FPGABee is a hardware emulation of a Microbee - the classic Aussie home computer of the 80's!

Building XulaBee - The Idea

Friday, July 26th, 2013     #xula #everything

For a while now I've been thinking about how to package FPGABee into a small case. It seems this is a step that many retro FPGA project's stop short off, but it's something I'd like to have a go at. The Nexys-3 board is an excellent development board but it's not very practical to setup and the soldered wires onto the PS2 keyboard socket keep breaking off.

A couple of weeks ago I stumbled upon these tiny XuLA boards from http://xess.com. There are two versions - the XuLA and XuLA2.

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The XuLA2 (on the left) costs USD$119 and has Spartan 6 LX25 with 1.5 million gates while the smaller XuLA costs USD$55 and has a Spartan 3A 200 (200 thousand gates). FPGABee will definitely fit on the XuLA2 but I'm not sure about the smaller XuLA so I ordered one of each to have a play with.

In order to reduce the unknowns (ie: my soldering/PCB design skills) and help get this up and running, I also ordered some prototyping pieces including a "Stick-It" board and add-ons for VGA and PS2. Conveniently the Stick-It board supports the same Pmod connectors as the Nexys-3, so I should be able to re-use my existing PmodSD.

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While waiting for the XuLA bits to arrive from the States I started on a case for it. It's a simple, but reasonably decent looking case from Jaycar and I've put in the VGA, PS2, speaker, power connectors and a power switch. There will also be a USB connector so the FPGA can be re-programmed without opening the case - this will either consist of a cable from the back panel to the XuLA, or I might try to redesign the PCB to have the XuLA's USB socket protrude through the back panel:

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The front panel will consist of a couple of LEDs, reset button and the SD card slot. (Note that although the XuLA2 has an on-board SD card slot, I won't be using it once it's all mounted in the case).

I've also had a go at designing a custom PCB for it. I've deliberately designed it so there's no surface mount components so that construction remains simple, but includes VGA, PS2, speaker, support for 3 leds, 3 push buttons, connector for the PmodSD and some tiny prototyping areas.

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I've been looking around for PCB manufacturers and http://iteadstudio.com looks promising - they can produce 100x100mm double sided boards at USD$25 for 10.

This is something I've never done before so it's all trial and error. I know there are issues with the design as it stands, but it's a start. Things that need to be addressed include:

  • as mentioned above, try a design where the XuLA USB socket protrudes through the rear panel.
  • re-design the audio circuitry to possibly support a sigma-delta DAC instead of the 1 bit audio. Need to test this with FPGABee to see if it will still work.
  • fix wiring errors to the socket for the SD card.

Well, that's the idea... I wonder if I can make it work?


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