Implementing USB 2.0 port sharing in smartphones

Mobile devices require many signal processing integrated circuits (ICs) to meet the various functional requirements of users. A typical smartphone contains a communication processor, an application processor, and a power management IC, all of which must share a single USB port and communicate at the high-speed USB data rate of 480Mbps. This article specifically introduces some solutions to this problem and compares various different solutions from USB hubs to simple analog switches.

One design approach for smartphones is to have the internal power management device control a single USB 2.0 port. This can be accomplished by using a 3:1 multiplexing USB switch to steer the USB 2.0 port to itself. By default, it can also steer to the application processor for most multimedia functionality (such as MP3 playback or video processing). It can also steer to the communications processor for radio communications, which can access data or make calls (see Figure 1). This architecture has the advantage of allowing the phone to go to sleep when the functionality is not in use. In addition, the power management unit can wake up the relevant processor when activity is detected on the USB 2.0 port or when either processor needs to use the USB port. After the USB receptacle is first plugged in, the power management IC can also interrogate the USB lines to determine if a dedicated USB charger or charging host port is connected so that the battery can be charged directly via the VBUS signal. When communicating with a USB host device (such as a PC), the physical layer (PHY) within the communications and application processors uses the full 480Mbps high-speed data bandwidth of the USB switch.

Figure 1: A multiplexed USB 2.0 switch used to share a single USB 2.0 port.

The latest trend in 4G mobile phones is to integrate two processing elements in the phone and access the USB port at the same time. For this application, a good choice is to use a hub. However, the trouble is that the wiring of the USB hub is usually capacitive and consumes a lot of power; on the other hand, there are not many opportunities to access the USB ports at the same time. Figure 2 shows a design approach for such a mobile phone, in which the 3G communication processor must be separated from the 4G processor to access the USB port separately. The capacitance can be minimized by using an isolation switch (FSUSB31) and very short PCB traces. In this way, a high-speed transmit USB data eye has a large margin compared to the USB specification on the path from the 3G communication processor to the USB host device. In this example, the application processor controls the USB 3:1 multiplexing switch, which allows the pull-up and pull-down resistors on its control lines to connect to the default application processor when the latter is in a low-power state in standby mode.

Figure 2: Multiplexing USB 2.0 switch and hub application with isolation switch.

These functions are accomplished with a switch that consumes very little power when powered and almost zero power when disabled. In the 4G mobile phone application shown in Figure 2, the FSUSB63 is always powered on and consumes only a few microamps. Meanwhile, in the application shown in Figure 1, when the power management IC turns off the FSUSB63 in standby mode, its power consumption drops to less than 1 microamp.

High speed and low power are often conflicting characteristics for most integrated circuits. Solutions that can achieve both often require lower voltages and use finer geometry; however, the USB specification requires high voltage signals. In robust cell phone designs, the ability of the D+ and D- signals to withstand shorts to the 5V VBUS signal line limits low voltage solutions. However, recently introduced low power charge pump USB switches address this issue and meet the stringent USB transmit eye diagram requirements, as shown in Figure 3. In fact, most of these designs can operate at more than twice the high-speed USB data rate (> 1 Gbps) without any changes to the architecture or selected components.

Figure 3: High-speed USB 2.0 transmit eye diagram.

To switch from one processor to another via a USB switch, all paths need to be disconnected first, and then enough time is allowed to ensure that the USB host port controller can recognize a disconnection and switch to another path; this allows the host device to reset and reconfigure the new USB device. This is all done in software on the phone and requires the involvement of software engineers to write the software according to the constraints of the selected hardware. However, the above methods are gradually being eliminated as more advanced USB switches allow for greater use of portable device software. This is because any change in the select control signal of these USB switches will automatically initiate a disconnect within a predetermined period of time. This time is determined by the USB 2.0 high-speed specification to allow the USB host device to recognize the disconnection and then generate a new connection internally.

As mentioned above, power saving is critical for most portable devices, so the processor supply voltage is often reduced to 1.2V or lower. Therefore, when interfacing with higher voltage devices that rely directly on battery power, the battery power consumption is considerable even if the input voltage is low. One way to reduce power consumption is to use a voltage converter. Since USB switching itself is a very slow process, by setting the input threshold based on the lowest supply voltage, the power consumption of these USB switches in such environments is negligible. In addition, input buffers are designed to achieve power saving in the worst voltage difference situation to greatly reduce the work intensity of system designers.

Figure 4: Audio headset using a single USB port.

For smaller portable devices, printed circuit board (PCB) space and cost are always important considerations. USB switches can now be designed in miniature packages with pin pitches of only 0.4mm, which takes up very little space on the PCB, thus far outperforming large-size proprietary connectors with dongles dedicated to USB connections. In addition, the cost of such switches is much lower than that of USB hubs.

Sometimes, it is necessary to cascade USB switches. In this application, a side loading controller uses the Media Transfer Protocol to download movies from a host PC to an SD memory card without burdening the application processor in the smartphone. In this case, the capacitance of the USB switch in the OFF state is critical to ensure that high-speed USB traffic is feasible for the fastest download speeds. The OFF capacitance of state-of-the-art USB switches is quite low (typically 2pF), which allows multiple switches to be added at the final stage of the phone design, thereby adding new features to the smartphone.

To minimize the number of connectors on smartphones, in addition to high-speed and/or full-speed USB, USB ports are sometimes designed to attach to analog audio headsets. Special USB switches meet this requirement by sending the headset microphone signal through the VBUS signal of the USB connector, while the D+ and D- signals are sent to the left and right speaker signals of the headset respectively. Taking the USB switch FSA800 as an example, after insertion, it determines whether the USB port is a USB charger using the USB battery charging specification algorithm. This allows the processor to control the switch path selection based on the state of the ID pin signal of the USB connector. Other USB switches use the ID pin to detect and automatically switch configurations and can help implement a large variety of complex accessory functions, including: a wide range of accessories, such as very special factory test cables that can use existing factory test equipment to obtain maximum test cost-effectiveness; very special music playing headphones equipped with all remote control buttons required for MP3 playback; and FM radio, or click-and-pop free call making/receiving, with seamless switching between different functions.

Today’s smartphones take advantage of high-speed USB switches to achieve tremendous functionality while reducing the thickness of the phone to a tiny micro-USB high-speed port connector. This USB port can be connected to application processors, communication processors, USB hubs, audio drivers and many other lower-speed functions required if USB communication with multiple devices is required simultaneously. This functionality is achieved thanks to the advanced circuitry built into the latest USB switches without adversely affecting power consumption, PCB chip area and cost. In addition, it also provides USB charger detection, which helps the global push towards a universal USB charger for all portable devices.