Mobile devices require many signal processing integrated circuits (ICs) to meet a variety of functional requirements of users. A typical smartphone includes a communications processor, an application processor, and a power management IC that all must share a single USB port and communicate at a high speed USB data rate of 480 Mbps.
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Dedicated to some solutions to this problem, and a variety of different solutions from USB hubs to simple analog switches.
One way to design a smartphone is to have an internal power management device that controls a single USB 2.0 port, which can be done by turning the USB 2.0 port to itself using a 3:1 multiplexed USB switch. By default, it can also be turned to an application processor for most multimedia functionality (such as MP3 playback or video processing). It can also be turned to a communications processor for radio communication for data access or calls (see Figure 1). This architecture has the advantage of allowing the phone to go to sleep when the function is not in use. In addition, the power management unit can wake up the associated processor when USB 2.0 port activity is detected, or when any processor needs to use a USB port. After the USB socket is first inserted, the power management IC can also interrogate the USB line to determine if there is a dedicated USB charger or charging master port connection to directly charge the battery through the VBUS signal. When communicating with a USB host device (such as a PC), the physical layer (PHY) within the communication and application processor uses the 480 Mbps full high speed data bandwidth of the USB switch.
Figure 1: Multiplexed USB 2.0 switch for sharing a USB 2.0 port.
The latest trend in 4G phones is to integrate two processing components in the phone while accessing the USB port. For this application, a better option is to use a hub. 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 port at the same time. Figure 2 shows a design method for such a handset in which the 3G communication processor must be separately accessible from the 4G processor to access the USB port separately. Capacitance can be minimized by an isolating switch (FSUSB31) with very short PCB leads. 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 multiplexer switch, which allows the pull-up and pull-down resistors on their control lines to be connected to the default when the latter is in a low-power state in standby mode. Application processor.
Figure 2: Multiplexed USB 2.0 switch and hub application with isolation switch.
The above functions can be realized by a switch. When power-on, the power consumption is extremely small, and the power consumption is almost zero when disabled. In the 4G mobile phone application shown in Figure 2, the FSUSB63 is always powered and consumes only a few microamps. At the same time, in the application shown in Figure 1, when the power management IC turns off the FSUSB63 in the standby mode, its power consumption drops below 1 microamp.
For most integrated circuits, high speed and low power are generally a pair of contradictory features. A solution that combines both often requires lowering the voltage and using a fine geometry process size; however, the USB specification requires a high voltage signal. In a robust handset design, the D+ and D- signals can withstand shorts to the 5V VBUS signal line, limiting the low voltage solution. However, the recently introduced low-power charge pump USB switch solves this problem and meets the stringent USB transmit eye diagram requirements, as shown in Figure 3. In fact, there is no need to make any modifications to the architecture or selected devices, and most of these designs can operate at speeds up to twice the high-speed USB data rate (> 1 Gbps).
Figure 3: Hi-Speed ​​USB 2.0 emission eye diagram.
To switch from one processor to another via a USB switch, you first need to disconnect all paths and then allow enough time to ensure that the USB host port controller recognizes a disconnect and switches to another path. This way, the master device can be reset and the new USB device can be reset. All of this is done on the phone using software and requires the involvement of a software engineer to write the software according to the constraints of the selected hardware. However, the more advanced USB switches now allow more use of portable device software, so the aforementioned methods are gradually being phased out. This is because any change in the selection control signals of these USB switches will automatically initiate a disconnect for a predetermined period of time. This time is determined by the USB 2.0 high speed specification so that the USB master recognizes the disconnect and then internally creates a new connection.
As mentioned above, for most portable devices, energy savings are critical, so the processor's supply voltage often drops to 1.2V or less. Therefore, when connected to a higher voltage device that is directly dependent on battery power, even if the input voltage is low, the battery consumes a considerable amount of power. One way to reduce power consumption is to use a voltage converter. Since the USB switch 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 negligibly small. In addition, an input buffer is designed to save energy in the worst case voltage conditions to greatly reduce the system designer's work intensity.
Figure 4: Audio headset with a single USB port.
For smaller portable devices, the footprint and cost on the printed circuit board (PCB) is always an important consideration. At present, USB switches can be designed and implemented in micro-packages with a pitch of only 0.4mm. The footprint on the PCB is very small, so it is far superior to those of large adapters with adapters dedicated to USB connections (Dongle). There are connectors. In addition, the cost of such a switch is much lower than that of a USB hub.
Sometimes, the USB switch needs to be cascaded. In this application, the side loading controller uses the Media Transfer Protocol to download movies from the host PC to the SD memory card instead of It puts a burden on the application processor in the smartphone. At this time, to ensure that the high-speed USB traffic is feasible for the fastest download speed, the capacitance of the USB switch is OFF. The state-of-the-art USB switch OFF capacitor is quite low (typically 2pF), which allows for the addition of multiple switches in the final stages of the phone design, adding new features to the smartphone.
In order to minimize the number of connectors on the smartphone, in addition to high speed and / or full speed USB, the USB port is sometimes designed to be attached to an analog audio headset. The special USB switch uses the headphone microphone signal sent via the VBUS signal of the USB connector to meet this demand, while the D+ and D- signals are sent to the left and right speaker signals of the headphones, respectively. Taking the USB switch FSA800 as an example, after plugging in, it will determine whether the USB port is a USB charger that uses the USB battery charging specification algorithm. This allows the processor to control the switching path selection based on the ID pin signal state of the USB connector. Other USB switches use the ID pin to detect and automatically switch configurations and can help implement a wide variety of complex accessory functions, including: a wide range of accessories, such as very special factory test cables that can utilize existing plants Test the device for maximum test cost-effectiveness; a very distinctive music player with all the remote buttons required for MP3 playback; and FM radio, or click-and-pop free to make/receive calls, Seamless switching between different functions.
Today's smartphones use the high-speed USB switch to achieve tremendous functionality while also reducing the phone to a small micro-USB high-speed port connector. If you need USB to communicate with multiple devices at the same time, this USB port can be connected to application processors, communication processors, USB hubs, audio drivers, and many other low-cost essential features. This functionality is achieved thanks to the advanced circuitry built into the latest USB switches without adversely affecting power consumption, PCB die area and cost. In addition, it also provides USB charger detection, which is conducive to the global push to the development of universal USB chargers for all portable devices.
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