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Hands On: Testing the $199.99 Digilent Digital Discovery Portable Logic Analyzer (based on a Spartan-6 FPGA)

Xilinx Employee
Xilinx Employee
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I don’t get to do too much hands-on work while writing Xcell Daily but today was an exception. Today, I took delivery of the new Digilent Discovery Portable Logic Analyzer and Digital Pattern Generator. This diminutive USB instrument measures about 3.25 inches on a side and costs a mere $199.99. For that, you get a 24-channel USB logic analyzer capable of 200Msamples/sec with simple single-ended leads and 800Msamples/sec on fewer channels with Digilent’s High Speed Adapter. (See “$199.99 Digital Discovery from Digilent implements 800Msample/sec logic analyzer, pattern generator. Powered by Spartan-6” for more information.)




Digilent Digital Discovery.jpg



Digilent Digital Discovery Logic Analyzer Kit




For comparison, my first logic analyzer was an engineering prototype of the HP 1615A, HP’s first timing/state analyzer. I borrowed this prototype instrument from HP’s Colorado Springs Division back in late 1977 to help debug a nagging problem with the I/O backplane of the HP 9845A Desktop Computer. The HP 1615A offered a “blazing” timing capture rate of 20Msamples/sec for 24 channels. Most important for me, it had a 5nsec “glitch capture” feature, which allowed me to see the I/O glitches taking place on the I/O backplane at the maxed-out DMA transfer rate of 400Ktransfers/sec. The HP 1615A logic analyzer cost $6800 back in the day and it sold like hotcakes. (For an affectionate history of HP’s entry into the logic analyzer business, see “Logic State Analyzer Birthing Pains” by Chuck House on the HP Memory Project Web site,




HP 1615 Logic Analyzer.jpg



The HP 1615A Logic Analyzer circa 1978 (Photo courtesy Hewlett-Packard Company)




Well, it’s exactly 40 years later and see what time, Digilent, and Xilinx FPGA technology have wrought. Digilent’s $199.99 Digital Discovery provides the same number of logic-analysis channels as the $6800 HP 1615A but runs 10x to 40x faster and has a 2Gbit acquisition buffer. (The HP 1615A had a 6Kbit acquisition buffer, but that was 40 years ago.) In addition, the Digilent Digital Discovery can not only display digital signals, it can decode and trigger on commonly used serial-bus protocols including SPI, I²C, UART, I2S, and CAN. Of course, none of those even existed 40 years ago except for the UART NRZ protocol.


So, is the Digilent Digital Discovery easy to use? The user interface for this instrument is Digilent’s GUI-based Waveforms 2015 application, which you download from the Web and install. I downloaded the Waveforms 2015 installer and immediately ran into a problem. My Xilinx-provided laptop lacked Microsoft’s Visual C++ 2013 and Visual C++ Redistributable Package. Waveforms 2015 generously offered to install the missing Microsoft package for me. I clicked “yes,” and Waveforms then tried to install the Microsoft C runtime package—and failed. Without too much trouble, I found the software on the Microsoft Web site, downloaded it, and installed it manually. Then I restarted the Waveforms 2015 installation and completed that successfully.


When I ran Waveforms 2015, it went looking for the Digilent Digital Discovery module, which I intentionally had not yet plugged into my laptop’s USB port. After getting the expected complaint from Waveforms 2015, I plugged the Digital Discovery into a USB port and everything started working just fine.


I’d known for a couple of weeks that Digilent was sending this instrument to me, so I had decided beforehand how I wanted to generate my initial test waveforms. It seemed to me that the fastest and simplest test-signal generator I could use that would be interesting was a low-cost Sparkfun RedBoard Arduino clone and an Adafruit Motor Control Shield v2, which Radio Shack had just put on sale as part of its “inventory reduction” program. (Fortunately, the Digital discovery is 5V-tolerant (I checked), because the RedBoard is a 5V board.)


When running the supplied “MotorParty” Arduino Sketch, the Sparkfun RedBoard continuously writes I2C commands to an H-bridge driver IC on the Adafruit Motor Control Shield to spin up a dc motor. It then then spins the motor down, reverses direction, rinses, and repeats. The MotorParty Sketch also generates a variable, TTL-level PWM signal to drive a hobby servo. I used the Digital Discovery to capture both waveforms.


By the way, I did all of this without taking a peek at the manual.


First, I hooked up the Digital Discovery to the Adafruit Motor Control Shield’s I2C pins (driven by the Sparkfun RedBoard) and set up the Logic Analyzer for I2C. It’s pretty easy. You just select I2C and the I2C clock and data lines show up on the first two channels of the logic analyzer. After initially messing up these two connections (hey, I’ve got a 50% chance of getting it right, plus I did get ground connected correctly), I was rewarded with this waveform display with the I2C traffic automatically decoded:




I2C Waveform Capture from Adafruit Motor Shield dc motor control.jpg



Digilent Digital Discovery I2C Waveform Display




OK, let’s see how well I do when I only need to connect one signal: the PWM signal driving the servo. Hobby servos use a very simple 1-wire PWM interface where the width of the logic pulse positions the servo. This protocol was created back in the pre-microprocessor days when we used RC oscillators to drive servos.


Here’s a 30-second video showing you how easy it is to look at continuously changing digital waveforms using the Digilent Digital Discovery (once you set the trigger correctly).






Again, setting up the Digital Discovery to capture this waveform is pretty simple, pretty intuitive.


As a reminder, these logic-analysis features are all implemented inside of one Xilinx Spartan-6 LX25 FPGA coupled to one DDR3 SDRAM serving as a really large capture buffer. I’ve yet to experiment with the other Digital discovery features including the digital pattern generator and digital I/O, but this is an amazingly powerful “bit of kit” with a nice GUI-based interface for $199.99.



Digilent Digital Discovery Module Board.jpg


Digilent’s Digital Discovery is based on a Xilinx Spartan-6 FPGA




Note: For more information about the Digital Discovery, contact Digilent directly.




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