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.The USB Implementors forum has yet to provide this service.In these cases you maywish to use one assigned to your development system's manufacturer.For example most chip manufacturerswill have a VID/PID combination you can use for your chips which is known not to exist as a commercialdevice.Other chip manufacturers can even sell you a PID to use with their VID for your commercial device.Another more notable feature of USB, is its transfer modes.USB supports Control, Interrupt, Bulk andIsochronous transfers.While we will look at the other transfer modes later, Isochronous allows a device toreserve a defined about of bandwidth with guaranteed latency.This is ideal in Audio or Video applicationswhere congestion may cause loss of data or frames to drop.Each transfer mode provides the designer trade-offs in areas such as error detection and recovery, guaranteed latency and bandwidth.ConnectorsAll devices have an upstream connection to the host and all hosts have a downstream connection to thedevice.Upstream and downstream connectors are not mechanically interchangeable, thus eliminating illegalloopback connections at hubs such as a downstream port connected to a downstream port.There arecommonly two types of connectors, called type A and type B which are shown below.USB in a Nutshell Page 4 www.beyondlogic.org Receptical Type BReceptacle Type A2 11 2 3 43 4Figure 1 : USB ConnectorsType A plugs always face upstream.Type A sockets will typically find themselves on hosts and hubs.Forexample type A sockets are common on computer main boards and hubs.Type B plugs are always connecteddownstream and consequently 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 insome computer stores.This is in contradiction of the USB specification.The only type A plug to type A plugdevices are bridges which are used to connect two computers together.Other prohibited cables are USBextensions which has a plug on one end (either type A or type B) and a socket on the other.These cablesviolate the cable length requirements of USB.USB 2.0 included errata which introduces mini-USB B connectors.The details on these connectors can befound in Mini-B Connector Engineering Change Notice.The reasoning behind the mini connectors came fromthe range of miniature electronic devices such as mobile phones and organisers.The current type B connectoris too large to be easily integrated into these devices.Just recently released has been the On-The-Go specification which adds peer-to-peer functionality to USB.This introduces USB hosts into mobile phone and electronic organisers, and thus has included a specificationfor mini-A plugs, mini-A receptacles, and mini-AB receptacles.I guess we should be inundated with mini USBcables soon and a range of mini to standard converter cables.Pin Number Cable Colour Function1Red VBUS (5 volts)2White D-3 Green D+4 Black GroundTable 1 : USB Pin FunctionsStandard internal wire colours are used in USB cables, making it easier to identify wires from manufacturer tomanufacturer.The standard specifies various electrical parameters for the cables.It is interesting to read thedetail the original USB 1.0 spec included.You would understand it specifying electrical attributes, butparagraph 6.3.1.2 suggested the recommended colour for overmolds on USB cables should be frost white -how boring! USB 1.1 and USB 2.0 was relaxed to recommend Black, Grey or Natural.PCB designers will want to reference chapter 6 for standard foot prints and pinouts.ElectricalUnless you are designing the silicon for a USB device/transceiver or USB host/hub, there is not all that muchyou need to know about the electrical specifications in chapter 7.We briefly address the essential points here.As we have discussed, USB uses a differential transmission pair for data.This is encoded using NRZI and isbit stuffed to ensure adequate transitions in the data stream.On low and full speed devices, a differential 1 istransmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5Kohm resistor pulled to 3.6V.A differential 0 on the other hand is a D- greater than 2.8V and a D+ less than0.3V with the same appropriate pull down/up resistors.The receiver defines a differential 1 as D+ 200mV greater than D- and a differential 0 as D+ 200mV less thanD-.The polarity of the signal is inverted depending on the speed of the bus.Therefore the terms J and Kstates are used in signifying the logic levels.In low speed a J state is a differential 0.In high speed a J stateis a differential 1.USB transceivers will have both differential and single ended outputs.Certain bus states are indicated bysingle ended signals on D+, D- or both.For example a single ended zero or SE0 can be used to signify aUSB in a Nutshell Page 5 www.beyondlogic.org device reset if held for more than 10mS.A SE0 is generated by holding both D- and D+ low ( [ Pobierz całość w formacie PDF ]
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.The USB Implementors forum has yet to provide this service.In these cases you maywish to use one assigned to your development system's manufacturer.For example most chip manufacturerswill have a VID/PID combination you can use for your chips which is known not to exist as a commercialdevice.Other chip manufacturers can even sell you a PID to use with their VID for your commercial device.Another more notable feature of USB, is its transfer modes.USB supports Control, Interrupt, Bulk andIsochronous transfers.While we will look at the other transfer modes later, Isochronous allows a device toreserve a defined about of bandwidth with guaranteed latency.This is ideal in Audio or Video applicationswhere congestion may cause loss of data or frames to drop.Each transfer mode provides the designer trade-offs in areas such as error detection and recovery, guaranteed latency and bandwidth.ConnectorsAll devices have an upstream connection to the host and all hosts have a downstream connection to thedevice.Upstream and downstream connectors are not mechanically interchangeable, thus eliminating illegalloopback connections at hubs such as a downstream port connected to a downstream port.There arecommonly two types of connectors, called type A and type B which are shown below.USB in a Nutshell Page 4 www.beyondlogic.org Receptical Type BReceptacle Type A2 11 2 3 43 4Figure 1 : USB ConnectorsType A plugs always face upstream.Type A sockets will typically find themselves on hosts and hubs.Forexample type A sockets are common on computer main boards and hubs.Type B plugs are always connecteddownstream and consequently 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 insome computer stores.This is in contradiction of the USB specification.The only type A plug to type A plugdevices are bridges which are used to connect two computers together.Other prohibited cables are USBextensions which has a plug on one end (either type A or type B) and a socket on the other.These cablesviolate the cable length requirements of USB.USB 2.0 included errata which introduces mini-USB B connectors.The details on these connectors can befound in Mini-B Connector Engineering Change Notice.The reasoning behind the mini connectors came fromthe range of miniature electronic devices such as mobile phones and organisers.The current type B connectoris too large to be easily integrated into these devices.Just recently released has been the On-The-Go specification which adds peer-to-peer functionality to USB.This introduces USB hosts into mobile phone and electronic organisers, and thus has included a specificationfor mini-A plugs, mini-A receptacles, and mini-AB receptacles.I guess we should be inundated with mini USBcables soon and a range of mini to standard converter cables.Pin Number Cable Colour Function1Red VBUS (5 volts)2White D-3 Green D+4 Black GroundTable 1 : USB Pin FunctionsStandard internal wire colours are used in USB cables, making it easier to identify wires from manufacturer tomanufacturer.The standard specifies various electrical parameters for the cables.It is interesting to read thedetail the original USB 1.0 spec included.You would understand it specifying electrical attributes, butparagraph 6.3.1.2 suggested the recommended colour for overmolds on USB cables should be frost white -how boring! USB 1.1 and USB 2.0 was relaxed to recommend Black, Grey or Natural.PCB designers will want to reference chapter 6 for standard foot prints and pinouts.ElectricalUnless you are designing the silicon for a USB device/transceiver or USB host/hub, there is not all that muchyou need to know about the electrical specifications in chapter 7.We briefly address the essential points here.As we have discussed, USB uses a differential transmission pair for data.This is encoded using NRZI and isbit stuffed to ensure adequate transitions in the data stream.On low and full speed devices, a differential 1 istransmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5Kohm resistor pulled to 3.6V.A differential 0 on the other hand is a D- greater than 2.8V and a D+ less than0.3V with the same appropriate pull down/up resistors.The receiver defines a differential 1 as D+ 200mV greater than D- and a differential 0 as D+ 200mV less thanD-.The polarity of the signal is inverted depending on the speed of the bus.Therefore the terms J and Kstates are used in signifying the logic levels.In low speed a J state is a differential 0.In high speed a J stateis a differential 1.USB transceivers will have both differential and single ended outputs.Certain bus states are indicated bysingle ended signals on D+, D- or both.For example a single ended zero or SE0 can be used to signify aUSB in a Nutshell Page 5 www.beyondlogic.org device reset if held for more than 10mS.A SE0 is generated by holding both D- and D+ low ( [ Pobierz całość w formacie PDF ]