Introduction
The Power Tracker option available on the Voyager M3i allows the user to dynamically monitor the VBUS voltage, current and power while the bus is in operation. More importantly this information is synchronised with the bus activity and so it can be correlated to specific bus events. Developers can precisely monitor actual VBUS power consumption as devices enter or /exit low power modes. Power Tracker can also be used to verify changes in bus power consumption pre and post configuration as well as monitor power draw by devices that utilize the VBUS for battery charging.
The Power Tracker is an optional feature on Teledyne LeCroy’s Voyager M3i system and operates for both USB 2.0 and 3.0. Power Tracker monitors voltage and current continuously and stores this and adds a cell to each packet to show the average of these values during each packets transmission.
In addition, a more detailed graph of the VBUS activity in a histogram format is available from the power tracker 
Icon. Power Tracker must be enabled in the recording options tab prior to recording USB activity.
Application Examples
USB Developers are generally aware of the minimum and maximum VBUS values which can usually be measured with a digital voltmeter. However, as the developer approaches compliance the need to understand additional factors - like inrush current becomes important. The Power Tracker option is designed to compliment USB electrical compliance test setups. It can provide and early insight into electrical compliance, although it is not designed to measure InRush current/voltage to a standard required for compliance. It is considered essential for some validation tasks, such as measuring current draw during the “suspend” state transition, because power analysis is automatically synchronised with the protocol events.
Figure shows the inrush current of a device peaking at 1.3A. This is significantly above the allowed value yet is just below the threshold that would typically cause most root ports to register an over-current condition. This condition would indicate the device may require further investigation using tools such as a scope to determine why it peaked so high. We can also see a little further in the graph that the device periodically demands around 1A of current which is double maximum defined in the USB2 specification. Assuming this design had passed USB IF certification, it’s possible one or more of the components inside the device could create this condition.
A similar problem can also happen when physical devices (usually a motor) starts up. The average operational state of the device may be within the 500mA budget. However the initial power needed to spin up or reach an operational state may be many times greater. Figure 3: Physical Disk Spin up shows a bus power mass storage device that incorporates and 2.5” disk drive.. The USB subsystem works perfectly until the disk drive is asked to spin up. This requires >800mA and causes the VBUS to drop, which in return causes the drive to stall. The power recovers and it re-tries multiple times. The power demands of this device are well beyond the ability of the USB host port to provide the required power.
When a USB device returns its device descriptors, it is communicating to the host its power draw during normal operation. In theory, the host uses this to determine if it can supply the power demands of the device and enables/disables it accordingly. No one wants to experience an over current PSU event when saving an important file, so it is important to avoid excessive current demands. However, some designs, are not strictly accurate about the amount of current they draw, occasionally relying on the idea that a root port can provide more power than the 500mA maximum defined in the USB 2.0 specification or the 900mA defined for SuperSpeed devices. Typically with 2.0 host ports this is based on the assumption that a root port pair each allow 500mA, however stacked USB sockets may for convenience share the VBUS source, so the trip has been set a little over 1A for the pair. It is not unusual for a device with a physically spinning subsystem to need more than 500mA to spin up the subsystem but once it is running the requirement falls to 500mA or less. Such devices often come with a tandem cable (noncompliant) to tap the power from two host ports to drive higher power to the device.
Since the early days of USB there have been features built into the specification designed to save power by allowing reduced current draw under certain conditions. With the introduction of USB 3.0, SuperSpeed devices are required to support the low power states below where each state incrementally lowers power usage while increasing the allowed exit latency.
Once the host issues LGO_U1, the device is supposed to go to low power state. While the host continues to supply full 5V to vbus, it is useful to view the actual power draw to verify the actual power savings. The specification imposes strict timing limits as to how quickly the device should return to recovery after U1.Exit. In the image below, the Power Tracker monitoring is synchronised with the trace display allowing users to verify and measure the amount of power saved during each power management state transition.
While supporting low power states is required for SuperSpeed devices, what steps devices actually implement to lower their power draw is left to the device vendor. It has been observed with bus powered devices, that xHCI ports (hosts) will supply the full 5V range even while devices are in low power states. Well designed devices will lower their current draw upon entering low power mode as shown in Figure 5.
As the number of flash storage devices has exploded in recent years, there is exponentially more testing needed with the bus powered implementations. In one case the flash device below would repeatedly “drop the link” during enumeration. The problem occurred immediately after the SET CONFIGURATION transaction where power attributes were assigned for the device. The device vendor asserted that the device was not receiving sufficient VBUS power after the SET CONFIGURATION.
The Power Tracker display above shows that voltage over VBUS was constant at 4.8v at the exact point where the DUT drops the link which confirms the xHCI port was supplying sufficient power over VBUS.
Battery Charging Over VBUS
Some USB systems are also asked to provide power for charging mobile devices. Power Tracker can be used to monitor and verify the power supplied to the range of devices connected to a USB port. Many Laptop ports can now supply Battery Charging Spec v1.2 amps for rechargeable devices. The Power Tracker can show this additional current.
Some USB systems are also asked to provide power for charging mobile devices. Power Tracker can be used to monitor and verify the power supplied to the range of devices connected to a USB port. The amount of current drawn may depend on the initial discharge state of the device. The picture below shows the power draw of a device with a discharged battery and the same device with battery close to full.
It is possible that a battery powered device may get sufficiently discharged that it demands more power than it should. This can have ramifications for the host which may have the local power temporarily pulled down to a point where the USB Clock is effected, causing it to send a response late. Understanding this type of issue is very difficult to debug if the VBUS power monitoring is not synchronised with the trace.
The emergence of USB as a universal connection for transmitting power as well as data has placed a new spotlight on VBUS power draw within portable systems. Developers of laptops, netbooks, smart phones, and tablets now scrutinize every amp of power usage at the system level in their drive for better power efficiency. USB validation tools like the Teledyne LeCroy Voyager M3i system with its integrated Power Tracker feature can help. By combining voltage meter functionality with protocol analyzer features, the Power Tracker helps users correlate actual VBUS power draw with protocol layer state changes. As USB expands its reach into new application areas, Teledyne LeCroy will continue to provide best of breed features allowing developer to deliver innovative USB products.
