Big, Ugly Pig: Bring-Up, Phase 1

Posted by: Dave Vandenbout 5 years, 11 months ago

(1 comment)

I got the prototype PCBs for the next version of the XuLA Board from @laen's service a few weeks ago. Here's the front and back of the bare board:

My goal with this version was to make the simplest changes that would bring the biggest perceived benefits to the customer. That's usually accomplished by just making everything bigger, so I did the following:

  1. Changed the FPGA from a Spartan-3A to a Spartan-6 with the following increases in on-chip resources:

    Resource  XC3S200AVQ100   XC6SLX25FTG256 
    #LUTs 4K 24K
    #BRAMs 16 52
    #Multipliers 16 38
    #DCMs 4 4
    #I/Os 68 186

  2. Increased the SDRAM from 4M x 16 to 16M x 16 Mb.
  3. Added a socket for a MicroSD flash to provide the FPGA with gigabytes of non-volatile storage to use after it is configured. 

Making room for the MicroSD socket required me to push some of the other XuLA components around. After assembling the parts onto a prototype, the board ended-up looking like this:

It's a bit cramped and it looks like the components have been forced in there, but that's the only way I could find to make it all fit in the same form factor as the original XuLA. Unfortunately, beauty takes a backseat here.

In order to get an FPGA with a lot more logic resources than the Spartan-3A, I had to use a Spartan-6 in a package with a lot more I/O. But even after connecting all the SDRAM and MicroSD signals to the FPGA, there were still more than 100 pins left-over on the BGA package. I couldn't add any more I/O to the board prototyping header and remain compatible with the original XuLA, so I just had to waste 'em. That's a first for me; I'm usually struggling to find enough FPGA pins to connect everything.

Given the things I did, I started referring to the new XuLA board as a "Big, Ugly Pig". Seems to fit.

After assembly, the first thing I like to do when I'm bringing-up a new FPGA board (after checking for power/ground shorts) is just download a simple bitstream that makes an LED blink. But the XuLA doesn't have any on-board LEDs that the FPGA can drive. Luckily, I already had some external seven-segment LED hardware built for the original XuLA, so I could just use that with the new board. Here's a video of me downloading a bitstream that makes the new XuLA output a string of digits:

Passing this first test shows a few things:

  1. The voltage regulators for the PIC 18F14K50 microcontroller and the FPGA are working.
  2. The PIC is managing the USB interface correctly.
  3. The JTAG signals from the PIC are loading the bitstream into the FPGA without errors.
  4. The FPGA is coming out of configuration mode and into user mode without a problem.
  5. The 12 MHz clock generated by the PIC is getting over to the FPGA and driving the state machine.
  6. The outputs from the FPGA are driving the LEDs.

That's pretty good for a first trial. Now I can start testing the other components connected to the FPGA.

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