【USB设备设计】-- CDC 设备开发（虚拟串口设备）

在嵌入式系统中，串行异步通信接口（UART）使用很频繁的接口，跟主机建立通信往往会用到USB转串口的设备，本章将介绍如何将USB虚拟成串口设备。

前期准备

1.带USB 功能的MCU (笔者使用的NXP RT1062)

2.串口调试助手

虚拟串口为cdc类（Communication Device Class），通常CDC类设备由两个子类接口组成：1个通信接口类接口（Communication Interface Class）和 0到多个数据接口类接口（Data Interface Class）。通常来说CDC设备一般需要至少2个接口。废话不多说，先贴上描述符：

Device Descriptor:
------------------------------
0x12    bLength
0x01    bDescriptorType
0x0200  bcdUSB
0xEF    bDeviceClass      (Miscellaneous device)
0x02    bDeviceSubClass
0x01    bDeviceProtocol
0x40    bMaxPacketSize0   (64 bytes)
0x1FC9  idVendor
0x00A3  idProduct
0x0101  bcdDevice
0x01    iManufacturer   "L17"
0x02    iProduct        "USB VCP"
0x03    iSerialNumber   "0123456789ABCDEF"
0x01    bNumConfigurationsConfiguration Descriptor:
------------------------------
0x09    bLength
0x02    bDescriptorType
0x004B  wTotalLength   (75 bytes)
0x02    bNumInterfaces
0x01    bConfigurationValue
0x00    iConfiguration
0xC0    bmAttributes   (Self-powered Device)
0x32    bMaxPower      (100 mA)Interface Association Descriptor:
------------------------------
0x08    bLength
0x0B    bDescriptorType
0x00    bFirstInterface
0x02    bInterfaceCount
0x02    bFunctionClass      (Communication Device Class)
0x02    bFunctionSubClass   (Abstract Control Model - ACM)
0x00    bFunctionProtocol
0x02    iFunction   "USB VCP"Interface Descriptor:
------------------------------
0x09    bLength
0x04    bDescriptorType
0x00    bInterfaceNumber
0x00    bAlternateSetting
0x01    bNumEndPoints
0x02    bInterfaceClass      (Communication Device Class)
0x02    bInterfaceSubClass   (Abstract Control Model - ACM)
0x01    bInterfaceProtocol   (ITU-T V.250)
0x00    iInterfaceCDC Header Functional Descriptor:
------------------------------
0x05    bFunctionalLength
0x24    bDescriptorType
0x00    bDescriptorSubtype
0x0110  bcdCDCCDC Call Management Functional Descriptor:
------------------------------
0x05    bFunctionalLength
0x24    bDescriptorType
0x01    bDescriptorSubtype
0x01    bmCapabilities
0x01    bDataInterfaceCDC Abstract Control Management Functional Descriptor:
------------------------------
0x04    bFunctionalLength
0x24    bDescriptorType
0x02    bDescriptorSubtype
0x06    bmCapabilitiesCDC Union Functional Descriptor:
------------------------------
0x05    bFunctionalLength
0x24    bDescriptorType
0x06    bDescriptorSubtype
0x00    bControlInterface
0x01    bSubordinateInterface(0)Endpoint Descriptor:
------------------------------
0x07    bLength
0x05    bDescriptorType
0x81    bEndpointAddress  (IN endpoint 1)
0x03    bmAttributes      (Transfer: Interrupt / Synch: None / Usage: Data)
0x0010  wMaxPacketSize    (1 x 16 bytes)
0x07    bInterval         (64 microframes)Interface Descriptor:
------------------------------
0x09    bLength
0x04    bDescriptorType
0x01    bInterfaceNumber
0x00    bAlternateSetting
0x02    bNumEndPoints
0x0A    bInterfaceClass      (CDC Data)
0x00    bInterfaceSubClass
0x00    bInterfaceProtocol
0x00    iInterfaceEndpoint Descriptor:
------------------------------
0x07    bLength
0x05    bDescriptorType
0x82    bEndpointAddress  (IN endpoint 2)
0x02    bmAttributes      (Transfer: Bulk / Synch: None / Usage: Data)
0x0200  wMaxPacketSize    (512 bytes)
0x00    bInterval         Endpoint Descriptor:
------------------------------
0x07    bLength
0x05    bDescriptorType
0x02    bEndpointAddress  (OUT endpoint 2)
0x02    bmAttributes      (Transfer: Bulk / Synch: None / Usage: Data)
0x0200  wMaxPacketSize    (512 bytes)
0x00    bInterval

通信接口

通信接口下拥有一个输入端点，用于报告主机一些状态。对于实现USB虚拟串口功能，需要将USB通信接口类的类型设置为：Abstract Control Model子类和Common AT Commands传输协议

通信接口描述符后面会跟，功能描述符（Functional Descriptors）,用于描述接口功能

描述符子类	基本功能信息
00h	功能描述符头
01h	Call Management 功能描述符
02h	抽象控制管理功能描述符
03h	Direct Line Management 功能描述符
04h	Telephone Ringer 功能描述符
05h	Telephone Call and Line State Reporting Capability 功能描述符
06h	Union 功能描述符
07h	Country Selection 功能描述符
08h	Telephone Operation Mode 功能描述符
09h	USB Terminal 功能描述符
0Ah	Network Channel 功能描述符
0Bh	协议单元功能描述符
0Ch	扩展单元功能描述符
0Dh	多通道管理功能描述符
0Eh	CAPI控制管理功能描述符
0Fh	以太网网络功能描述符
10h	ATM功能描述符
11-FFh	保留

数据接口

数据接口，没啥好说的，接口类定义为：0x0A 。端点描述符，注意传输类型为：Bulk 块传输

虚拟串口相关类请求

GET LINE CODING

主机获取当前串口属性请求，包括波特率、停止位、校验位及数据位的位数。CTL请求编码格式

返回7Byte参数，分别为: [0:3]波特率，[4]停止位，[5]校验位，[6]数据位。可知上图获取波特率为：115200

SET LINE CODING

类似GET LINE CODING，用于主机设置从机当前属性，可修改波特率、停止位、校验位及数据位

SET CTRL LINE STATE

该请求没有数据输出阶段，作用是设置设备的DTR和RTS引脚电平，D0位表示DTR，D1位表示RTS

类请求回调函数内容如下：

usb_status_t USB_DeviceCdcVcomCallback(class_handle_t handle, uint32_t event, void *param)
{uint32_t len;uint8_t *uartBitmap;usb_cdc_acm_info_t *acmInfo;usb_device_cdc_acm_request_param_struct_t *acmReqParam;usb_device_endpoint_callback_message_struct_t *epCbParam;volatile usb_cdc_vcom_struct_t *vcomInstance;usb_status_t error = kStatus_USB_InvalidRequest;acmReqParam = (usb_device_cdc_acm_request_param_struct_t *)param;epCbParam   = (usb_device_endpoint_callback_message_struct_t *)param;vcomInstance = &g_deviceComposite->cdcVcom;acmInfo      = vcomInstance->usbCdcAcmInfo;switch (event){case kUSB_DeviceCdcEventSendResponse:{if ((epCbParam->length != 0) && (!(epCbParam->length % vcomInstance->bulkInEndpointMaxPacketSize))){/* If the last packet is the size of endpoint, then send also zero-ended packet,** meaning that we want to inform the host that we do not have any additional** data, so it can flush the output.*/error = USB_DeviceCdcAcmSend(handle, vcomInstance->bulkInEndpoint, NULL, 0);}else if ((1 == vcomInstance->attach) && (1 == vcomInstance->startTransactions)){if ((epCbParam->buffer != NULL) || ((epCbParam->buffer == NULL) && (epCbParam->length == 0))){/* User: add your own code for send complete event *//* Schedule buffer for next receive event */error = USB_DeviceCdcAcmRecv(handle, vcomInstance->bulkOutEndpoint, vcomInstance->currRecvBuf,vcomInstance->bulkOutEndpointMaxPacketSize);}}else{}}break;case kUSB_DeviceCdcEventRecvResponse:{if ((1 == vcomInstance->attach) && (1 == vcomInstance->startTransactions)){vcomInstance->recvSize = epCbParam->length;if (!vcomInstance->recvSize){/* Schedule buffer for next receive event */error = USB_DeviceCdcAcmRecv(handle, vcomInstance->bulkOutEndpoint, vcomInstance->currRecvBuf,vcomInstance->bulkOutEndpointMaxPacketSize);}}}break;case kUSB_DeviceCdcEventSerialStateNotif:((usb_device_cdc_acm_struct_t *)handle)->hasSentState = 0;error                                                 = kStatus_USB_Success;break;case kUSB_DeviceCdcEventSendEncapsulatedCommand:break;case kUSB_DeviceCdcEventGetEncapsulatedResponse:break;case kUSB_DeviceCdcEventSetCommFeature:if (USB_DEVICE_CDC_FEATURE_ABSTRACT_STATE == acmReqParam->setupValue){if (1 == acmReqParam->isSetup){*(acmReqParam->buffer) = vcomInstance->abstractState;*(acmReqParam->length) = COMM_FEATURE_DATA_SIZE;}else{/* no action, data phase, s_abstractState has been assigned */}error = kStatus_USB_Success;}else if (USB_DEVICE_CDC_FEATURE_COUNTRY_SETTING == acmReqParam->setupValue){if (1 == acmReqParam->isSetup){*(acmReqParam->buffer) = vcomInstance->countryCode;*(acmReqParam->length) = COMM_FEATURE_DATA_SIZE;}else{/* no action, data phase, s_countryCode has been assigned */}error = kStatus_USB_Success;}else{/* no action, return kStatus_USB_InvalidRequest */}break;case kUSB_DeviceCdcEventGetCommFeature:if (USB_DEVICE_CDC_FEATURE_ABSTRACT_STATE == acmReqParam->setupValue){*(acmReqParam->buffer) = vcomInstance->abstractState;*(acmReqParam->length) = COMM_FEATURE_DATA_SIZE;error                  = kStatus_USB_Success;}else if (USB_DEVICE_CDC_FEATURE_COUNTRY_SETTING == acmReqParam->setupValue){*(acmReqParam->buffer) = vcomInstance->countryCode;*(acmReqParam->length) = COMM_FEATURE_DATA_SIZE;error                  = kStatus_USB_Success;}else{/* no action, return kStatus_USB_InvalidRequest */}break;case kUSB_DeviceCdcEventClearCommFeature:break;case kUSB_DeviceCdcEventGetLineCoding:*(acmReqParam->buffer) = vcomInstance->lineCoding;*(acmReqParam->length) = LINE_CODING_SIZE;error                  = kStatus_USB_Success;break;case kUSB_DeviceCdcEventSetLineCoding:{if (1 == acmReqParam->isSetup){*(acmReqParam->buffer) = vcomInstance->lineCoding;*(acmReqParam->length) = LINE_CODING_SIZE;}else{/* no action, data phase, s_lineCoding has been assigned */}error = kStatus_USB_Success;}break;case kUSB_DeviceCdcEventSetControlLineState:{error                     = kStatus_USB_Success;vcomInstance->usbCdcAcmInfo->dteStatus = acmReqParam->setupValue;/* activate/deactivate Tx carrier */if (acmInfo->dteStatus & USB_DEVICE_CDC_CONTROL_SIG_BITMAP_CARRIER_ACTIVATION){acmInfo->uartState |= USB_DEVICE_CDC_UART_STATE_TX_CARRIER;}else{acmInfo->uartState &= (uint16_t)~USB_DEVICE_CDC_UART_STATE_TX_CARRIER;}/* activate carrier and DTE. Com port of terminal tool running on PC is open now */if (acmInfo->dteStatus & USB_DEVICE_CDC_CONTROL_SIG_BITMAP_DTE_PRESENCE){acmInfo->uartState |= USB_DEVICE_CDC_UART_STATE_RX_CARRIER;}/* Com port of terminal tool running on PC is closed now */else{acmInfo->uartState &= (uint16_t)~USB_DEVICE_CDC_UART_STATE_RX_CARRIER;}/* Indicates to DCE if DTE is present or not */acmInfo->dtePresent = (acmInfo->dteStatus & USB_DEVICE_CDC_CONTROL_SIG_BITMAP_DTE_PRESENCE) ? true : false;/* Initialize the serial state buffer */acmInfo->serialStateBuf[0] = NOTIF_REQUEST_TYPE;                /* bmRequestType */acmInfo->serialStateBuf[1] = USB_DEVICE_CDC_NOTIF_SERIAL_STATE; /* bNotification */acmInfo->serialStateBuf[2] = 0x00;                              /* wValue */acmInfo->serialStateBuf[3] = 0x00;acmInfo->serialStateBuf[4] = 0x00; /* wIndex */acmInfo->serialStateBuf[5] = 0x00;acmInfo->serialStateBuf[6] = UART_BITMAP_SIZE; /* wLength */acmInfo->serialStateBuf[7] = 0x00;/* Notify to host the line state */acmInfo->serialStateBuf[4] = acmReqParam->interfaceIndex;/* Lower byte of UART BITMAP */uartBitmap    = (uint8_t *)&acmInfo->serialStateBuf[NOTIF_PACKET_SIZE + UART_BITMAP_SIZE - 2];uartBitmap[0] = acmInfo->uartState & 0xFFu;uartBitmap[1] = (acmInfo->uartState >> 8) & 0xFFu;len           = (uint32_t)(NOTIF_PACKET_SIZE + UART_BITMAP_SIZE);if (0 == ((usb_device_cdc_acm_struct_t *)handle)->hasSentState){error = USB_DeviceCdcAcmSend(handle, vcomInstance->interruptEndpoint, acmInfo->serialStateBuf, len);if (kStatus_USB_Success != error){//                    usb_echo("kUSB_DeviceCdcEventSetControlLineState error!");}((usb_device_cdc_acm_struct_t *)handle)->hasSentState = 1;}/* Update status */if (acmInfo->dteStatus & USB_DEVICE_CDC_CONTROL_SIG_BITMAP_CARRIER_ACTIVATION){/*  To do: CARRIER_ACTIVATED */}else{/* To do: CARRIER_DEACTIVATED */}if (acmInfo->dteStatus & USB_DEVICE_CDC_CONTROL_SIG_BITMAP_DTE_PRESENCE){/* DTE_ACTIVATED */if (1 == vcomInstance->attach){vcomInstance->startTransactions = 1;}}else{/* DTE_DEACTIVATED */if (1 == vcomInstance->attach){vcomInstance->startTransactions = 0;}}}break;case kUSB_DeviceCdcEventSendBreak:break;default:break;}return error;
}

测试自发自收功能

串口调试工具使用MobaXterm，底层自发自收，测试结果如下