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146 Universal Serial Bus USB Overview Universal Serial Bus or USB, is the latest peripheral connection bus scheme from the computer manufacturers. Developed by a consortium of companies, USB promises faster data transfer, simplified hookup and easier setup of hardware devices. Data transfer speed may be misleading. As a comparison, Figure 10-1 shows relative bandwidth of the some data transfer methods. Note USB is slightly faster than a normal T1 communications line but not as fast as other internal computer bus structures, such as IDE and SCSI. The Universal Serial Bus resulted from an industry-wide initiative to standardize peripheral attachments to personal computers, and to improve the speed, performance and ease of use of any PC peripheral.
PORT
Serial P o rt ISDN Stand ard P arallel Po rt: T1 C o mmunicatio ns Line US B -Lo w EC P /EP P P arallel P o rt: IDE SC S I- 1 SC S I- 2 (F ast S C S I, F ast N arro w S C S I): US B -Hig h F ast Wid e S C S I (Wid e SC S I) Ultra S C S I (SC S I- 3 , F ast- 2 0 , Ultra N arro w) UltraIDE Wid e Ultra SC S I (F ast Wid e 2 0 ) Ultra2 S C S I IEEE- 1 3 9 4 (F irewire) Wide Ultra2 SC S I Ultra3 S C S I Wide Ultra3 SC S I F C - AL F ib er C hannel
D ata R a te (M bits /s e c)
0 . 0 1 4 3 MBYTES/s (11 5 k b its/s) 0 . 0 1 6 MBYTES 0.115MBYTES /s (11 5 k BYTES /s) 0 . 1 9 3 MBYTES /s 1 .5 M B YTES/s 3 MBYTES /s 3 .3- 16.7MBYTES /s 5 MBYTES /s 1 0 MBYTES /s 1 2 M B YTES /s 2 0 MBYTES/s 2 0 MBYTES/s 3 3 MBYTES/s 4 0 MBYTES /s 4 0 MBYTES /s 1 2 . 5 - 5 0 MBYTES /s 80MBYTES/s 8 0 MBYTES/s 160MBYTES /s 1 0 0 - 4 0 0 MBYTES /s
Figure 10-1, Data Rate Comparison
Universal Serial Bus 147 Co-developed by Compaq, Digital, IBM, Intel, Microsoft, NEC and Northern Telecom, and supported by a consortium of 450 technology companies, USB ports at the time of this writing are standard on most new PC's, including laptops shipping to the retail market. USB is currently supported in the latest version of Windows 95 (OSR 2.1), and Windows 98. In addition, according to Microsoft, USB will also be supported in the next release of Windows CE and Windows NT 5.0. Support for USB is currently available on the Apple platform and may be available shortly on the Sun and Digital platforms. USB will improve the connection of add-on peripherals in a number of ways: Plug-in Installation In non-USB enabled PCs, each peripheral device requires its own port, usually gained through one of the few expansion slots available on the PC motherboard. To install all but the most fundamental peripherals, keyboard, monitor, printer and modem, a user must open the case and install an add-on board. Generally, switches must be set, jumper wires configured or the different physical connectors, such as serial or parallel cables, matched. These are steps that frequently discourage an average user from bothering with any new peripherals. Once the motherboard expansion slots are filled, there is very little way to add new peripherals without removing an old one. The promise of USB is simplicity and ease of use. Using intelligence from the host PC, the Universal Serial Bus detects when a device is added or removed while power is on and without having to re-boot the system. This is unlike conventional expansion slots, where power must be completely removed, the device card installed and the system restarted. The Universal Serial Bus automatically determines what host resources, including driver software and bus bandwidth, each peripheral requires, then makes those resources available...with little user intervention. The Universal Serial Bus specification also defines a standardized connector and socket which all peripherals claiming USB compatibility will use, eliminating the existing mixture of connector and cable styles. With USB, one peripheral device, such as the keyboard, monitor or another hub, plugs directly into the PC. Other peripherals may simply connect into either an expansion hub built into the keyboard or monitor, or into a stand-alone USB box. Such expansion hubs provide additional connection sockets and may be connected into a tiered tree arrangement. Each peripheral device can extend up to five meters (about 15 feet) from each other or from the expansion hub. In all, the USB can connect up to 127 different devices to a single PC and also carry a +5 volt power supply line which could eliminate the bulky AC power pack currently required by many peripherals. At a cost equivalent to today's connector technologies, USB's 12 Mbit/sec data rate offers ample throughput for nearly all peripheral devices for which higher performance, higher cost technologies exist.
148 Universal Serial Bus Unlike the previous generation of peripherals, no add-in expansion cards are required to use USB peripherals. For example, USB video cameras and speakers can be used with just a USB connector and the appropriate software. No extra video cards or sound cards are required. This removes complexities introduced by opening the box to install a card and reconfiguring the system (setting IRQs, DMAs, jumpers, dip switches, etc.). High Bandwidth At 12 Mbits/sec, USB is up to 100 times faster than a standard serial port, and nearly 10 times faster than a standard parallel port (See Figure 10-1). It is faster than any port currently standard on the PC platform. In practical terms, this means that eventually all low-speed peripherals may well be USB devices. These peripherals include keyboards, mice, printers, joysticks, modems, scanners, digital cameras, speakers, and even telephones and fingerprint ID systems. Digital speakers, graphics tablets, multimedia digital cameras or futuristic virtual reality goggles and data gloves could be operated on a USB. Universal Serial Bus technology now makes it easy for PC users to connect monitors, printers, digital speakers, modems and input
PC Motherboard
To Other Hubs or Devices
Hub
Upstream Connector Downstream Connector
Hub Device
Device
Device Device
To Other Hubs or Devices
Device
Device
Figure 10-2, USB Device/Hub Hookup
Universal Serial Bus 149 devices like graphics tablets, scanners, digital cameras, joysticks and multimedia game equipment. Although in theory, USB allows faster operation, moving data at 12 megabits (1.5 MBytes) per second, it may still be too slow for devices that require ultra-high throughput, such as camcorders or heavy-duty storage and backup devices. USB has two standard data rates; 12Mbits/sec for devices requiring increased bandwidth, and 1.5 Mbit/sec for lower-speed devices such as joysticks, keyboards and game pads. Built-in Power Supply Elimination of the AC power adapter ("Wall Wart") for some peripherals is another benefit of USB technology. USB can recognize the power requirement for a device, (up to 500 mA), and supply it automatically. Video conferencing cameras and other small peripheral devices available today are bus-powered. Some manufacturers have elected to redesign products with lower power requirements to take advantage of this feature. Two-way Data Support for Telephony Telecommunications devices require two-way (asynchronous) data transfers, not supported by serial, parallel or SCSI bus technologies. USB supports asynchronous and isochronous data transfers so telephones may be integrated with a PC to share voice mail and other features. Smaller System Footprints Today, USB ports share PC and notebook rear panels with other port technologies. It is anticipated that USB will reduce the requirement for PC slots, allowing footprint reduction for desktop systems and replacing the standard serial, parallel and PS2 ports over time. The technology is expected to free up needed space on laptops as well. Easier Expansion As previously stated, theoretically, a USB-compatible PC can run 127 daisy-chained peripherals from a single root hub (normally a PC) port. For technical reasons, it is not practical for a single hub to have more than eight ports per hub or for hubs to be stacked more than five deep. Five hubs daisy-chained together would cut the number of attached USB devices to 36 per port. Devices may be added and removed without restarting the computer after each addition or removal. (This feature is known as "hot swapability.") As long as a device remains connected to the PC any other device in the chain may be removed or added without interference of operation. Although just a single device may be plugged into a USB port, USB hubs may also be attached. Much like a power strip which allow multiple components to be connected into a single AC outlet, a USB hub allows multiple devices to be connected to a single USB port. HID (Human Interface Device) USB supports HID applets needed to provide software control of device functions from the host computer. These applets must be written by the hardware provider or a third party developer.
150 Universal Serial Bus Hubs and Devices USB Devices are separated into two main categories called Hub and Device. Hubs are further divided into passive bus-powered, or unpowered, and powered. There is only one "root" hub in any chain providing all application software commands. This is normally the PC. Expansion hubs simply pass on commands from the "root" hub to the proper device. Devices may also be hubs, providing expansion ports for other downstream devices. A passive hub divides available bus power from an upstream USB port among it's own ports. They must limit current to 100 mA for any single downstream port and have no more than 4 downstream ports. They may or may not include overcurrent detection. Passive hubs would be sufficient for low-power devices such as mice and keyboards, whose current demands are meager. Scanners, modems, data storage and similar devices will demand more power than a passive hub can distribute. Those types of devices will require either a powered hub or contain their own internal power supply. Powered hubs are capable of supplying a maximum of 500 milliamps to each downstream device and are limited to 7 downstream ports. A Device is a peripheral under control of a USB hub. The USB ports communicate with each other to pass commands between the PC and the device to provide the different functions or actions required to satisfy the software application. Monitors, such as the MM101, are natural devices to combine with a powered hub because they have a large power supply available. Monitor hubs, unlike a PC which is usually on the floor or whose USB ports may be otherwise inaccessible, are normally always easy to reach when users decide to connect or disconnect devices. Monitors such as the MM101, are also USB devices, allowing setup through the use of software applets instead of requiring use of front panel buttons and OSD displays. The MM101 supports the adjustment of brightness, contrast, tilt and horizontal/vertical positioning only. The ability of a user to adjust these controls via USB is contingent upon the commercial availability of a monitor control applet. The same advantage for USB speakers means that users will be able to adjust volume, tone or effects settings using software control instead of knobs on the front of speakers. Branching Hubs are used to provide branching with one upstream port and a number of downstream ports. The convention is that those ports going towards the computer are "upstream" while those going away are "downstream". Upstream connectors are square to differentiate them from the rectangular shaped downstream connectors. All interconnecting cables have a square upstream connector at one end and a rectangular downstream connector at the other, making it impossible to connect ports incorrectly. USB Topology The USB topology has three elements that work together to enable four different types of data transfer. The three elements are: Host, Hub, and Function. Within a USB system, the Host controls the flow of data and control information over the bus. This capability normally resides with the PC on the motherboard. Functions provide capabilities from devices to the host system or application. These can include typical
Universal Serial Bus 151 PC activities such as keyboard or joystick input and monitor controls, or more advanced activities such as digital telephony and image transfer. Finally, Hubs provide an expansion port for USB by supplying a connection to other USB functions and devices. There are four types of data transfer over the USB. Control, (bi-directional), is used to set up the USB structure and address allocation. Interrupt provides a time-critical, uni-directional link from the device to the host PC for a mouse, keyboard or joystick. Bulk, has a variable bandwidth, adjusted by the host PC depending on the amount of traffic on the USB. It is used for devices such as a scanners or printers, where delivery of data is not time-critical. Isochronous has a fixed bandwidth pre-negotiated with the host PC for devices that must have a specific bandwidth available at all times. This might be data such as an audio or MPEG video stream that has maximum permitted delay in the signal. Bulk and Isochronous are uni-directional links but can be either from the computer to the device or vice versa, but not both. USB Hubs USB uses a "tiered star topology" which means that some USB devices, called USB "hubs", can serve as connection ports for other USB peripherals. Only one device needs to be plugged into the PC. Other devices can then be plugged into the hub. Hub controller IC's may be embedded in such devices as monitors, printers and keyboards. Stand-alone hubs could also be made available, providing a handful of convenient USB ports, right on the desktop. Hubs feature an upstream connection (pointed toward the PC) as well as multiple downstream ports to allow the connection of additional peripheral devices. USB hubs play an integral role in the expansion of the PC. With device connections furnished by embedded hubs in keyboards, monitors, printers, and other devices, attaching or removing a new peripheral might be as simple as connecting the plugs. For even simpler connectivity, the USB cable consists of only four wires: Vcc, D+, D-, and GND. The connector cables have two different size and shape plugs to assure downstream and upstream connectors cannot be crossed. Data is differentially driven over D+ and D- at a bit rate of 12 Mbit/s for full-speed signaling, or a rate of 1.5 Mbit/s for the USB low-speed signaling mode.
+Vcc D+
1 2 3 4
MM101 USB Hub Panel
DGnd
USB Cable Connectors
Figure 10-3, USB Port Pin Out
152 Universal Serial Bus
HOST
Software Application Management
LOGIC DATA FLOW
DEVICE
Function: Interface for Peripheral Action
USB System Drivers (Manage Devices)
LOGIC DATA FLOW
USB Logical Device (Collection of Endpoints)
USB Bus Interface Host Controller
UPSTREAM (A Port) USB PORT DOWNSTREAM (B Port)
USB Bus Interface Peripheral Controller
USB PORT
PHYSICAL CONNECTION FOR DATA FLOW
Figure 10-4, USB Data Flow USB Operation Figure 10-4 illustrates USB data flow on three logical levels: application software to function, USB driver to device controller, and the USB physical connection and interface, where actual transmission of data and/or power occurs. On the highest logical level, USB is the transport agent for data transfers moving between application software and desired function and results required of the device. This is the endpoint for the data. There is one logical endpoint for each softwarefunction pipe. Devices are managed by USB system software drivers. Every device on a USB pipeline has a unique "endpoint" address for control transfers. These define USB function configurations. Control transfers allow the host to access different parts of a device and communicate between application software and desired functions using configuration/command/status control type information. Data content includes standard, class and vendor type requests. USB host controllers, normally found on the PC motherboard, manage and control the driver software and bandwidth (data flow) required by each peripheral connected to the bus. User intervention in the control process is not required, because all configuration steps are automatic. The USB host controller allocates electrical power to the USB devices. Like USB host controllers, USB hubs can detect attachments and detachments of peripherals occurring downstream and supply appropriate levels of power to downstream devices. Other endpoints are optionally determined by implementation requirements, with a total of 16 endpoints available to each device.
Universal Serial Bus 153 Device Address When the PC is switched on, every device and hub connected to the USB assumes address #0 and all downstream connections are disabled. Remember, these are software, not hardware addresses! There are no switches or jumpers on USB compatible devices. The PC then interrogates the USB and finds the first device (which may be a device or a hub). It then allocates address #1 (reserving address #0) to this hub and activates the first downstream connector. If there are more devices on this hub, they are interrogated and given the next logical address, #2. When the end of the first hub is reached, the PC interrogates the next connection to find more devices or hubs, assigning them the next highest logical address. The process continues with each device or hub being identified in turn and allocated a unique address number. Once all devices or hubs are found (or the limit of 127 is reached) interrogation stops. At the same time as the allocation of addresses, the PC also determines what drivers should be loaded for each device. Hubs only require USB drivers. The USB continuously interrogates the bus for changed or unresponding addresses. NOTE: The actual address allocation scheme may vary from host to host and is dependent upon the devices that have been previously "registered" (information available in the system registry). By reserving address #0, adding or removing devices without turning off the PC or upsetting the existing configuration is possible. This is known as "hot plug-n-play" or "hot swapability". The USB always recognizes address #0 as a new device. Any device connected to the bus has a default address #0, since it has just been powered up. When the USB sees a device at address #0, it interrogates it to determine what it is, loads the appropriate drivers, then allocates to it the first unused logical address number in its device list. The removal of a device is also noted by the computer as a non-responding address and its logical address number is put back into the list of available addresses. If the device is reconnected to the computer, it may or may not be assigned the same logical address number. Figure 10-5 shows a simple address assignment scheme. Packets, Data Transfers and Endpoints Information to USB devices is carried back and forth in data packets. Each packet contains a 7-bit address that identifies the target device. In addition, there is a 4-bit endpoint address and an I/O bit, giving 16 in and 16 out endpoints. Each endpoint is an addressable buffer within the device for different functions, adjustment, or data storage. Endpoints are associated with only one transfer type, bulk. They are used to send or receive data to or from a specific subsystem within a device. For instance, a monitor may contain an addressable buffer that controls brightness. The packet would contain the logical address of the device on the USB, the 4-bit address of the brightness endpoint and the data to be placed in that endpoint buffer.
154 Universal Serial Bus
PC Motherboard
Hub
Address #9
Device
Address #1
Endpoints for Functions
Hub Device
Address #5
Address #2
Address #3
Device
Address #4
Device Device
Address #6
Device
Address #7
Device
Address #8
Figure 10-5, USB Address Assignments
Universal Serial Bus 155 MM101 USB Overview The MM101 adheres to USB specification, Revision 1.0 and USB Hub Specification Revision 1.1 RC2. Three downstream ports and a hub connection are supplied for peripheral and root hub (PC) hookup. As shown in Figures 10-6 and 10-7, these are all standard USB connections.
Figure 10-6, Front Panel USB Connections The front panel connectors are for downstream (USB) devices only. These would most likely be used for connection of joysticks, keyboards and a mouse.
Figure 10-7, Rear Panel USB Connections
Rear panel connectors provide for one upstream (PC/HUB) and one downstream (DEVICE/HUB) port.
156 Universal Serial Bus
+8V USB (From Standby Supply) +3.3V USB 2
U13204 +3.3V REG
3
1 +5V USB 9 USB 46 RESET USB INT 22 +5Vs 1 STDBY 44 DATA STDBY CLK 43 46 13 D C IIC D 12 RUN 1 C 1 A B IIC D USB C 15 2 18 D +5V USB
1 R13229 47K R13237 1000 R13234 33 R13239 470
+5V USB
30 OVER 10 CURRENT
4
R13226 22K
R13242 2 2W
RESET
Q13202 +3.3V USB
R13230 100 R13233 4.7
R13238 680
USB 17 INT
R13235 47K
Q13204
Q13203
R13240 51
R13241 0.2 2W
IIC D 14 RUN 2 C 5
+5V USB
R13232 3.3K
SUPPLY ON-OFF
11
R13228 4700
3 10
3 Q13201 U13206 2
1
SELECT A SELECT B
U13201 USB
19 C 2 3 5 20 29 DSP-3P DSP-3M 25 26
R13231 3.3K
45
9
U13101 SYSTEM CONTROL
U13203 IIC BUS MULTIPLEXER
2 3 4
Device/Hub
5
DSP-2M 28 DSP-2P 27
6 7 8
Device/Hub
Front Panel
1
USP USP P M 31 32
DSP-4M DSP-4P
24 23
1 2 3 4
2 3 4
Device/Hub
PC/Hub
Rear Panel
Figure 10-8, USB Block Diagram USB Supply Operation To be considered a "Powered" host hub, the MM101 must be capable of providing a +5V, 500 mA supply to downstream devices. The regulated supply is derived from +8V generated by the standby supply. Q13204 is the main regulator. When the USB IC, U13201 requests power to downstream devices, pin 11 goes low, shutting off Q13201. With Q13201 off, a high (>6.5V) is placed on Q13202-B from the +8V supply, turning it on. Q13202-E follows Q13202-B which places about +5.9V on Q13204-B. That turns on Q13204 which passes current from the +8V supply to the USB devices. U13206 provides regulation by monitoring the USB supply line through a voltage divider network consisting of R13231 and R13232. They are selected to provide exactly half the regulated supply voltage to U13206-1, a +2.5V reference device. If the supply rises above +5V, the half supply point also increases above +2.5V and U13206 turns on and its internal impedance goes toward zero. The decreased impedance lowers Q13204-B voltage and decreases current flow through the device, reducing output voltage on the emitter (+5V USB).
Universal Serial Bus 157 As the output voltage now begins to drop, when the half supply point on U13206-1 drops below +2.5V it approaches infinite internal impedance, decreasing current flow through the device. The voltage on Q13204-B now increases, increasing current flow in the device raising output voltage on Q13204-E. As the +5V supply begins increasing the process continues with Q13204-B voltage increasing and decreasing to regulate output voltage. The output device is protected against overcurrent by Q13203. If excessive current flows in Q13204, the voltage drop across R13241 will increase. If it increases such that the Q13203 bias voltage between E-B becomes greater than 0.6V, Q13203 turns on. A current path now exists between the output voltage and Q13204-B drawing the voltage closer to Q13204-E, which is the bias voltage of the output. As this bias voltage decreases, current flow in the output device also decreases. When current decreases enough to lower the E-B voltage of Q13203, it turns off and normal operation returns. USB devices are also protected against overcurrent by the USB IC, U13201. Pin 10 monitors the regulated USB supply via a voltage divider consisting of R13226 and R13235. If the regulated supply decreases, indicating heavy current demand, the USB IC shuts off the supply by shutting off the output signal on pin 11. The +3.3V supply now turns on Q13201 reducing Q13202-B voltage to less than +1.0V. Q13202 now shuts off, removing base drive from the output device, Q13204. Current flow through the output now ceases removing the +5V USB supply from the USB ports. The MM101 USB supply only protects against overcurrent from any device, not individual devices and can only interrupt all current flow. Some USB hubs can sense each port and shut off an individual port when overcurrent is indicated, leaving the other ports to operate normally. The MM101 will shut down all downstream current flow when any indication of over current is sensed. If the USB supply shuts down, disconnect downstream devices one by one until the defective device is isolated. The remaining devices may then be reconnected. USB-MM101 Communications All communications from a PC or USB Hub to USB devices is administrated by the USB controller IC, U13201. Communications between the USB controller and the MM101 microprocessor occurs over the IIC bus. The MM101 may initiate a reset of the USB controller or jettison the device during a "Batten" routine. The USB controller provides data and control isolation between any device connected to one of the MM101's ports and the chassis. U13201 is a self-contained IC capable of all communications on the USB without MM101 intervention. Once the MM101 USB hub has been recognized and enumerated (assigned an address), activities between downstream devices and the upstream host (PC) are reasonably independent. The MM101 is required to respond to a host computer query from time to time. Although USB supports interactive control of monitor functions, a suitable HID (Human Interface Device) applet from a third party vendor is required. TCE does not supply an applet at retail delivery.
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