The universal serial bus (USB) was first introduced in 1995.
USB delivered a new way to connect personal computers and devices and allows users to easily connect many different types of devices and peripherals. As a result, USB has essentially replaced ageing serial and parallel ports as the connection of choice for device manufacturers and end users.
One of the biggest factors in the incredible success of USB has been the successful interoperability of USB devices and hosts. Interoperable products are an indirect result of the compliance testing programme developed and maintained by the USB-Implementers Forum (USB-IF), which publishes compliance test specifications and has developed a comprehensive set of compliance tests that are used to verify quality and conformance of USB designs.
If a manufacturer can demonstrate that their products meet these specifications then their customers can be confident that their respective products will work together. Proving that one’s products comply with the USB-IF specification requires a test methodology that meets the definitions contained within the CTS (compliance test specification) published by the USB-IF.
Successful compliance testing provides many benefits. Firstly, logo certification – a product will only gain accreditation to use the official USB logos when proof is given that it meets the CTS. Secondly, interoperability – a product designer cannot predict nor control the user’s environment. Products must be submitted to a robust testing methodology in order to minimise the potential for interoperability problems when connected to a real device in the real world. Finally, customer confidence – one’s customers need to be assured that the products one delivers will work in their OEM environments. By testing to a proven method with approved test solutions, customers have confidence that the products conform to the CTS and will work effectively in their designs.
The USB platform
Since the introduction of USB, the USB-IF has continued to launch new variants of the interface standard. In 2000, the USB-IF announced USB 2.0 to provide increased data throughput for higher-bandwidth devices such as video-conferencing cameras and high-resolution printers.
USB 2.0 provided the choice of three data rates – Low Speed at 1,5 Mbps, Full Speed at 12 Mbps and High Speed at 480 Mbps. A further development has seen the introduction of wireless USB, allowing users to wirelessly interconnect up to 127 devices. Wireless USB delivers a bandwidth of up to 480 Mbps at 3 metres and 110 Mbps at 10 metres.
Now a further enhancement to the USB platform has been announced with the release of SuperSpeed USB 3.0. USB 3.0 is backward compatible with earlier versions and has been designed to address the high bandwidth transport requirements of rich media and large digital files. USB 3.0 provides a 10-fold increase in the data transfer rates compared with USB 2.0, with a data rate specified up to 5 Gbps and data throughput greater than 200 Mbps.
The architecture of USB is such that all of these data rates can co-exist at the same time within a configuration, all controlled by a common PC host. This gives the flexibility to mix and match peripherals with different needs in terms of speed and throughput.
The family of USB specifications, USB 2.0, wireless USB and now USB 3.0, meet the needs of different applications and it is anticipated that all will co-exist in the market. Designers will choose the specification to meet the particular needs of their product. Whatever choice is made, a robust test methodology is required to ensure that the end product is compliant to the relevant standards.
For designers of USB devices and systems, the successful completion of the compliance test regime is essential to ensure conformance to the standards and interoperability.
Each variant of USB has its own compliance test specification. A number of the tests are common between variants, however, as the data rates increase, the requirement for a more thorough testing of high-speed effects increase. RF effects need to be considered when validating the high-speed modes of USB 2.0 and for USB 3.0.
Agilent Technologies offers a portfolio of solutions for testing USB, addressing all of the current variants. The company participates actively in the USB-IF, allowing it to influence and respond rapidly to changes in the test specifications.
Agilent’s USB-IF approved test solutions are used in compliance workshops and by test labs the world over. With Agilent’s oscilloscopes, BERTs, network and logic analysers together with a range of USB specific test applications and fixtures, engineers can quickly and accurately test USB designs to ensure compliance.
USB 2.0 test solutions
The USB 2.0 cabling system comprises four wires, two signal (D+ and D-), power (Vbus) and ground. D+ and D- are configured as a differential signal pair used in NRZI half-duplex mode and serve as the primary information carrier between the host, hubs and peripherals. Devices may be either bus powered with a limit of 500 mA current drain or self powered using a built-in power supply.
The flexibility inherent in USB is a direct result of the stringent specifications, regulations and compliance testing mandated by the USB-IF. To prove electrical compliance requires that signal integrity, current and voltage parametric, and data integrity testing is undertaken. High-speed USB 2.0 in addition requires that high-speed signal quality, receiver sensitivity, CHIRP timing (high-speed handshake) and packet parameters are tested.
Signal quality tests
The basic USB 2.0 electrical test suite includes signal quality, in-rush current and voltage droop/drop tests. For signal quality testing, an oscilloscope is used to observe the signals being transmitted and received. The oscilloscope allows a range of parameters to be measured including signal eye, end of packet width, signalling rate, rise/fall times, cross-over voltage and jitter.
Agilent Infiniium 9000 series oscilloscopes together with Agilent probes and test fixtures, are used to reproduce the USB-IF specified tests. The Agilent USB Compliance Test Software makes USB signal integrity compliance testing as simple as making an automatic measurement. By integrating a run-time version of MATLAB within the oscilloscope for use with the USB-IF test scripts, the software provides a one-box solution to speed signal quality testing as well as provide confidence that results will be consistent to those obtained at certification events or USB-IF approved test labs.
Data integrity test
Determining the cause of a data error or a performance problem in a USB network can be challenging. Logic and protocol analysis allows one to see data traffic, make critical timing measurements and verify protocols, making it the ideal tool for detecting and debugging higher-level data integrity problems. The Infiniium 9000 series mixed signal oscilloscopes include integrated built-in logic and protocol analysis capabilities. The in-scope protocol viewer and debug application software with protocol triggering and USB serial decode allow users to extend debug and testing capabilities without the use of additional test equipment.
High-speed tests
High-speed USB 2.0 requires that high-speed signal quality, receiver sensitivity, CHIRP timing and packet parameters are tested. Since the connection scheme for USB 2.0 uses a single pair of wires for both transmit and receive paths, high-speed signal quality testing has to be sequenced between sending a stimulus and then measuring the response. Agilent’s USB 2.0 Receiver Test Library utilises the Agilent 81134A pattern generator and an Infiniium oscilloscope for extensive receiver testing. The device under test is put into the SEO_NAK mode and the pattern generator is used to apply an amplitude and bit pattern controlled stimulus to test the sensitivity level of the receiver.
The Infiniium oscilloscope test software automatically controls the signal source to easily and quickly sequence through the required tests. Additionally, Agilent provides an automated jitter tolerance test solution for USB 2.0 using the N5990A software platform. The receiver sensitivity and squelch thresholds are then measured using the oscilloscope. Together with the USB Compliance Test Software, this ensures that products are tested to be fully compliant to the USB 2.0 specification requirements.
Agilent has simplified the process of pre-compliance testing by installing a run-time version of MATLAB software in the scope and integrating the USBIF USB test option into the Infiniium oscilloscope’s menu structure. Once the test is executed, the test results appear on the Infiniium display in an HTML-formatted window. Each test also automatically saves the PNG, HTML and TSV files required by the USB. The HTML reports are complete with test configuration, all measurements made, pass/fail status, margin analysis, waveforms and the USB-IF approved MATLAB outputs.
Additionally, with full automation, the oscilloscope is set up automatically for each test to ensure correct measurement configuration. For example, Agilent’s receiver tests communicate between the pattern generator and oscilloscope to automatically load the correct test patterns into the pattern generator, adjust voltages and automatically trigger and measure NAK responses to find pass/fail points for each receiver test as described by USB-IF.
Protocol triggering and decode
Agilent’s 9000 series oscilloscope offers protocol triggering and decode capabilities, allowing users to trigger on and quickly view USB packets, payload, header and detail information. Powerful time-correlated views of waveform and symbol, to the bit level, make it easy to isolate communication faults to logic or analog sources. Engineers can efficiently debug HSIC, UTMI, ULI or other MAC/ PHY interface quickly to track down bit error locations, and quickly move between physical and protocol layer information using the time-correlated tracking marker. The oscilloscope displays protocol content using waveform symbols and a multitab protocol viewer. The packets tab shows a high level view of the packet over time.
USB 3.0 test solutions
USB 3.0 has been introduced to address some of the performance bottlenecks of USB 2.0. High Speed USB 2.0 provides a data rate of 480 Mbps. However, the real data throughout is typically much less with I/O performance limitations typically allowing less than 35 Mbps. When downloading larger files, higher throughput is required to allow manageable transfer times. USB 3.0 provides up to 5 Gbps data rate and greater than 200 Mbps data throughput.
To achieve this higher level of performance, USB 3.0 adds 4 additional wires to the interconnection. The cabling system comprises eight wires in total. Four of these conform to USB 2.0 and are used to ensure backward compatibility. The additional wires are configured as two additional pairs that are dedicated to USB 3.0 communications. These operate in full simplex mode with one pair for transmit and one for receive.
The tenfold increase in data rate of USB 3.0 poses new challenges in testing the transmitter, receiver and cabling system.
Transmitter tests
For USB 3.0 transmitter testing, compliance must be measured at the end of a ‘compliance channel’ with SMA termination for the transmitter signals using a phase matched SMA cable. Agilent’s USB 3.0 test fixture provides a high performance USB 3.0 signal breakout with support for both transmitter and receiver testing with the SMA terminations.
When testing the transmit path, a high-speed oscilloscope must be used to measure the transmitted waveform using compliance patterns. This will allow eye diagram analysis and the measurement of signal amplitudes, jitter, average data rate and rise/fall times. Agilent’s 90000 series of oscilloscopes provide up to 13 GHz of bandwidth. Together with the U7243A USB 3.0 Transmitter Compliance Test Application, the oscilloscope can be used to perform transmitter compliance and validation testing as defined by the USB 3.0 specification.
Receiver tests
USB 3.0 specifies an error counter within the device that can internally check the error performance of the receiver. The specification also provides test modes that include the ability to loop back the received data over the device’s transmitter. Testing receiver sensitivity requires a controlled stimulus or pattern generator, which can provide a calibrated jitter input to the receiver under test. Measuring using the integrated error counter requires the use of an oscilloscope or BERT to read and decode the resulting value. Without an internal error counter, testing in asynchronous loopback mode requires a BERT or protocol analyser to evaluate the resulting bit stream for errors.
The error counter approach is limited to the compliance pattern that can be recognised by the internal device. Although this is adequate for simple pass/fail testing, it does not allow for debugging and characterisation. Generally, receiver stress testing is used to characterise the performance of the USB device under varying conditions of amplitude and jitter. It requires a pattern generator that can apply these different conditions, including the ability to generate true random jitter for the target BER.
Agilent’s portfolio of test solutions addresses all of these issues. For USB 3.0 the N4903B J-BERT can generate the patterns necessary to stimulate the device, including the ability to stress the device for jitter tolerance testing. The captured result can be decoded using an oscilloscope together with the N5990A test automation software platform. Alternatively, Agilent’s BERT together with SuperSpeed protocol analysis solutions from Ellisys can be used in loopback mode to characterise the resulting performance of the device. The Ellisys EX280 is integrated in the N5990A Test Automation Software; therefore, both the JBERT and the EX280 are fully automated.
Channel tests
Due to the increased bandwidth and longer channel lengths of USB 3.0, more testing is required for cables, connectors and channel validation. TDR/TDT measurements are essential to ensure that designs meet the differential impedance and skew targets. VNA measurements are necessary to allow accurate verification of return loss, insertion loss and near/far end crosstalk requirements.
Agilent’s test portfolio allows users to test all aspects of USB 3.0 – the transmitter, the receiver, the channel and the cabling system. The J-BERT N4903B offers a complete jitter tolerance testing for receivers up to 12,5 Gbps. It characterises the jitter tolerance and margins of receivers by providing half-rate clocks with variable duty-cycle distortion to emulate effects of non-ideal clocking. This allows highly accurate characterisation, enabling more robust designs.
Automated Rx testing
Jitter tolerance curves are key to understanding the real jitter performance of a receiver. The N5990A Test Automation Platform offers not only automated receiver compliance testing, but also automated receiver characterisation including jitter tolerance. Realistic conditions are emulated and certain properties of the signal deliberately stress specific building blocks of the DUT. Unlike other time consuming solutions that create the files in software to load waveforms for each individual jitter test point, the N5990A sweeps using the proper stimulus signal including calibrated jitter, thus reducing measurement time. Automated calibration capabilities are critical to ensuring that normal variations in instrument jitter characteristics and interconnect tolerances are compensated prior to testing.
Test fixture
The U7242A USB 3.0 test fixture helps simplify the USB 3.0 measurement process by providing access to the transmitter and receiver measurement points required for USB 3.0 compliance testing. It has been designed for direct SMA connections for easy and accurate measurements with direct connections to the oscilloscope and J-Bert SMA connections. It also includes probing connections for InfiniiMax active differential probes for the characterisation and testing of active bus signalling of USB 3.0 and USB 2.0 traffic.
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