T030

I figured I would need a programmer for the AM29F040B, so I made one using my trusty old ATmega128 development board and a ZIF socket:

With a simple interface:

Very retro as well I guess, haven't used XMODEM since the early 90's :-)

The programmer is pretty slow and it only supports 16-bit addressing, but it is ok for a start...

After having further studied the available documentation and having played aorund with a preliminary board design in KiCad, I have decided to revise the design a bit.

Since I will need to have the PCB fabricated (no breadboard PCB this time), I have settled for a eurocard sized (160x100mm) board with two layers to keep the cost at a reasonable level. Since there is not enough space on such a board for the amount of RAM I will eventually need, I have decided to dial things back a bit for the first part of the project.

Second, the 32 bit RAM banks and their associated selection, decoding and bus cycle termination signals is still something I am in the process of getting my head around.

So for now, I am aiming for a simpler design with just 512 KB flash and 512 KB RAM using an 8 bit data bus. If I can manage to get that to work I can implement more advanced stuff on a second board later on.

I have also decided to use the multifunction peripheral MC68901 which I have found in a PLCC package instead of the 68681/68230 combo in order to save space and limit complexity of the board.

Motorola has published Application Note ANE426 describing a minimal system based on the MC68030.

It consists of the following:

• MC68030 running at 20 Mhz
• MC68681 Dual UART
• MC68230 Parallel Interface / Timer
• 8 x 6164 8 KB SRAM
• 2 x 27512 64 KB EPROM
• PAL16L8 for RAM byte select signals
• 555 timer for reset generation
• A bunch of 74- TTL IC's for glue logic

I thougt of using this application note as a starting point for my board with a few exceptions.

All glue logic will be implemented in a CPLD. Having never used CPLD's or FPGA's before, I'm sure there will be a lot for me to learn about those devices.

I recently bought a cheap CPLD development board with an Altera MAX II EPM240 along with a USB-Blaster cable to play around with the Altera Quartus II software. As far as I can understand from the documentation, the Altera MAX II series is not really 5V tolerant, so for the T030 project I have settled for the older MAX 7000 series, to be specific the EPM7128SLC4 which comes in a nice PLCC package.

Instead of the EPROMS, I will use a more modern flash IC for ROM: AMD AM29F040B

From the other projects I have found, It seems like the Alliance Memory AS6C4008 is a good choice as a relatively large capacity 8 bit parallel SRAM as it is available as a DIP package.

The next step in the project is choosing which CPU in the m68k series to use.

I have a board from an old payment terminal with a 68008, but I quickly decided that 1 MB address space is simply not enough.

In the attic I found and old Amiga 500 from which I could pull a 68000, but the idea of eventually being able to run "real" Linux or NetBSD on a homebrew board necessitates the addition of a MMU.

The next logical choice would be a 68020 with a 68851 MMU, but to keep the design of the board as simple as possible I chose the 68030 instead. The 68030 has an on-chip MMU but still supports dynamic bus sizing, which makes interfacing with 8-bit peripherals a lot easier.

I believe I can do without an FPU since both Linux and NetBSD support FPU-emulation for the m68k platform.

I also found a printed manual...

I got the idea for this project when I found my old Z80 Single Board Computer in the attic. It turned out that it actually still works, and I suddenly remembered how great a learning experience it was to design and build it from scratch, making a lot of errors and fixing them along the way.

After having finished a couple of AVR based projects after I haven't touched electronics for quite a while apart from simple repair tasks, I got the urge to create something else.

I thought it could be fun to learn something new by designing and building a more advanced single board computer that eventually should be able to run a variant of a modern day OS.

Originally I thought about starting with something like a 80386, but I seems like the x86 architecture and hobbyist projects don't mix very well.

Then I discovered that the Motorola 68k platform is much more suited for hobby projects if you want to do something a little more advanced than classic 8-bit stuff like 6502 and Z80.

After reading about these two very impressive projects:

Steve Chamberlin's 68 Katy:

http://www.bigmessowires.com/68-katy/

Luis Alves' 68000 project:

http://mc68k.blogspot.com/

(Both projects were originally implemented on breadboard(!) and both are capable of running uClinux.)

I decided to go for a m68k based project.