Case study: Considerations for USB-based data acquisition and control to extend test capabilities in semiconductor validation
USB universality bodes well for the future of the technology in DAQ and other OEM and embedded applications.
New, high-performance USB-based data acquisition products are assisting the influx of USB into new markets. Simplicity, availability, flexibility, and a focus on embedded and OEM applications all contribute to this growing trend. USB is now the simplest and most interoperable choice for vendor neutrality,
compatibility, and flexibility throughout the design cycle.
Despite the broad selection of new bus technologies over the decades, none ever achieved the heights of ubiquity and flexibility now enjoyed by the USB interface. USB is available on effectively every computer shipped in the last 10 years. Unlike even the ISA bus, which was available on every IBM PC, the USB bus even crosses the “PC species boundary” and is provided on computers from Apple as well. One might compare USB’s broad availability to the PCI bus, which is also available on both platforms, and at its peak was even available on the DEC Alpha system – built right onto the CPU package. Only USB, however, is available on virtually every computer, including Small Form Factor (SFF) systems that don’t have the space for older “slot” cards. Even the 100 mm x 72 mm Pico-ITX standard offers at least two USB ports. In addition to the SFF, desktop, workstation, and server systems from every vendor, USB is present in the latest handheld devices – including Android tablets and phones.
USB’s vast popularity is due to a combination of many factors that lined up uniquely in USB’s favor. Low implementation cost, true plug-and-play, true hot-plug, fast setup times, the ability to power devices over the cable, and a flexible, extensible set of software and protocol standards all combined to take the peripheral market by storm. Due to its popularity and features, USB has revolutionized the way we use – and even design – computer-based systems.
An emerging interface in data acquisition and control
This revolution has also been felt in the Data Acquisition (DAQ) and Control market. USB interconnects the majority of embedded platforms and makes adding system-specific I/O painless. The previously perceived consumer orientation of USB products is changing. Various SFF system vendors now offer specific and targeted USB-based data acquisition products for use in the DAQ industry, their product lines aimed directly at the embedded and OEM marketplace. Features such as custom USB cables with smaller, non-rigid miniature embedded connectors and high-retention designs prevent accidental disconnection of the cable. Steel enclosures with various mounting provisions such as DIN rail and mounting plates, extended-temperature options, vibration, EMI/ESD management, and even NEMA ratings are now commonplace. USB has become the simplest and most interoperable choice for vendor neutrality, compatibility, and flexibility throughout the design cycle.
As USB-based DAQ continues to evolve, industries requiring a broad range of measurement and instrumentation products are able to reduce costs and benefit greatly from modular and easily upgradeable USB solutions. One such industry is semiconductor manufacturing (Figure 1). This highly specialized and multi-step process requires devices to be subjected to a great deal of electrical testing to ensure proper function. Prior to the advent of PC-based solutions, semiconductor testing relied on expensive, proprietary, rack-mounted, or stand-alone instruments. Such legacy products are costly and not easily upgradeable. USB solutions provide both low-cost and simple, highly flexible upgrades.
Semiconductor validation and debug concerns
Recently, a major semiconductor manufacturer was looking for an effective and cost-efficient solution to implement in their semiconductor validation lab. Primary concerns involved dramatically lowering the costs of the current proprietary hardware solution without sacrificing performance. Additionally, the desire to continue to use their existing software architecture was imperative. As chip designs become more complex and volumes increase, advanced test and validation systems are required to manage cycle time and control costs. A dedicated yet platform-independent instrumentation and test platform is essential to balance the ongoing and ever-growing demands of semiconductor validation with the cost-control of capital equipment purchases.
Specifically, the system needed to be capable of exercising load and test parameters via digital and analog outputs on coolant flow, thermal heads, power supplies, and perform Unit Under Test (UUT) stimulation. Tantamount to this was the requirement to precisely monitor the process by signal conditioning and calibrating the outputs of very low-level sensors such as Type T thermocouples, junction temperature diodes, and non-standard Resistance Temperature Detectors (RTDs). Across the measurement spectrum were pressure transducer and flow sensor outputs, and the attenuation of power supply outputs from 0 V to 30 V down to 0 V to 10 V.
All control hardware needed to be dependable and fail-safe to prevent yield losses in case some part of the system becomes inoperable. Specific hardware requirements were generally in groups of eight per system, with scalability of up to 72 such input channels:
- Analog outputs to control the validation process variables
- Type T thermocouple inputs with cold junction compensation
- Junction-temperature diodes
- 1,000 ohm RTDs
- Power supply output level analog inputs
- Digital outputs to control thermal heads and flow controllers
- 0-1 VDC digital inputs to monitor semiconductor signals directly
USB-based DAQ solution
Validation lab engineers quickly realized that the best solution was a USB-based DAQ system. The 16-bit, high-speed USB DAQ system they developed included hardware auto-calibration, using factory-configured signal conditioning cards to interface with each signal type on the requirements list, including input attenuation to reduce the power supply outputs to a level suitable for introduction to the A/D.
Significant cost and resource savings were realized by replacing the expensive, proprietary modular instrumentation approach with a platform-independent USB DAQ system. This system would not be tied to a specific hardware platform or software platform, working in virtually all system environments. The fail-safe requirements of the test system threatened to derail the project, but the flexibility afforded by the USB design allowed for the USB I/O modules to detect a loss of communication and set all analog and digital outputs to neutral values – even if the host computer was unplugged. The DAQ-PACK from ACCES I/O Products, Inc. (Figure 2) offered high levels of customization and flexibility out-of-the-box, and was integrated into a custom system enclosure with thermocouple connectors and a docking station for a netbook or laptop. Further cost savings were realized by the customer integrating the OEM version of the DAQ-PACK into existing enclosures and systems by simply using an available USB port (Figure 3).
The pre-existing control software package used in this case was National Instruments’ LabVIEW, which offers simple integration with thousands of hardware devices. This seamless integration with the DAQ-PACK hardware minimized development time and permitted a similar, familiar software interface to be used.
The best hardware solution oftentimes requires engineers to have cumulative years of measurement and control expertise. The industrial marketplace for USB is still unfolding and it is important to be educated on what will work and what won’t work for a particular application. Applications may require specifications that are not attainable when using USB hardware. These borderline applications are centered around high-speed synchronous measurement – anything needing low-latency “real-time” data. In this case, the customer’s throughput and latency requirements were a perfect match for a USB DAQ solution.
A long future for USB
USB in general provides many advantages for DAQ applications. Simplicity, performance, space-saving, low-cost, availability, installation, and servicing make USB I/O a natural and easy fit for numerous embedded applications. All of this is fine, but what happens tomorrow? Busses come and go, and USB for embedded and industrial I/O is a new idea. How long will designs incorporating USB last?
Given the market penetration and amazing breadth of devices currently available for USB, unmatched by any bus in history, USB will be available on PCs and SBCs for the foreseeable future. The USB Implementers Forum continues to develop the standard, improving and expanding upon the underlying technologies. Wireless USB and USB 3.0 will fill useful roles in the future, further reducing cabling costs, increasing bandwidth, and improving latency.
All of the planned directions for USB growth retain backward compatibility with the existing mountain of devices and software available today. USB 1.1 or 2.0 I/O devices can be plugged into a USB 3.0 computer port and run the same software applications without modification. This compatibility eliminates any concern about the lifespan USB may have.
USB continues to develop in the OEM and embedded marketplace. Current popular form factors are quickly being supplanted in the hearts and minds of the industry by newer and ever-smaller systems such as the Pico-ITX and nanoETXexpress form factors. USB’s OEM and embedded features include the tiny (72 mm x 60 mm) USB/PICO form factor, PC/104 mounting compatibility, high retention USB connectors, and micro-fit embedded USB header. This re-use of the existing mechanical standards allows for maximum flexibility in OEM applications by leveraging existing enclosure and mounting infrastructures while avoiding the limitations of pass-through stacking modules and slot-cards. Future generations of embedded systems will only get better as underlying USB technologies continue to improve.
ACCES I/O Products, Inc. firstname.lastname@example.org email@example.com firstname.lastname@example.org www.accesio.com