Holst Centre is an independent R&D centre – set up by imec Belgium and TNO Netherlands – that develops generic technologies for wireless autonomous sensor technologies and flexible electronics. A key characteristic of Holst Centre is its partnerships with industry and academia based around shared roadmaps and programs.
The Department of Ultra-Low-Power Digital Signal Processing (DSP) focuses on ultra-low-power chip design for wireless, battery-powered devices. Low-power application-specific integrated circuit (ASIC) designs are greatly needed to make devices less dependent on power supplies, enable smaller devices, and provide more embedded processing capabilities.
In our department we research, design, and test prototypes of integrated circuit (IC) cores. Several low-power techniques are applied, including using lower supply voltages and multiple voltage and power domains that can be switched off during idle times. Our goal is to develop a wireless sensor node that contains a sensor, front end, microprocessor, DSP, and a radio that also keeps the power level below 100 µW (see figure 4).
To test our new IC designs and characterize their behavior, we need to measure the current consumption very accurately into the nano-ampere (nA) range. Because we also simulate behavior, we need an automated method to input the test vectors. Up to 96 digital lines have to generate or acquire data through digital I/O synchronously at up to 200 MHz. Additionally, the board has to be powered at different voltages, and finally, all tests have to be performed automatically.
Before the automated test system, we did not have other means of testing our designs. Testing 100 devices by hand is impractical and labor intensive. We discovered that our imec branch in Leuven applied an automated PXI system that could perform these tests. They automated their tests using LabVIEW. With their expertise and the existing LabVIEW code, we quickly set up our own system. They also showed us that a lot of the code they used is taken from the LabVIEW example library and adapted to their needs.
We chose to apply PXI Express instead of PXI to our system because of the increased bandwidth to the digital instruments. Currently, our IC designs do not need to run above 100 MHz clock rates, but we thought we would limit ourselves if the device could not handle higher data rates. The boards we chose can handle up to 200 MHz or even up to 400 MHz in double data rate (DDR2) mode. PXI Express also has the benefit of having dedicated bandwidth to the devices, instead of the 133 MHz shared bandwidth of PXI.
For the digital I/O, the number of data lines and the sample rate were important selection criteria. In some applications with three digital boards totaling 96 lines, there are only eight unused lines. All the lines have to be synchronized. The PXI Express platform provides a modular test platform and an easy way to accurately synchronize the instruments through its backplane.
When developing the ultra-low-power ICs, accuracy is important, especially for current measurements. We looked at various power supplies and advanced digital multimeters (DMMs) and chose the NI PXI-4110 programmable, triple-output, precision DC power supply and the 7.5-digit NI PXI-4071 DMM with a minimum current range of 1 µA.
We first applied the PXI Express test system to our new Bio-DSP. This small chip receives electrocardiograph (ECG) and electroencephalography (EEG) signals, processes these signals, and reports them. We also use the system for automatically testing memory chips. These IC designs apply CMOS technology, but at lower voltages than are usually applied.
The PXI system automates our digital tests as well as performs current measurements. The measurements can be in the nA range, but in some cases we need to have more data points. For example, when we measure the power consumption during read/write cycles, we measure at rates up to 10 kHz. During these measurements we can clearly see the current consumption of a memory bank rise from the 400 nA range all the way up to 13 to 160 µA.
We use NI CB-2162 connector blocks that are connected to the 400 Mbit/s NI PXIe-6548 digital stimulus response boards and interface through flat cables to a small test printed circuit board (PCB) that can fit the IC. Two NI PXI-4110 programmable power supplies provide power to the test PCB. We use dip-switches to power or disconnect areas of the board.
For some of our tests, the IC operates at 50 MHz, so we apply a 4X higher sample rate; the digital board samples at 200 MHz. We use one of the data lines as a clock line to generate the 50 MHz clock to the IC. The acquired data is verified against simulated data. To simulate the ASIC design, we use Cadence’ NC Sim software, which generates either .VCD (value change dump file) or .csv (comma separated file) formatted files. We reformat these files to .hws (hierarchical waveform storage data files) in LabVIEW because we discovered this format is much more efficient in conjunction with the NI PXI-6548 digital instruments. In the LabVIEW application, errors are displayed if the simulation data is different from the measurement. If the measurement is lagging one sample behind, this is usually caused by small alignment errors in the simulation and can be filtered out. After saving the raw data we can further filter out these errors using a small PERL script.
We use LabVIEW to control our tests and PXI instruments. The LabVIEW application can process different test scripts for different applications, like memory test, bit error rate test (BERT), scan chain test, structural tests, as well as functional tests, one after another. For some of the tests, we have 20 to 30 .csv files that have to be processed. Testing a complete IC now takes less than three minutes and generates an enormous amount of data.
Successfully Applying PXI for Automated Validation and Characterization
We have already used the PXI Express automated test system for multiple projects with great satisfaction. The system saves a lot of time in testing the chips and the fast, accurate current measurements are a great asset in our lab. We can clearly see current consumption during different operations of the IC.
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