David Harrison, Design Engineer for Model Sounds Inc.

Transcription

1 David Harrison, Design Engineer for Model Sounds Inc. 1

2 History -1 In 1994 an alliance of four industry partners (Compaq, Intel, Microsoft and NEC) started to specify the Universal Serial Bus (USB). The bus was originally designed with these intentions: Connection of the PC to the telephone Ease-of-use Port expansion Compete with Apple Desktop Bus The specification (v1.0) was first released in January 1996 and the official version 1.1 was released in August USB bus speed of v1.1 was Low Speed (1.5MBits/sec) or Full Speed (12Mbits/sec). Note: These rates are the raw signalling rates, not typical data throughput rates. 2

3 History-2 Version 2.0 was released in April It introduced USB High Speed (480Mbits/sec) signalling rate in addition to the v1.1 Low Speed and Full Speed. Due to bus access constraints, the effective throughput of the High Speed signalling rate is limited to 35 MByte/s or 280 Mbits/sec. USB 3.0 was released in November Added Super Speed (5Gbits/sec) signalling rate with a data throughput of typically 4Gbits/sec. USB 3.1 was released in July Added Super Speed+ (10Gbits/sec) signalling rate with a data throughput of typically 7.2Gbits/sec. 3

4 USB Specifications USB.ORG Can download many specifications e.g. USB MB Is a ZIP file containing tons of documents. The main USB 2.0 spec. alone is 650 pages. HID spec. is 100 pages USB OTG and Embedded host spec. is 100 pages USB 3.1 Spec. is another 57MB and so on 4

5 Basic USB Concepts Best on-line resource for USB information : USB is strictly hierarchical and is controlled by one host. The host uses a master/slave protocol to communicate with attached USB devices. This means that every communication is initiated by the host and USB devices cannot establish any direct connection to other USB devices. This seems to be a drawback in comparison to other bus architectures but it is not because the USB was designed as a compromise of costs and performance. The master/slave protocol solves implicit problems like collision avoidance or distributed bus arbitration. USB 2.0 allows 127 devices to be connected at the same time. 5

6 USB Devices A device can be self powered, bus powered or both. The USB bus can provide a power supply up to 500mA for its devices. If there are only bus powered devices on the bus the maximum power dissipation could be exceeded and therefore self powered devices exist. They need to have their own power supply. Devices that support both power types can switch to self powered mode when attaching an external power supply. The maximum communication speed can differ for particular USB devices. The USB specification differentiates between low speed, full speed, high speed and super speed devices. Low speed devices (such as mice, keyboards, joysticks etc.) communicate at 1.5MBit/s and have only limited capabilities. Full speed devices (such as audio and video systems) can use up to 90% of the 12Mbit/s which is about 10Mbit/s including the protocol overhead. 6

7 USB Connectors 1.x/2.0-1 All devices have an upstream connection to the host and all hosts have a downstream connection to the device. Upstream and downstream connectors are not mechanically interchangeable, thus eliminating illegal loopback connections at hubs such as a downstream port connected to a downstream port. There are commonly two types of connectors, called type A and type B which are shown below. Type A Type B Type A plugs always face upstream. Type A sockets will typically find themselves on hosts and hubs. Type B plugs are always connected downstream and so type B sockets are found on devices. It is interesting to find type A to type A cables wired straight through and an array of USB gender changers in some computer stores. This is in violation of the USB specification. The only type A plug to type A plug devices are bridges which are used to connect two computers together. 7

8 USB Connectors - 1.x/2.0-2 Standard, mini, and micro USB plugs (not to scale). The white areas in these drawings represent hollow spaces. Pin numbering looking into receptacles is mirrored from plugs, such that pin 1 on plug connects to pin 1 on the receptacle. USB 1.x/2.0 mini/micro pinout Pin Name Wire color Description USB 1.x/2.0 standard pinout Pin Name Wire color Description 1 V BUS Red (or orange) +5 V 2 D White (or gold) Data 3 D+ Green Data+ 4 GND Black (or blue) Ground 1 V BUS Red (or orange) +5 V 2 D White (or gold) Data 3 D+ Green Data+ 4 ID N/A Cable end detect 5 GND Black (or blue) A connector (host): ID connected to the signal ground B connector (device): ID not connected 8

9 USB 3.0/3.1 Standard Connectors Pin Color Signal name ("A" Connector) Signal name ("B" Connector) 5 Blue StdA_SSRX StdB_SSTX 6 Yellow StdA_SSRX+ StdB_SSTX+ Description SuperSpeed transmitter differential pair 7 N/A GND_DRAIN Ground for signal return 8 Purple StdA_SSTX StdB_SSRX 9 Orange StdA_SSTX+ StdB_SSRX+ SuperSpeed receiver differential pair 9

10 USB 3.0/3.1 Micro Connectors micro-b USB 3.0 plug 1. Power (V BUS ) 2. USB 2.0 data (D ) 3. USB 2.0 data+ (D+) 4. GND 5. USB 3.0 transmit (SSTx ) 6. USB 3.0 transmit+ (SSTx+) 7. GND_DRAIN 8. USB 3.0 receive (SSRx ) 9. USB 3.0 receive+ (SSRx+) 10. DPWR provided by device (Powered-B only) 10

11 USB Speed Identification Low/Full Speed A USB device must indicate its speed by pulling either the D+ or D- line high to +3.3 volts. (NOT 5V USB PWR). A full speed device will use a pull up resistor attached to D+ to specify itself as a full speed device. A low speed device will use a pull up resistor attached to D- to specify itself as a full speed device. These pull up resistors at the device end will also be used by the host or hub to detect the presence of a device connected to its port. Without a pull up resistor, USB assumes there is nothing connected to the bus. Some devices have this resistor built into its silicon, which can be turned on and off under firmware control, others require an external resistor. 11

12 USB Speed Identification High Speed A USB 2.0 High speed device initially identifies itself as a Full Speed device (with a pull-up from D+ to a microcontroller pin initially set to +3.3V). Then, as part of the high speed enumeration process, the device identifies its high speed capabilities, then disconnects the pull-up to balance the D+/D- lines. 12

13 USB Device Architecture - 1 A physical USB device may consist of several logical sub-devices that are referred to as device functions. A single device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function) OR A Mass Storage Device and a HID device. This kind of device is called a USB Composite Device. 13

14 USB Device Architecture - 2 USB device communication is based on pipes (logical channels). A pipe is a connection from the host controller to a logical entity, found on a device, and named an endpoint. Because pipes correspond 1-to-1 to endpoints, the terms are sometimes used interchangeably. A USB device could have up to 32 endpoints (16 IN, 16 OUT), but it's rare to have so many. An endpoint is defined and numbered by the device during initialization (the period after physical connection called "enumeration") and so is relatively permanent, whereas a pipe may be opened and closed. USB endpoints reside on the connected device : the channels to the host are referred to as pipes. 14

15 USB Device Architecture - 3 There are two types of pipe: stream and message. A message pipe is bi-directional and is used for control transfers. Message pipes are typically used for short, simple commands to the device, and a status response used, for example, by the bus control pipe number 0. A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an isochronous, interrupt, or bulk transfer: isochronous transfers: at some guaranteed data rate (often, but not necessarily, as fast as possible) but with possible data loss (e.g., real-time audio or video). interrupt transfers: devices that need guaranteed quick responses (bounded latency) (e.g., pointing devices and keyboards). bulk transfers: large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g., file transfers). 15

16 USB Device Architecture - 4 An endpoint of a pipe is addressable with a device_address and endpoint_number as specified in a TOKEN packet that the host sends when it wants to initiate a data transfer session. If the direction of the data transfer is from the host to the endpoint, the host sends an OUT packet having the desired device address and endpoint number. If the direction of the data transfer is from the device to the host, the device sends an IN packet to the host. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g. the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet is ignored. Otherwise, it is accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets. 16

17 Endpoints and Interfaces Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface. A single device function composed of independently controlled interfaces is called a Composite Device. A composite device only has a single device address because the host only assigns a device address to a function. 17

18 Device Enumeration When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by the host sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address (hence max. 127 devices). If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. The device identifies itself by a 2 byte VendorID and a 2 byte DeviceID. These are used to identify the correct device driver to load. 18

19 USB Bus Speed With Different Device Speeds The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host. In USB 2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to that port. High-speed USB 2.0 hubs contain devices called transaction translators that convert between high-speed USB 2.0 buses and full and low speed buses. When a high-speed USB 2.0 hub is plugged into a high-speed USB host or hub, it operates in high-speed mode. The USB hub then uses either one transaction translator per hub to create a full/low-speed bus routed to all full and low speed devices on the hub, or uses one transaction translator per port to create an isolated full/low-speed bus per port on the hub. 19

20 Device Classes The functionality of USB devices is defined by class codes, communicated to the USB host to affect the loading of software driver modules for each connected device. This provides for adaptability and device independence of the host to support new devices from different manufacturers. Class Usage Description Examples 0x00 Device Unspecified 0x01 Interface Audio Speaker, microphone, sound card, MIDI 0x02 Both Communications and CDC Control 0x03 Interface Human Interface Device (HID) 0x05 Interface Physical Interface Device (PID) Modem, Ethernet adapter, Wi-Fi adapter. CDC is Comms Device Class. Mouse, keyboard, joystick Force feedback joystick 0x06 Interface Image Webcam, scanner 20

21 More USB Device Classes Class Usage Description Examples 0x07 Interface Printer Laser printer, inkjet printer, CNC machine 0x08 Interface Mass Storage USB flash drive, memory card reader, digital audio player, digital camera, external drive 0x09 Device Hub Full bandwidth hub 0x0B Interface Smart Card USB smart card reader 0x0D Interface Content Security 0x0E Interface Video Webcam Fingerprint reader 0xFF Both Vendor Specific Vendor has to provide device specific drivers. ETC. 21

22 USB On-The-Go 1. USB On-The-Go, often abbreviated to USB OTG or just OTG, is a specification first used in late 2001, that allows USB devices such as digital audio players or mobile phones to act as a host, allowing other USB devices like a USB flash drive, digital camera, mouse, or keyboard to be attached to them. 2. Use of USB OTG allows these devices to switch back and forth between the roles of host and client devices. For instance, a mobile phone may read from removable media as the host device, but present itself as a USB Mass Storage Device when connected to a host computer. 3. USB On-The-Go introduces the concept that a device can perform both the master and slave roles whenever two USB devices are connected and one of them is a USB On-The-Go device, they establish a communications link. 22

23 Designing Embedded USB Devices 1. Choose a microcontroller that has a hardware USB peripheral. Most microcontrollers have a 2.0 Full Speed 12Mbits/s USB peripheral. A very few have a High Speed Mbit/s USB peripheral. 2. There are some purely SW USB 1.1 low speed implementations for AVR 8 bit chips. E.G. V-USB Most microcontroller manufacturers provide libraries of examples to illustrate how to use their USB peripherals. 4. If at all possible, choose a pre-defined USB device/interface class that is a standard one supported natively by your OS. E.G. for file transfers choose the USB MSD (Mass Storage Device). If necessary, use a USB composite device having two or more interfaces. 23

24 The Versatile HID USB Device Class 1. Originally designed for low speed Human Interface Devices such as keyboards, mice and game controllers. 2. HID Protocol allows a maximum of 64 Bytes to be sent as OUT or IN packets using the standard control endpoint These 64 byte packets can carry any data payload you like. 4. The uc manufacturers usually have a HID mouse emulator example using a variety of sensors to simulate mouse movement. 5. Once you ve figured out the right library calls to make a mouse, you can simply extend the example to transfer any data you like. 24