by Brig Asay, Keysight Technologies
USB-C cords promise to be all-in-one cables for the next decade thanks, in no small part, to the way this new spec handles electrical power.
Over the next few years, the ubiquitous USB connector will move from the versions familiar today, called Type-A and Type-B, to a new version called Type-C. This is much more than a mere cosmetic change; widely used connection technologies that include DisplayPort (for video), MHL (for audio and video), and Thunderbolt (for data and video) have all announced they’ll move into the Type-C connector.
Consumers probably think the consolidation of multiple connector styles into one sounds like a great idea. But there are many other reasons to be excited about the change. Some of the reasons include better usability (yes, a USB connector can be plugged in either way), higher power handling power (up to 100 W), and devices using Type-C can be either a supplier or a consumer of power. Additionally, the Type-C connector is smaller.
Excitement around the Type-C connector has brought visibility to a specification that has been around for many years, but is at the heart of many of Type-C connector’s key features. This technology is the USB power delivery specification. USB, DisplayPort, MHL and Thunderbolt are pushing data rates of up to 20 Gb/sec. The USB power delivery specification only runs at 300 kHz, yet there is as much excitement about it as for the other technologies. The reason for all the excitement is that power delivery is the backbone behind the Type-C revolution.
What is power delivery?
The USB-IF website says, “The USB Power Delivery Specification enables the maximum functionality of USB by providing more flexible power delivery along with data over a single cable. Its aim is to operate with and build on the existing USB ecosystem.” Given the revolution within Type-C, the power delivery specification becomes increasingly important.
Power delivery resolves cable orientation to establish data bus routing, establishes the “host” and “device” roles between two attached ports, discovers and configures the Vbus pin, configures Vconn, and discovers and configures the optional alternate mode. One example from the USB-IF website shows just how configurable the power is: Battery powered devices implementing Type-C can both charge up from a hub and then give power back temporarily if need be.
The Type-C connector now has 24 pins. These pins include a CC line (channel configuration), which is a single-ended, BMC encoded, 4b/5b signal used for PD (Power Delivery) negotiation. The CC line is where much of the power delivery testing takes place.
A few details about the concept of the alternate mode: Simply put, it refers to running a technology other than USB 3.1 over the Type-C connector. Not surprisingly, alternate mode really depends on the power delivery technology, because the power delivery technology provides the hand shaking between USB and other technologies such as MHL, DisplayPort and Thunderbolt.
Part of the large interest in power delivery stems from the more complicated Type-C connector. Previously, power negation was simple: The connector could only connect one way, power management was straight forward, and only one technology was ever used on a USB connector. Now, the Type-C connector is much smarter in the management of power. Plus, the device must recognize which way the connector is plugged in. The connection won’t work if the devices think the connector is right side up when it is upside down. The increased complexity means there is a need for more test specifications.
The power delivery specification considers such factors as supply drift and static load response. The specification also looks at high-frequency transients and noise and the programmable rail response. The CC line alone has a whole new level of testing, including eye diagram testing, rise time testing and a CRC check. Some of the other tests on the CC line include fall-time testing, bit rate testing, voltage swing testing and inter-frame gap testing. The CC line also undergoes full protocol testing, along with the monitoring of the CC line to ensure that what is being programmed is what is actually being passed through the line.
The testing of power delivery focuses on three chapters from the power delivery specification: chapter 5, physical layer test; chapter 6, protocol test; and chapter 7, power supply. Oscilloscope vendors and third-party test houses have begun to develop automated tests for the power delivery specification. For instance, Granite River Labs now sells a complete power delivery test suite, and other vendors will shortly follow. The advantage of these compliance applications is that validation engineers needn’t deeply understand the many power delivery tests. And they can spend more time analyzing the data and less time writing complicated tests.
The power delivery controller is another important component of the power delivery test specification. This controller “toggles” the devices into and out of the different modes that the Type-C connector can handle. Currently, there are a number of power delivery controllers on the market, and all have different functions.
Engineers who validate Type-C functions should look to see that the power delivery controller has been integrated into the test and measurement company’s compliance software. More complete integration between the PDC and the compliance software makes possible more automation in testing. The PDC, at bare minimum, should support Alt mode link negotiation and Power Delivery handshaking needed for other tests such as high-speed PHY testing.
The hardware needed for testing the power delivery specification depends on the chapter of interest. For instance, chapter 5 includes an oscilloscope (such as a Keysight 10-bit S-series), two probes (such as the Keysight 1147B current probe and N2837A passive probes), and two power delivery coupons (which can be purchased from companies such as Granite River Labs or Luxshare). The aforementioned software from Granite River Labs will run all the tests in chapter 5 in an automated fashion. The power delivery controller will toggle the device into the mode needed for each of the individual tests to permit their automation.
Chapter 7 testing is more typical of conventional power supply testing. It includes the need for a dc power module, a power analyzer and a dc load. Again, automated software can handle chapter 7 procedures.
There are many challenges in Type-C. In particular, a couple worth considering are power supply drift and incorrect traffic being passed across the power delivery bus. Note the CC line is the key communication/handshaking device for Type-C. Fixtures designed to look at the CC line should give easy access to it.
Tools such as Keysight’s N8837A USB-PD Protocol and Triggering software make it possible to monitor this traffic through the S-series scope used for chapter 5 testing. The N8837A software provides access to a rich set of integrated protocol level triggers. The application includes a suite of configurable protocol-level trigger conditions specific to USB-PD.
When serial triggering is selected, the application enables special real-time triggering hardware inside the Infiniium S-series scope. Hardware-based triggering ensures the scope never misses a trigger event when armed. This hardware takes signals acquired using either scope or digital channels and reconstructs protocol frames. It then inspects these protocol frames against specified protocol-level trigger conditions and triggers when the condition is met.
From the power supply drift side, the Type-C connector supports up to 100 W of power (5 V at 20 A). Devices typically haven’t been designed to handle this type of power, so it is increasingly important to test the power supply. Power supply drift can be monitored easily with equipment featuring low noise and high dc offset. There are many candidates in this category, including Keysight’s N7020A voltage rail probe, which provides dc offset up to 20 V.
Over the next few years, many high-speed digital standards will move to Type-C. While not running at really high speeds, power delivery is at the heart of the Type-C connector. It is the technology that makes all the flexibility of the connector possible. As a result, a technology running at only 300 kHz has become one of the hottest technologies in the high-speed digital industry today.
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