an11018 usb composite device on the lpc134x rev. 1 ? 17 december 2010 application note document information info content keywords usb, virtual com, mass storage, comp osite, microcontroller, descriptor, interface association descriptor, iad, cdc, acm, msd abstract this document describes how to create a composite usb device with an interface association descriptor for the lpc134x usb port.
nxp semiconductors an11018 usb composite device on the lpc134x an11018 all information provided in this document is subject to legal disclaimers. ? nxp b.v. 2010. all rights reserved. application note rev. 1 ? 17 december 2010 2 of 7 contact information for more information, please visit: http://www.nxp.com for sales office addresses, please send an email to: salesaddresses@nxp.com revision history rev date description 1 20101217 initial version.
nxp semiconductors an11018 usb composite device on the lpc134x 1. introduction t his is an example of a composite device driver for the usb device block on the lpx134x microcontrollers. a composite devic e is one that has multiple interfaces controlled independently of each other. in this device the independent interfaces include: ? v irtual com device (cdc/acm) ? ma ss storage device 2. driver architecture thi s driver was created by combining the code in the usbcdc (virtual com port) and usbmsd (mass storage) example projects of the lpcxpresso ide. as these two examples were already very similar to ea ch other the merge of the functional code included few changes, the bulk of cha nges were made to the descriptors. 2.1 functional code the code in this example driver is very simple. it includes a main() function with an endless loop and a collection of library support routines that implement the basic usb device stack and provide s upport for the cdc/adm a nd msd device classes. the main() function performs a onetime initia lization of the usb stack and classes and then drops into a loop that processes charac ters over the communications channel. all mass storage and character tx/rx activity is interrupt driven. the ma ss storage read and write handlers oper ate on a 4k ram buffer only, the device?s internal flash is not used in this example. 2.2 descriptors during the enumeration process a usb device will at some point send a collection of descriptors to the host that provide detailed information about itself, such as what class or classes the device su pports and how it will interface to the host. th is section will discuss thos e specific areas in the descriptors that make this a composite device. reference the usb 2.0 specif ication at usb.org for a full description of the descriptors. 2.2.1 descriptor types and ordering this device produces a device descriptor with a miscellaneous device class code, a single configuration descriptor, a single interf ace association descript or (iad), and three interface descriptors. th e ordering of these descriptors in memory is important. they are ordered like this: 1. device descriptor 2. configuration descriptor 3. interface descriptor (msd) 4. interface association descriptor (cdc) 5. in terface descriptor (cdc communications class) 6. interface descriptor (cdc data class) an11018 all information provided in this document is subject to legal disclaimers. ? nxp b.v. 2010. all rights reserved. application note rev. 1 ? 17 december 2010 3 of 7
nxp semiconductors an11018 usb composite device on the lpc134x 2.2.2 device descriptor the device descriptor describes general info rmation about the device. the key fields in this descriptor that define this device as composite are the device class, subclass, and protocol. these three fields are not filled with zeros as is usually the case with composite devices because the configuration desc riptor contains an iad descriptor. /* usb standard device descriptor */ const uint8_t usb_devicedescriptor[] = { usb_device_desc_size, /* blength */ usb_device_descriptor_type, /* bdescriptortype */ wbval(0x0200), /* 2.0 */ /* bcdusb */ usb_device_class_miscellaneous, /* bdeviceclass */ 0x02, /* bdevicesubclass */ 0x01, /* bdeviceprotocol */ usb_max_packet0, /* bmaxpacketsize0 */ wbval(usb_vendor_id), /* idvendor */ wbval(usb_prod_id), /* idproduct */ wbval(usb_device), /* 1.00 */ /* bcddevice */ 0x01, /* imanufacturer */ 0x02, /* iproduct */ 0x03, /* iserialnumber */ 0x01 /* bnumconfigurations: one possible configuration*/ }; note: the constant usb_device_class_miscellaneous is equal to 0xef. key fields 2.2.3 interface association descriptor the interface association descri ptor associates multiple interfaces with a single logical function. in this device the multiple interfaces are the two cdc interfaces, communications and data, and the single l ogical function is the cdc device class. this device includes one of these descriptor s because the cdc interfaces are part of a composite device class that includes other interfaces. had this device not been a composite device then the device class code defined in the device descriptor would have been set to usb_device_class_communications , which automatically makes the association, and there would have been no need to use this descriptor. without the iad the os would attempt to load a separate dr iver for each of the cdc interfaces. /* iad to associate the two cdc interfaces */ usb_interface_association_desc_size, /* blength */ usb_interface_association_descriptor_type, /* bdescriptortype */ usb_cdc_cif_num, /* bfirstinterface */ 2, /* binterfacecount */ usb_device_class_communications, /* bfunctionclass */ cdc_abstract_control_model, /* bfunctionsubclass */ 0, /* bfunctionprotocol */ 0, /* ifunction (index of string descriptor describing this function) */ see http://www.usb.org/developers/ whitepapers /iadclasscode_r10.pdf for more details about iads. an11018 all information provided in this document is subject to legal disclaimers. ? nxp b.v. 2010. all rights reserved. application note rev. 1 ? 17 december 2010 4 of 7
nxp semiconductors an11018 usb composite device on the lpc134x an11018 all information provided in this document is subject to legal disclaimers. ? nxp b.v. 2010. all rights reserved. application note rev. 1 ? 17 december 2010 5 of 7 this iad associates the 2 interfaces by including the number of the first interface to be associated in the bfirstinterface field and the count of interfaces to be associated together in the binterfacecount field. in this case there are two interfaces that are to be associated with each other and t hey start with the one numbered usb_cdc_cif_num. no te: only contiguously numbered interfaces c an be associated. also note that this iad descriptor must be positioned just bef ore the interfaces it references. 3. windows os support the file lpc134x-vcom.inf exists to assist in the loading of the proper usb serial drivers in the windows operating system for this device to operate correctly as a virtual com interface. there is a line in this file titl ed [devicelist] which looks like this: %description%=lpc134xusb, usb\vid_1fc9&pid_0003&mi_01 this line associates the cdc device with the operating system?s usbser.sys driver file and causes the os to load this driver during the enumeration process. vid_1fc9 and pid_0003 are the vendor and product ids for this device. the mi_01 portion of the line (mi stands for multiple in terface) corresponds to the second interface in the device and tells the os to associate this interface with the driver that is loaded. the cdc device is the second interface becau se it was the second interface descriptor defined after the configuration descriptor as shown in 2.2.1 descriptors . sin ce this composite device includes a mass storage device, and since this mass storage device is enumerated befor e the cdc device, this .inf file can be stored in the device and accessed directly by the opera ting system when the cdc device is being enumerated.
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nxp semiconductors an11018 usb composite device on the lpc134x please be aware that important notices concerning this document and the product(s) described herein, have been included in the section 'legal information'. ? nxp b.v. 2010. all rights reserved. for more information, please visit: http://www.nxp.com for sales office addresses, please send an email to: salesaddresses@nxp.com date of release: 17 december 2010 document identifier: an11018 5. contents 1. introduction .........................................................3 2. driver arch itecture ..............................................3 2.1 functional code..................................................3 2.2 descripto rs .........................................................3 2.2.1 descriptor types and orderi ng ............................3 2.2.2 device descr iptor ...............................................4 2.2.3 interface associat ion descrip tor..........................4 3. windows os support ..........................................5 4. legal information ................................................6 4.1 definiti ons ..........................................................6 4.2 disclaime rs.........................................................6 4.3 trademar ks ........................................................6 5. contents...............................................................7
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