn't seem that hard . . . :-)

Motorola seemed to run out of 68K gas in the late 1980s. Maybe they were distracted? They developed the 88000 RISC, and at Apple we actually had some NuBus boards that we were urged to write software for, but that effort wasn't terribly well organized and ultimately fizzled (one fine day we were told to immediately rip out all our 88K dev boards and return them. Something contractual probably fell apart, and we guessed that some lawyers had gone non-linear...)

marktangotango• 12 years ago

Attempting to control the consumer market by choosing a lesser chip does sound like something IBM would do. They were all about protecting their turf (as400 and mainframes).

lbruder• 12 years ago

That's why I loved the CBM64 floppy drive. The 1541 didn't have a light barrier to find its base position, so the stepper motor had to move against a physical barrier long enough to ensure it was at a known position. Brrrrrrrrttttt...pkTTTTTTTTRRRRRRRRRRRRRRRRmkkkkkKKKKKKKKKKrtttttt... Loading... Ready. God I miss that sound.

unoti3• 12 years ago

You worked on the ST firmware! That's so cool!

ido2• 12 years ago

I never understood why the ST wasn't more successful - in the 80s these were more or less the same as macs for less than 1/2 (1/3?) the price. Why get a mac plus with 1mb ram for $2500 when an ST with the same amount of ram and higher res (monochrome - like the plus) monitor was $1000?

Considerably cheaper than comparable and even slower PC clones too.

foobarge• 12 years ago

Your blog is answering so many young adult-hood questions I had about many things Atari, it's unbelievable.

wglb5• 12 years ago

Writing boot ROM code for the 68K

So this was for the Atari ST? I was at MWC when that came out and it was a hacker's delight.

userbinator• 12 years ago

The 6809 would have been a better choice for a micro than the 8086 was.

An 8-bit CPU with a 64K address space, compared to a 16-bit one with 1MB? The 6809 is more comparable to the Z80.

As for why 68K didn't make it, at the time it was just too "big" and expensive of a processor for the market IBM wanted to target with the PC - more powerful than the 6502 and Z80/8085 systems that were common at the time, but not at the level of minicomputers.

Interesting article about this here: http://trixter.oldskool.org/2012/12/27/the-ibm-pc-5150-what-...

jacquesm9• 12 years ago

Have a look at OS/9 and what the 6809 was capable of, it's not nearly as small as it would make you believe. Sure it's an '8 bitter' and the 8086 nominally 16 bits but on a bang-per cycle comparison it comes off pretty good and memory limits could be fixed by using bankswitching (which the 8086 did too but a bit closer to the processor).

When the PC was designed the 68K was already available, my point is that even a 6809 would have been a better choice, given that the 8086 was 8 bits external and memory never went over 256K for the original PC, easily addressed using 16 banks of 16K.

Anyway, it hardly matters but the 8086 was a weird choice, the explanation elsewhere that they chose it over the 68K because it was considered 'too powerful' makes pretty good sense.

http://en.wikipedia.org/wiki/OS-9

srcmap• 12 years ago

embedded software project chooses a new cpu architecture has a lot of risks:

hw risks: When will the chip be ready? Will it work, are there any engineering samples, eval boards? How long will my hw engr learn the new design rules for the new chip?

sw risks: Is my favorite RTOS (Linux ports ) available? How stable is the port? Are my software engineers familiar with the system? In not, how long would it take?

Each of the risk listed can add a lot time and money to the project.

AnimalMuppet• 12 years ago

And, in the case of the IBM PC, Intel was actively sowing FUD about Motorola's ability to deliver the 68000 in enough volume to meet IBM's needs.

unoti3• 12 years ago

Back when I was a Cub Scout, I learned assembly language on my 6809-based TRS-80 Color Computer. A few years later at my first professional job we ran 300+ users on a single 68030 based minicomputers from Alpha Micro. I ended up learning 68000 assembly there, and later, C.

And while that may sound old school, it's not. When I was coming up, every time we talked old school someone would bring up paper tape and punched cards which totally predate me. I wonder where those people are now!

JoeAltmaier3• 12 years ago

I'm right here!

robomartin• 12 years ago

I was exposed to paper tape for about six months during my first job. I thought it was amazingly cool technology. You could actually read the one's and zero's and enter them by hand if needed into the PDP 11/34's front panel keypad. No head crashes or corrupted sectors. Even if the tape ripped you could reproduce the data.

wglb5• 12 years ago

Right here, still coding.

krylon• 12 years ago

every time we talked old school someone would bring up paper tape and punched cards which totally predate me. I wonder where those people are now!

I once met this university professor - he is the husband of a friend of my mother's, they had trouble with their computer, I fixed it, and while we sat there, we talked, and he told me that he had in the 1980s a colleague down the hall who carried data around on paper tape.

During my training, I knew this guy who still had a stack of punched cards sitting on his desk he used to take notes.

Sigh Some part of me is really glad I can now carry gigabytes of data on a flash drive smaller than my finger, but another part of me wants to have been there.

jacquesm9• 12 years ago

Did you ever clue in to the CoCo having 64K ram that you could use if you messed with the video/memory controller for a bit to copy the ROM to RAM first? After that you could enable the full 64K and modify the (former) ROM data! Very neat and I used that trick quite a bit to relocate the basic to the highest 16K (giving much more free space in basic) and for all kinds of other tricks requiring more memory. Adding a bunch of new commands/functions to basic was also a fun project.

JoeAltmaier3• 12 years ago

Memory. The x86 line has anisotropic instruction encoding; more frequently-used instructions are shorter. Memory was very expensive. It took 1.5X - 2X the memory to write a program in 68K asm.

jacquesm9• 12 years ago

That's an interesting observation that I had not considered, you're right memory back then was absolutely crazy expensive. I recall paying $500 or thereabouts for 64K of ram. (4164's).

zwieback• 12 years ago

I had an Apple ][ and was disappointed when the Mac came out and left hobbyists stranded. Even though the Intel CPU in the IBM PC was horrible I much preferred IBM clones to Macs, which seemed expensive, closed and goofy. If there had been a 68K based continuation of the Apple ][, with a well documented and accessible design things might have turned out different for 68K.

Once the 386 came out things started looking up but the 8086 and 80286 years were painful.

Of course the 68K is still super popular in its Coldfire incarnation.

wglb5• 12 years ago

Intel understood something that Motorola did not. Intel went for design wins, not for units shipped.

I worked only briefly with the 6809 from the compiler side. Reminded me a lot of the pdp-11 instruction set.

cp512• 12 years ago

Guys check out the ex natami project and apollo core project http://www.apollo-core.com/ also check out http://majsta.com

unwind2• 12 years ago

The Apollo project looks awesome, but the site is very shallow and doesn't provide much detail about the actual delivarables of the project, or its licensing.

It sounds as if maybe aims to be commercial, which sounds ... I don't know, like a hard sell these days.

It would be so awesome to pair it with some graphics processor in a suitable FPGA and ... uh ... write demos, I guess. :)

zackmorris• 12 years ago

I have fond memories of writing assembly for the 68000. It had many RISC-like properties (a large number of registers, few specialized instructions like the looping and string handling of the x86, not having to worry about memory segments, etc). I wrote some interesting B&W scrolling blitters after seeing the full screen labyrinth scrolling in Beyond Dark Castle. I can't remember if they used bit shifting or rolling, but I don't think I managed 30 fps because the Mac Plus had such a slow bus that only vertical scrolling at 30 or 60 fps was possible by reading from scan line offsets of the source image. I used Apple’s BlockMove(), which was like an even more optimized version of memcpy(). The horizontal scrolling jittered when it hit 8 bit multiples because I wrote an optimization that skipped the shifting. I actually commented that out just to get consistent speed (I later synced to the vertical refresh instead).

I often feel that Apple's jump to PowerPC was not worth it. Losing hand-written assembly was a major blow and took Apple out of gaming for years because no Mac port came even close to the optimized innermost loops of games like Descent. I remember running games on a friend’s 486 at full speed that only got 10 fps on my 60 MHz PowerPC. Those were bleak years. Luckily video cards eventually leveled the playing field (but brought their own issues, which we still struggle with until DSP instructions are brought on-chip the way the FPU was).

tzs• 12 years ago

A single board 68K system is fun to make. My senior year at Caltech, in 1982, I took a microprocessor lab and for my project built a general purpose 68K system (as a joint project with a friend--we shared the design, and then each built a system).

Here are photos of my system: http://imgur.com/a/jS42c

I don't recall the specs exactly, but it was 6 MHz CPU clock speed, something like 4K of EEPROM and 2K of static RAM, and two serial ports.

We almost weren't allowed to do this, because in prior years there had always been a few general purpose systems, and they wanted people to try something that hadn't been done to death. However, those were all 8-bit systems. This was the first year they had obtained some 16-bit processors, and decided those were sufficiently different that they would let us get away with general purpose systems.

Some of the part choices might seem odd. That was due to cost. They gave us the 68Ks, and I think we got the EEPROM and static RAM from them, too. Everything else was on our dime, which meant there was a lot of scrounging around at surplus stores to find things that fit into a broke student budget.

One design feature I really liked was the way we did the serial port connectors. Note that the connection to the board uses a standard DIP socket. That has two advantages:

1. It was cheap!

2. It is NOT a keyed connector. That allowed plugging it in two ways. We wired them up so that both worked, but one was equivalent to inserting a null modem.

My friend's system had a funny problem. His and mine should have worked identically, since they were the same design and same layout. However, for some reason his serial ports would not run faster than 2400 bps, whereas mine went up to at least 19200 (the speed of the fastest terminal we had access to). If he tried to go faster, some characters would work, and some would not.

It turns out that when he went to the EE stockroom to buy bypass capacitors for the serial lines, the stockroom gave him the wrong capacitors. They were much much much larger than they were supposed to be, and so were filtering out as line noise frequencies that were low enough to affect the data. If a character only had a couple 0/1 or 1/0 transitions, not too close together, it would work. If it had more, or they were too close together, it got messed up.

There was also an hilarious incident with the development system. The EEPROM burner was hooked up to some HP system. The 68K cross assembler ran on Caltech's IBM 360. So, we'd write our assembly code on the VAX, then there was a process to submit it to be cross assembled on the 360, then you could go to the HP and download it from the 360 and burn it into EEPROM.

There were strict orders to delete your files from the HP as soon as you burned your EEPROM, because the disk was near full. The thing had been there a long time, and was full of junk from old projects, and no one knew what was really junk (such as past student files) that could be deleted, and what might be someone's research files. The space crunch on the machine was starting to get very annoying.

Also, no one really knew much about the OS on the HP. Everyone just learned a set of commands by rote to transfer files, burn EEPROMs, and things like that.

One fine day, I happened to notice that there was a place in all the commands that manipulated my files that a "1" or an "A" (I forget which) appeared near the file name. Curious as to what it might mean, I tried copying a file but changing that to a "2" (or "B") for the destination, and it worked. I then asked for a listing, again using "2" (or "B"), and there was my file, alone on an empty disk.

I had discovered that there were actually TWO drives in the computer--and the second was empty (except for my file)! So...a bunch of (allegedly) bright people at Caltech had spent months, or even years, struggling with low disk space in the lab, and all that time there was an empty second disk in the computer...wow

Designing a 68K Single Board Computer

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