好玩的电子设备（2）

这次介绍一下USB逻辑分析仪。USB协议是比较复杂的通讯协议，使用差分信号进行传输，速度很快，编码上采用 NRZI 【参考1】，这样导致根本无法使用示波器之类的进行Debug （速度快，数据多，一直在变化）。因此，要想知道线路上究竟跑了什么数据只能使用专用的逻辑分析仪进行分析。下面是 Lecroy 出品的 Advisor T3 型USB分析仪，可以进行USB 2.0/3.0 协议的分析（不支持 USB Type C ,  有专门额外的设备可以用来分析这个协议）。

下面是设备的正面，A口和B口分别连接在设备和主机上，对于USB HOST 和 Salve来说是透明的。

这是背面的照片，捕获的数据会通过这个接口输出。这是一个2.0的接口，因为内部存储空间有限，这个设备不适合长期抓取大量的数据。

下面尝试抓取 Arduino USB 串口（16U2）的数据，我们先编写一个简单的代码，一直向串口输出字符串：

 

void setup() {

  // put your setup code here, to run once:

   Serial.begin(9600);

}



void loop() {

  // put your main code here, to run repeatedly:

  Serial.println("www.lab-z.com");

  delay(5000);

}

 

启动工作机上的逻辑分析仪软件之后就可以开始抓取数据了，这是软件界面

 

抓取的过程是：

1. 开始记录
2. 插入 Arduino UNO
3. 打开串口工具记录
4. 从串口工具对 Arduino 发送数据

 

第一步：设置设备的地址，这里可以看到分配给 Arduino 的USB地址是 0x0F

第二步：HOST要求设备送出 Device Descriptor

 

REQUEST SUMMARY

Setup Data	80 06 00 01 00 00 12 00
Direction	Device-to-host
Type	Standard
Recipient	Device
bRequest	GET_DESCRIPTOR
wValue	DEVICE
wIndex	0x0000
wLength	0x0012

 

DECODING INFORMATION

 

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bRequest	8	8	GET_DESCRIPTOR	0x06	bRequest HexVal: 0x06
wValue	16	16	DEVICE type	0x0100	Type of Descriptor
wIndex	16	32	0x0000	0x0000	index info

DEVICE Descriptor

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	0	0x12	0x12	Descriptor size is 18 bytes
bDescriptorType	8	8	0x01	0x01	DEVICE Descriptor Type
bcdUSB	16	16	0x0110	0x0110	USB Specification version 1.10
bDeviceClass	8	32	0x02	0x02	The device belongs to the Communication Device/Interface Class
bDeviceSubClass	8	40	0x00	0x00	Subclass 0x00 not defined
bDeviceProtocol	8	48	0x00	0x00	The device uses the No class specific protocol required Protocol
bMaxPacketSize0	8	56	0x08	0x08	Maximum packet size for endpoint zero is 8
idVendor	16	64	0x2341	0x2341	Vendor ID is 9025: No Company Matches Id
idProduct	16	80	0x0043	0x0043	Product ID is 67
bcdDevice	16	96	0x0001	0x0001	The device release number is 0.01
iManufacturer	8	112	0x01	0x01	The manufacturer string descriptor index is 1
iProduct	8	120	0x02	0x02	The product string descriptor index is 2
iSerialNumber	8	128	0xDC	0xDC	The serial number string descriptor index is 220
bNumConfigurations	8	136	0x01	0x01	The device has 1 possible configurations

 

第三步，上面的描述符提到这个设备有1个 Configuration Descriptor, 主机要求设备送出 Configuration Descriptor

REQUEST SUMMARY

Setup Data	80 06 00 02 00 00 FF 00
Direction	Device-to-host
Type	Standard
Recipient	Device
bRequest	GET_DESCRIPTOR
wValue	CONFIGURATION, Index 0
wIndex	0x0000
wLength	0x00FF

 

DECODING INFORMATION

 

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bRequest	8	8	GET_DESCRIPTOR	0x06	bRequest HexVal: 0x06
wValue	16	16	CONFIGURATION type, Index 0	0x0200	Type of Descriptor
wIndex	16	32	0x0000	0x0000	index info

CONFIGURATION Descriptor (9 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	0	0x09	0x09	Descriptor size is 9 bytes
bDescriptorType	8	8	0x02	0x02	CONFIGURATION Descriptor Type
wTotalLength	16	16	0x003E	0x003E	The total length of data for this configuration is 62. This includes the combined length of all the descriptors returned Warning : The Value of wTotalLength is not equal to real length
bNumInterfaces	8	32	0x02	0x02	This configuration supports 2 interfaces
bConfigurationValue	8	40	0x01	0x01	The value 1 should be used to select this configuration
iConfiguration	8	48	0x00	0x00	The device doesn't have the string descriptor describing this configuration
bmAttributes	8	56	0xC0	0xC0	Configuration characteristics : Bit 7: Reserved (set to one) 1 Bit 6: Self-powered 1 Bit 5: Remote Wakeup 0
bMaxPower	8	64	0x32	0x32	Maximum power consumption of the device in this configuration is 100 mA

INTERFACE Descriptor (9 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	72	0x09	0x09	Descriptor size is 9 bytes
bDescriptorType	8	80	0x04	0x04	INTERFACE Descriptor Type
bInterfaceNumber	8	88	0x00	0x00	The number of this interface is 0.
bAlternateSetting	8	96	0x00	0x00	The value used to select the alternate setting for this interface is 0
bNumEndpoints	8	104	0x01	0x01	The number of endpoints used by this interface is 1 (excluding endpoint zero)
bInterfaceClass	8	112	0x02	0x02	The interface implements the Communication Device/Interface class
bInterfaceSubClass	8	120	0x02	0x02	The interface implements the Abstract Control Model Subclass
bInterfaceProtocol	8	128	0x01	0x01	The interface uses the Common AT commands Protocol
iInterface	8	136	0x00	0x00	The device doesn't have a string descriptor describing this iInterface

Communication Class Specific INTERFACE Descriptor (5 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength....	8	144	0x05	0x05	Size of the descriptor, in bytes
bDescriptorType	8	152	0x24	0x24	CS_INTERFACE Descriptor Type
bDescriptorSubType	8	160	0x00	0x00	Header Funtional Descriptor Subtype
bcdCDC	16	168	0x1001	0x1001	USB Class Definitions for Communications the Communication specification version 10.01

Communication Class Specific INTERFACE Descriptor (4 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength....	8	184	0x04	0x04	Size of the descriptor, in bytes
bDescriptorType	8	192	0x24	0x24	CS_INTERFACE Descriptor Type
bDescriptorSubType	8	200	0x02	0x02	Abstract Control Management Functional Descriptor Subtype
bmCapabilities	8	208	0x06	0x06	Bit 0: Whether device supports the request combination of Set_Comm_Feature, Clear_Comm_Feature, and Get_Comm_Feature 0 Bit 1: Whether device supports the request combination of Set_Line_Coding, Set_Control_Line_State, Get_Line_Coding, and the notification Serial_State 1 Bit ...

Communication Class Specific INTERFACE Descriptor (5 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength....	8	216	0x05	0x05	Size of the descriptor, in bytes
bDescriptorType	8	224	0x24	0x24	CS_INTERFACE Descriptor Type
bDescriptorSubType	8	232	0x06	0x06	Union Functional Descriptor Subtype
bControlInterface	8	240	0x00	0x00	The interface number of the Communications or Data Class interface
bSubordinateInterface0	8	248	0x01	0x01	Interface number of subordinate interface in the Union

ENDPOINT Descriptor (7 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	256	0x07	0x07	Descriptor size is 7 bytes
bDescriptorType	8	264	0x05	0x05	ENDPOINT Descriptor Type
bEndpointAddress	8	272	0x82	0x82	This is an IN endpoint with endpoint number 2
bmAttributes	8	280	0x03	0x03	Types - Transfer: INTERRUPT Low Power: No Pkt Size Adjust: No
wMaxPacketSize	16	288	0x0008	0x0008	Maximum packet size for this endpoint is 8 Bytes. If Hi-Speed, 0 additional transactions per frame
bInterval	8	304	0xFF	0xFF	The polling interval value is every 255 Frames. Undefined for Hi-Speed

INTERFACE Descriptor (9 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	312	0x09	0x09	Descriptor size is 9 bytes
bDescriptorType	8	320	0x04	0x04	INTERFACE Descriptor Type
bInterfaceNumber	8	328	0x01	0x01	The number of this interface is 1.
bAlternateSetting	8	336	0x00	0x00	The value used to select the alternate setting for this interface is 0
bNumEndpoints	8	344	0x02	0x02	The number of endpoints used by this interface is 2 (excluding endpoint zero)
bInterfaceClass	8	352	0x0A	0x0A	The interface implements the Data Interface class
bInterfaceProtocol	8	368	0x00	0x00	The interface uses the No class specific protocol required Protocol
bInterfaceSubClass	8	360	0x00	0x00	The Subclass code is 0
iInterface	8	376	0x00	0x00	The device doesn't have a string descriptor describing this iInterface

ENDPOINT Descriptor (7 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	384	0x07	0x07	Descriptor size is 7 bytes
bDescriptorType	8	392	0x05	0x05	ENDPOINT Descriptor Type
bEndpointAddress	8	400	0x04	0x04	This is an OUT endpoint with endpoint number 4
bmAttributes	8	408	0x02	0x02	Types - Transfer: BULK Pkt Size Adjust: No
wMaxPacketSize	16	416	0x0040	0x0040	Maximum packet size for this endpoint is 64 Bytes. If Hi-Speed, 0 additional transactions per frame
bInterval	8	432	0x01	0x01	The polling interval value is every 1 Frames. If Hi-Speed, 1 uFrames/NAK

ENDPOINT Descriptor (7 bytes)

Field	Length (bits)	Offset (bits)	Decoded	Hex Value	Description
bLength	8	440	0x07	0x07	Descriptor size is 7 bytes
bDescriptorType	8	448	0x05	0x05	ENDPOINT Descriptor Type
bEndpointAddress	8	456	0x83	0x83	This is an IN endpoint with endpoint number 3
bmAttributes	8	464	0x02	0x02	Types - Transfer: BULK Pkt Size Adjust: No
wMaxPacketSize	16	472	0x0040	0x0040	Maximum packet size for this endpoint is 64 Bytes. If Hi-Speed, 0 additional transactions per frame
bInterval	8	488	0x01	0x01	The polling interval value is every 1 Frames. If Hi-Speed, every 1 uFrames

 

第五步，主机请求 String Descriptor

我不熟悉USB CDC 协议，因此上面只能做一个简单的使用实例。

最后，多说两句：这款分析仪的价格在 20万人民币,个人基本上不用考虑购买一个玩玩儿了。此外，近两年有一些国产的逻辑分析仪，价格只有国外的十分之一，但是我没有实验过。期待随着时间的推移，国产分析设备有所长进，甚至有一天能够对 USB协议产生影响。大多数情况下，USB设备的调试还是依靠Slave Device 本身的Debug 输出（比如：Uart），普通开发者也不会遇到传输上面的问题。

参考：

1. http://baike.baidu.com/view/1367029.htm NRZ-I编码