The Idle modes may be entered due to inactivity on a downstream port for a programmable period of time or may be initiated by the device, based on scheduling information received from the host. Such information is indicated by the host to the device using the flags “Packet pending,” “End of burst,” and “Last packet.” Based on these flags, the device may decide to enter an Idle mode without having to wait for inactivity on the bus.
When a link is in one of these Idle states, communication may take place via low-frequency period signaling (LFPS), which consumes significantly lower power than SuperSpeed signaling. In fact, the Idle mode can be exited with an LFPS transmission from either the host or device.
The USB 3.0 standard also introduces the “Function Suspend” feature, which enables the power management of the individual functions of a composite device. This provides the flexibility of suspending certain functions of a composite device, while other functions remain active.
Additional power savings are achieved via a latency tolerance messaging (LTM) mechanism implemented by USB 3.0. A device may inform the host of the maximum delay it can tolerate from the time it reports an ERDY status to the time it receives a response. The host may factor in this latency tolerance to manage system power.
Thus, power efficiency is embedded into all levels of a USB 3.0 system, including the link layer, protocol layer, and PHY. Figure 2 compares the system power consumed during a SuperSpeed and a Hi-Speed data transfer.
A USB 3.0 system requires more power while active. But due to its higher data rate and various power-efficiency features, it remains active for shorter periods. A SuperSpeed data transfer could cost up to 50% less power than a Hi-Speed transfer. This is crucial to the battery life of mobile handset devices.
STREAMS AND MASS STORAGE ACCESS
A new model of “Streams” supports the SuperSpeed raw data rate. Multiple buffers of data may be set up and organized as Streams to a single bulk endpoint. Up to 64k Streams may be multiplexed per endpoint.
Streams are available on both IN and OUT endpoints, and each Stream is tagged with a Stream ID. Both the host and the device can establish the “Current Stream” associated with an endpoint. The host or device may also truncate a Stream (Fig. 3) when necessary.
Streams make it possible to realize an out-of-order execution model required for command queuing. Currently, the USB Mass Storage Class (MSC) standard is the protocol of choice to communicate with storage devices.
But the MSC protocol imposes certain limitations. For example, an MSC host can only issue one command at a time. Also, the host and device require frequent intervention during command processing. These inherent restrictions lead to significant bottlenecks in MSC transfers. They limit throughput in current USB 2.0 systems and would severely impair throughput in future USB 3.0 systems.
The concept of Streams would enable more powerful mass storage protocols. A typical communication link would consist of a command OUT pipe, an IN and OUT pipe (with multiple data streams), and a status pipe. The host would be able to queue commands, i.e., issue a new command without waiting for completion of a prior one, tagging each command with a Stream ID. As a result, Streams would be essential to alleviate MSC bottlenecks.
COMPARISON WITH OTHER INTERFACES
Several other serial communication standards boast a data rate similar to that of USB 3.0. For example, PCIe Gen 2.0 has a data rate of 5 Gbits/s, and Serial ATA (SATA) III has a data rate of 6 Gbits/s. Although PCIe inspired the USB 3.0 PHY architecture, several key distinctions exist.
The USB 3.0 PHY must implement an equalizer required to compensate for cable loss. This enables USB cables to be as long as 3 meters. The PCIe and SATA PHYs do not have such an equalizer. The LFPS feature described earlier is also unique to the USB 3.0 PHY. And, a PCIe link may consist of multiple lanes (i.e., pairs of transmit and receive differential lines) to increase bandwidth, whereas a USB 3.0 link supports only a single lane.
Of these three interfaces, only USB enables true plug and play. The SATA standard does support hot-plug, but it’s contingent on the SATA controller functioning in Advanced Host Controller Interface (AHCI) mode.
PCIe is typically used to connect peripheral function cards (such as graphic cards) directly onto the motherboard of a PC. SATA is an interface of choice for mass storage devices such as hard-disk drives and optical drives. Hence, most PCs integrate a SATA host adapter. USB, on the other hand, serves as a general-purpose SuperSpeed or Hi-Speed bridge between a host and virtually any interface.
USB 3.0 also lends itself well to be an alternative interface for transferring video given the increased bandwidth. This could herald some interesting applications in the digital living room, including TV, set-top boxes (STBs), monitors, and gaming consoles. The key advantage of USB versus video interfaces like VGA, DVI, DisplayPort, and HDMI is its ubiquity. With HDCP 2.0 including USB as one of the interfaces, USB 3.0 also can enable content protection. Another benefit is the fact that USB is royalty free.
USB CHARGING
Charging over USB has inherent benefits compared to having discrete chargers for every device. Not only does it lower manufacturing cost by limiting vendor-specific chargers, it also has a significant impact on the environment in the long run by eliminating the need for individual chargers for each USB-based device.
USB 3.0 hosts and hubs enable the battery charging schemes as defined by the USB Battery Charging Specification. It currently defines three types of usage modes for charging:
- Host charger: A host charger is a USB 2.0 host that provides up to 500 mA to a downstream port and implements charger detection. A Hi-Speed device may draw only up to 900 mA from the host charger.
- Hub charger: A hub charger is a USB 2.0 hub that provides up to 500 mA to a downstream port and supports charger detection. The charging functionality is very similar to a host charger.
- Dedicated charger: A dedicated charger provides power over the USB interface but does not enumerate the device. The charging current is limited to 1.5 A.
The specification also provides for dead battery charging, allowing a dead or very weak battery to draw up to 100 mA from a host or hub until it is charged to a reasonable threshold. USB 3.0 systems will continue to leverage the distinct advantage of being able to charge over USB.
USB 3.0 is anticipated to be the ubiquitous solution for many bandwidth-hungry applications. Its evolution from the highly successful USB 2.0 is fueled by increasing multimedia consumption demands and higher-density storage technologies.
USB 3.0 is an enabling technology, but it will still have to prove that it can survive the inflated expectations that precede mass market adoption. Given USB’s track record, the electronics industry can count on the fact that the day is fast approaching when end users can get all the content they want on their way to the airport in less than a minute.