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New board designs underway

A project log for rhombus

An open-source minimalist 68020 based single board computer

Jason WesterveltJason Westervelt 01/22/2016 at 21:3512 Comments

I've finished the schematic for the two boards and once the parts that I have on order arrive, I will begin making wire-wrapped prototypes prior to sending the boards off to be fabricated. Overall, the design is a lot more flexible and a lot less complicated that I thought it would be.

The VME board itself will be somewhat expensive due to the physical size of the board. For this reason, I want to make the board as flexible as possible to allow for future changes without having to spin off a new board. While rhombus is a 68020 board, I do plan on moving up to a 68040 for my next project, and I'd prefer having a VME interface that will accept the new CPU board with no physical changes. The board itself is rather simple... a CPLD, an EPROM for doing the address modifier lookups, and the bus buffer/transceivers.

With regard to rhombus, the board is actually going to end up being smaller than my current wire-wrapped prototype, mainly due to switching to the PLCC-packaged MC68901 and ROM. RAM is going to be tested with DIP parts, however it will be replaced with 4x 44-TSOP packaged ICs once I run off copies of the board. There will be a need for pull-up resistors as some of the CPLD outputs are needing to be changed to open collector as they will also be potentially driven from the VME bus header.

I also plan on adding a flash configuration rom and creating a basic configuration utility, but i'm currently debating a few design choices. Primarily the flash rom will be useful for storing information about add-in cards on the VME bus, and thus generating the necessary address modifier codes. Additionally, it will store things found in traditional PC BIOS, such as date/time, serial port configuration, etc. As such, there is potential need for the ROM to exist on the stand-alone CPU board as well as on the VME adapter board. I'm thinking of going with the latter, and allowing the CPU board to read data from the flash ROM if it is present and reverting to either jumpered values or values programmed into the CPU-board's EPROM if the flash device is not present.



Discussions

Jason Westervelt wrote 01/22/2016 at 22:16 point

Actually, it will remain minimal.  :) I am about to break out the VME board as a separate project.  The core board is gaining an expansion header and an expanded RAM and ROM bus width, but other than that, it remains the same.

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Yann Guidon / YGDES wrote 01/22/2016 at 22:50 point

@Stefan Lochbrunner prepares TSOP II-44 adapters that could be very useful for your SRAM too :-)

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Jason Westervelt wrote 01/22/2016 at 23:04 point

Thanks for the heads-up. I plan on having them soldered directly to my board once I get to the PCB step. In the mean time, I have 4 KM681000 SRAMs which are DIP32 parts that I can prototype with.

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Yann Guidon / YGDES wrote 01/22/2016 at 23:08 point

TSOP44 ASRAMs are 16-bits wide so it's not the same thing... It's better to prototype with the final parts ;-)
Are you using 5V or 3.3V parts ?

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Jason Westervelt wrote 01/24/2016 at 09:12 point

I am using K6X8008C2B... these are 1Mx8 SRAM

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Yann Guidon / YGDES wrote 01/24/2016 at 11:22 point

K6X8008C2B is in TSOP44, I see. 5V. Slow but low power, good for battery backup.

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Jason Westervelt wrote 01/24/2016 at 16:58 point

Fair point.  I was planning on using what I had on hand and thought that 55nS would be fast enough, but I'll look around for a better option to ensure 0-waitstate operation.  

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Yann Guidon / YGDES wrote 01/24/2016 at 17:03 point

You can easily find 3.3V parts in TSOP44 with 512K×16 @<20ns, now is the logic level compatible ?

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Jason Westervelt wrote 01/24/2016 at 17:42 point

Looks like I might end up ordering a few CY7C1041D... 10ns should be sufficient.

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Yann Guidon / YGDES wrote 01/24/2016 at 17:50 point

Is the CY7C1041D 5V-compatible ?

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Stefan Lochbrunner wrote 01/22/2016 at 23:02 point

You can check out the design here. I'm about to order a panel and could throw some on there for you as well if you're interested.

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Yann Guidon / YGDES wrote 01/22/2016 at 21:59 point

This is not a minimal computer anymore :-)

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