Re: 68030 SBC project ideas

[lynchaj <lyn...@yahoo.com>]

Hi

I've been thinking about how to proceed with the 68040 project and
here are my thoughts.  Probably there will have to be a couple of
rounds of prototyping to get the design hammered out.  It took two
iterations with the N8 and this is project is at least as complex.

For now, I am thinking to keep to just making simple prototype PCBs
and ask to the prototype builders to install their own SMT components
rather than getting stuffed boards.  Stuffing the PCBs will drive the
cost up unreasonably high and the builders interested in this project
build and test phase will likely have the skills and means to install
their own SMT components.

We need a preliminary design to capture a schematic and lay out a
PCB.  I am inclined to go with the MC68040RC (PGA) for the CPU and the
MC68360EM (QFP) since it appears to ease the integration and
eliminates the need for custom programmed CPLD and/or FPGA for bus
integration logic.  Also the MC68360 is relatively low cost (<$30 for
the QFP) and includes UARTs, timers, interrupt controller, and
parallel IO pins which simplifies the design considerably.  We'd still
need an IO Propeller connected to the MC68360 parallel IO pins for the
VGA, PS/2 keyboard, and SD.

Memory is going to be a tricky issue.  I am thinking for system RAM we
use a couple of the 72 pin SIMM sockets.  They are plain DRAM in a
convenient package and require only four 74F157s to interface (I
think).  Using SDRAM or later would require a more sophisticated
controller not found on the MC68360.

For the boot ROM the 68040 User Manual has various memory connections
and its shows a 32 bit wide path which is two of the 16 bit wide
EPROMs (27C400 -- 40 pin DIP).  It seems that when in "MC68040
companion mode" the data path is fixed at 32 bits.

Basically seven "capital" chips (MC68040, MC68360, Propeller, two
27C400 EPROMs, and two SIMMs), glue logic, and the rest being
passives, connectors, RS-232 level shifters, power, etc.  Assuming
there is some room left on the PCB there would be a prototyping area.

KiCAD seems to support the MC68040RC component and we can reuse the
Propeller subsystem in the PropIO board.  The MC68360 would be a KiCAD
custom part and with 240 pins is no small undertaking!

How do we proceed with this project?

Is this practical/feasible to do?

Is there sufficient interest to make such a board knowing it will be
considerably more costly/complex than past projects?

At a minimum we'll need some basic designs to discuss and even some
schematics/PCB layout drawings.

Ideas?  Thoughts?  Comments?

Thanks and have a nice day!

Andrew Lynch