Tuesday, August 26, 2014

Updates and pending projects

It's been a while since I've written anything here so here's a bit of a brain-dump on upcoming stuff that will find its way here eventually.

Thesis stuff

This has been eating the bulk of my time lately. I just submitted a paper to ACM Computing Surveys and am working on a conference paper for EDSC that's due in two weeks or so. With any luck the thesis itself will be finished by May and I can graduate.

Lab improvements

I'm in the process of fixing up my lab to solve a bunch of the annoying things that have been bugging me. Most/all of these will be expanded into a full post once it's closer to completion.
  • Racking the FPGA cluster
    The "raised floor" FPGA cluster was a nice idea but the 2D structure doesn't scale. I've filled almost all of it and I really need the desk space for other things.

    I ordered a 3U Eurocard subrack from Digikey and once it arrives will be making laser-cut plastic shims to load all of my small boards into it. The first card made for the subrack is already inbound: a 3U x 4HP 10-port USB hub to replace several of the 4-port hubs I'm using now. It will be hosted by my Beaglebone Black, which will function as a front-end node bridging the USB-UART and USB-JTAG ports out to Ethernet.

    The AC701 board is huge (well over 3U on the shortest dimension) so I may end up moving it into one of the two empty 1U Sun "pizza box" server cases I have lying around. If this happens the Atlys boards may accompany it since they won't fit comfortably in 3U either.
  • Ethernet - JTAG card
    FTDI-based JTAG is simple and easy but the chips are pricey and to run in a networked environment you need a host PC/server. I'm in the early stages of designing an XC6SLX45 based board with a gigabit Ethernet port, IPv6 TCP offload engine, and 16 buffered, level-shifted JTAG ports. It will speak the libjtaghal jtagd protocol directly, without needing a CPU or operating system, for ultra-low latency and near zero jitter.
  • Logo
    I've gone long enough without having a nice logo to put on my boards, enclosures, etc. At some point I should come up with one...

Test equipment

I've gradually grown fed up with current test equipment. Why would I want to fiddle with knobs and squint at a tiny 320x240 LCD when I could view the signal on my 7040x1080 quad-screen setup or, better yet, the triple 4K displays I'm going to buy when prices come down a bit? Why waste bench space on dials and buttons when I could just minimize or close the control application when it's not in use? As someone who spends most of his time sitting in front of a computer I'd much prefer a "glass cockpit" lab with few physical buttons.

I'm now planning to make a suite of test equipment based on the Unix philosophy: do one thing and do it well. Each board will be a 3U Eurocard with a power input on the back and Ethernet + probe/signal connections on the front. They will implement the low-level signal capture/generation, buffering, and trigger logic but then leave all of the analysis and configuration UI to a PC-based application, connected over 1- or 10-gigabit Ethernet depending on the tool. Projects are listed in the approximate order that I plan to build them.
  • 4-channel TDR for testing cat5e cable installs
    This design will be based on the same general concept as a SAR ADC, with the sampling matrix transposed. Instead of gradually refining one sample before proceeding to the next, the entire waveform will be sampled once, then gradually refined over time.

    Each channel of the TDR will consist of a high-speed 100-ohm differential output from a Spartan-6 FPGA to generate a pulse with very fast rise time, AC coupled into one pair of a standard RJ45 jack which will plug into the cable under test.

    On the input stage, the differential signals will be subtracted by an opamp, then the single-ended differential voltage compared against a reference voltage produced by a DAC using a LMH7324SQ or similar ultra-fast comparator. The comparator will have LVDS outputs driving a differential input on the Spartan-6, which can sample DDR LVDS at up to 1 GHz. This will produce a single horizontal slice across a plot of impedance mismatch/reflection intensity vs time/distance.

    By sending multiple pulses in sequence with successively increasing reference voltages from the DAC, it should be possible to reconstruct an entire TDR trace to at least 8 bits of precision for a fraction of the cost of even a single 1 GSa/s ADC.

    Given the 5ns/m nominal propagation delay of cat5 cable (10us/m after round trip delay), the theoretical spatial resolution limit is 10cm although I expect noise and sampling issues to reduce usable positioning accuracy down to 20-50, and the TDR will need to be calibrated with a known length of cable from the same lot if exact propagation delays are needed to compute the precise location of a fault.
  • 10-channel DC power supply

    Offshoot of the PDU. Ten-channel buck converter stepping 24 VDC down to an adjustable output voltage, operating frequency around 1.5 MHz. Digital feedback loop with support for soft-start, state machine based current limiting and overcurrent shutdown, etc.

    More details TBD once I have time to flesh out the concept a bit.
  • Gigabit Ethernet protocol analyzer
    Spartan-6 connected to three 1gbaseT PHYs. Packets coming in port A are sent out port B unchanged, and vice versa. All traffic going either way is buffered in some kind of RAM, then encapsulated inside a TCP stream and sent out port C to an analysis computer which can record stats, write a pcap, etc.

    The capture will be raw layer-1 and include the preamble, FCS, metadata describing link state changes and autonegotiation status, and cycle-accurate timestamps. Error injection may be implemented eventually if needed.

  • 128-channel logic analyzer
    This will be based on RED TIN, my existing FPGA-based ILA, but with more features and an external 4GB DDR3 SODIMM for buffering packet data. A 64-bit data bus at 1066 MT/s should be more than capable of pushing 32 channels at 1 GHz, 64 at 500 MHz, or 128 at 250 MHz. The input standards planned to be supported are LVCMOS from 1.5 to 3.3V, LVDS, SSTL, and possibly 5V LVTTL if the input buffer has sufficient range. I haven't looked into CML yet but may add this as well.

    The FPGA board will connect to the host PC via a 10gbit Ethernet link using SFP+ direct attach cabling. Dumping 4GB (32 Gb) of data over 10gbe should take somewhere around 4 seconds after protocol overhead, or less if the capture depth is set to less than the maximum.

    The FPGA board will connect via matched-impedance 100-ohm parallel cables (perhaps something like DigiKey 670-2626-ND)) to eight active probe cards. Each probe card will have a MICTOR or similar connector to the DUT providing numerous grounds, optional SSTL Vref, 16 digital inputs, and two clock/strobe inputs with optional complement inputs for differential operation. An internal DAC will allow generation of a threshold voltage for single-ended LVCMOS inputs.

    The probe card input stage will consist of the following for each channel:
    • Unity-gain buffer to reduce capacitive load on the DUT
    • Low-speed precision analog mux to select external Vref (for SSTL) or internal Vref (for LVCMOS). This threshold voltage may be shared across several/all channels in the probe card, TBD.
    • High-speed LVDS-output comparator to compare single-ended inputs against the muxed Vref.
    • 2:1 LVDS mux for selecting single-ended or differential inputs. Input A is the LVDS output from the comparator, input B is the buffered differential input from this and the adjacent channel. To reduce bit-to-bit skew all channels will have this mux even though it's redundant for odd-numbered channels.
    The end result will be 16 LVDS data bits and 2 LVDS clock bits, fed over 18 differential pairs to the FPGA board. The remaining lines in the ribbon will be used for shielding grounds, analog power, and an I2C bus to control the DAC and drive an I/O expander for controlling the mux selectors.
LA input stage for two single-ended or one differential channel
  • 4-channel DSO
    This will use the same FPGA + DDR3 + 10gbe back end as the LA, but with the digital input stage replaced by an AFE and two of TI's 1.5 GSa/s dual ADCs with interleaving support.

    This will give me either two channels at 3 GSa/s with a target bandwidth of 500 MHz, or four channels at 1.5 GSa/s with a target bandwidth of 250 MHz. The resulting raw data rate will be 3 GSa/s * 8 bits * 2 channels or 48 Gbps, and should comfortably fit within the capacity of a 64-bit DDR3 1066 interface.

    I have no more details at this point as my mixed-signal-fu is not yet to the point that I can design a suitable AFE. This will be the last project on the list to be done due to both the cost of components and the difficulty.

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