AN197_技术文档

AN197

SERIAL COMMUNICATIONS GUIDE FOR THE CP210X

Relevant Devices

This application note applies to the following devices:

CP2101, CP2102, CP2103

1. Introduction

This document is intended for developers creating products based on the CP210x USB to UART Bridge Controller.

It provides information about serial communications and how to obtain the port number for a specific CP210x

device. Code samples are provided for opening, closing, configuring, reading, and writing to a COM port. Also

included are GetPortNumWinXXXX() functions that can be copied and used to determine the port number on a

CP210x device by using its Vendor ID (VID), Product ID (PID), and serial number.

2. Opening a COM Port

Before configuring and using a COM port to send and receive data, it must first be opened. When a COM port is

opened, a handle is returned by the CreateFile() function that is used from then on for all communication. Here is

example code that opens COM3:

HANDLE hMasterCOM = CreateFile("\\\\.\\COM3",

GENERIC_READ | GENERIC_WRITE,

0,

OPEN_EXISTING,

FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,

0);

The first parameter in the CreateFile() function is a string that contains the COM port number to use. This string will

always be of the form "\\\\.\\COMX" where X is the COM port number to use. The second parameter contains flags

describing access, which will be GENERIC_READ and GENERIC_WRITE for the example in this document, and

allows both read and write access. Parameters three and four must always be 0, and the flag in parameter five

must always be OPEN_EXISTING when using CreateFile() for COM applications. The sixth parameter should

always contain the FILE_ATTRIBUTE_NORMAL flag. In addition, the FILE_FLAG_OVERLAPPED is an optional

flag that is used when working with asynchronous transfers (this option is used for the example in this document).

If overlapped mode is used, functions that read and write to the COM port must specify an OVERLAPPED

structure identifying the file pointer, which is demonstrated in section 3.1. and section 3.2. (more information on

overlapped I/O is located at http://msdn.microsoft.com/library/en-us/dnfiles/html/msdn_serial.asp?frame=true -

serial_topic4). The seventh, and last, parameter must always be 0.

If this function returns successfully, then a handle to the COM port will be assigned to the HANDLE variable. If the

function fails, then INVALID_HANDLE_VALUE will be returned. Upon return check the handle and if it's valid, then

prepare the COM port for data transmission.

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3. Preparing an Open COM Port for Data Transmission

Once a handle is successfully assigned to a COM port several steps must be taken to set it up. The COM port must

first be purged and its initial state should be retrieved. Then the COM port's new settings can be assigned and set

up by a device control block (DCB) structure (more information is provided on the DCB structure in section 3.3. and

at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/devio/base/dcb_str.asp).

3.1. Purging the COM Port

First the COM port should be purged to clear any existing data going to or from the COM port using the

PurgeComm() function:

PurgeComm(hMasterCOM, PURGE_TXABORT | PURGE_RXABORT | PURGE_TXCLEAR | PURGE_RXCLEAR);

The first parameter in the PurgeComm() function is a handle to the open COM port that will be purged. The second

parameter contains flags that further describe what actions should be taken. All four flags, PURGE_TXABORT,

PURGE_RXABORT, PURGE_TXCLEAR, and PURGE_RXCLEAR should always be used. The first two flags

terminate overlapped write and read operations, and the last two flags clear the output and input buffers.

If this function returns successfully then a non-zero value is returned. If the function fails, then it returns 0. Upon

return, check the return value; if it is non-zero, continue to set up the COM port (more information on the

PurgeComm() function is located at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/devio/base/

purgecomm.asp).

3.2. Saving the COM Port's Original State

Since the COM port settings can be modified to meet different needs, it is good practice to obtain the COM port's

current state and store it so that when the COM port is closed, the COM port can be restored back to its original

state. This can be done using the GetCommState() function:

DCB dcbMasterInitState;

GetCommState(hMasterCOM, &dcbMasterInitState);

The first parameter in the GetCommState() function is a handle to the open COM port to obtain settings from. The

second parameter is an address to a DCB structure to store the COM port's settings. This DCB structure should

also be used as the initial state when specifying new settings for the COM port (see section 3.3.).

If this function returns successfully then a non-zero value is returned. If the function fails, then it returns 0. Upon

return, check the return value; if it is non-zero, continue to set up the COM port (more information on the

GetCommState() function is located at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/devio/base/

getcommstate.asp).

3.3. Setting up a DCB Structure to Set the New COM State

All of a COM port's settings are stored in a DCB structure. In section 3.2. a DCB structure was retrieved that

contained the initial settings of the COM port by using the GetCommState() function. To change a COM port's

settings, a DCB structure must be created and filled out with the desired settings. Then the SetCommState()

function can be used to activate those settings:

DCB dcbMaster = dcbMasterInitState;

dcbMaster.BaudRate= 57600;

dcbMaster.Parity= NOPARITY;

dcbMaster.ByteSize= 8;

dcbMaster.StopBits= ONESTOPBIT;

SetCommState(hMasterCOM, &dcbMaster);

Delay(60);

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Here a new DCB structure dcbMaster has been initialized to dcbMasterInitState, which are the current settings of

the COM port. After it has been initialized to the current settings, new settings can be assigned.

3.3.1. Baud Rate

The baud rate property is set to 57600 bps, but can be set to any of the baud rates supported by the CP210x. (See

the current data sheet for the list of supported baud rates for the CP210x.)

3.3.2. Parity

The parity is set to NOPARITY, however it can also be set to ODDPARITY, EVENPARITY, SPACEPARITY, and

MARKPARITY if supported by the CP210x. (See the current data sheet for the list of supported parities for the

CP210x.)

3.3.3. Byte Size

The byte size is set to 8, so there are 8 data bits in every byte of data sent. This can also be set to 5, 6, or 7 if

supported by the CP210x. (see the data sheet for the list of supported byte sizes for the CP210x.)

3.3.4. Stop Bits

The stop bits are set to ONESTOPBIT, but could also be set to TWOSTOPBITS or ONE5STOPBITS (1.5). (See the

current data sheet for the list of supported stop bits for the CP210x.) All combinations of data and stop bits can be

used except for the combination of 5 data bits with 2 stop bits and the combination of 6, 7, or 8 data bits with 1.5

stop bits.

After each of these settings is set to the desired value, the SetCommState() function can be called to set up the

COM port. The first parameter in the SetCommState() function is a handle to the open COM port to change the

settings on. The second parameter is an address to a DCB structure containing the COM port's new settings 2

(more information on serial settings using DCB structures is located at http://msdn.microsoft.com/library/en-us/

dnfiles/html/msdn_serial.asp?frame=true - serial_topic6).

If this function returns successfully, a non-zero value is returned. If the function fails, it returns 0. Upon return,

check the return value; if it is non-zero, delay for 60 ms to allow time for the settings to change and then continue to

set up the COM port. This delay is not required; however, a conservative time of 60 ms is good practice to ensure

that the settings are changed before any other operations take place.

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4. Transmitting Data Across the COM Port

Once the COM port is successfully opened and configured, data can be written or read.

4.1. Writing Data

There are several things that need to happen in a write, so it is a good idea to create a function for the writes to be

called whenever a write must occur. Here is an example of a write function:

bool WriteData(HANDLE handle, BYTE* data, DWORD length, DWORD* dwWritten)

{

bool success= false;

OVERLAPPED o= {0};

o.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);

if (!WriteFile(handle, (LPCVOID)data, length, dwWritten, &o))

{

if (GetLastError() == ERROR_IO_PENDING)

if (WaitForSingleObject(o.hEvent, INFINITE) == WAIT_OBJECT_0)

if (GetOverlappedResult(handle, &o, dwWritten, FALSE))

success = true;

}

else

success = true;

if (*dwWritten != length)

success = false;

CloseHandle(o.hEvent);

return success;

}

The parameters passed in to this function are the handle to an open COM port, a pointer to an array of bytes that

will be written, the number if bytes that are in the array, and a pointer to a variable to store and return the number of

bytes written.

Two local variables are declared at the beginning of the function: a bool named success that will store the success

of the write (this is initialized to false, and only set true when the write succeeds) and an overlapped object o which

is passed to the WriteFile() function and alerts if the transfer is complete or not (this is always initialized to {0}

before the hEvent is assigned). Creating an event with the CreateEvent(NULL, FALSE, FALSE, NULL) function

sets the hEvent property of o to prepare it to be passed to the WriteFile() function (more information on

CreateEvent() is located at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/

createevent.asp).

Next, the WriteFile() function is called with the handle, data, length of the data, and variable to store the amount of

data that was written (more information on WriteFile() is located at http://msdn.microsoft.com/library/

default.asp?url=/library/en-us/fileio/base/writefile.asp). If this function returns successfully, a non-zero value is

returned. If the function fails, it returns 0. The if statement will determine if the write succeeded and if it did not, the

last error is retrieved to see if there really was an error or the write just wasn't finished. If ERROR_IO_PENDING is

returned then object o is then waited on until either the write finishes or fails (if something other than

ERROR_IO_PENDING is returned by the GetLastError() function, then there is the possibility of surprise removal;

see “8. Application Design Notes” for comments on surprise removal). When the wait is over, the result is obtained

so that the amount of bytes written is updated. The success variable will then be assigned with the appropriate

value, and the handle of o.hEvent is closed. Then the amount of bytes written is checked, and finally the function

returns the success of the write, which will be true if the write successfully completed.

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4.2. Reading Data

There are several things that need to happen in a read, so it is a good idea to create a function for the reads to be

called whenever a read must occur. Here is an example of a read function:

bool ReadData(HANDLE handle, BYTE* data, DWORD length, DWORD* dwRead, UINT timeout)

{

bool success= false;

OVERLAPPEDo= {0};

o.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);

if (!ReadFile(handle, data, length, dwRead, &o))

{

if (GetLastError() == ERROR_IO_PENDING)

if (WaitForSingleObject(o.hEvent, timeout) == WAIT_OBJECT_0)

success = true;

GetOverlappedResult(handle, &o, dwRead, FALSE);

}

else

success = true;

CloseHandle(o.hEvent);

return success;

}

The parameters passed in to this function are the handle to an open COM port, a pointer to an array of bytes that

will be read, the number if bytes that are in the array, a pointer to a variable to store and return the number of bytes

read, and a timeout value.

Two local variables are declared at the beginning of the function: a bool named success that will store the success

of the read (this is initialized to false, and only set true when the read succeeds), and an overlapped object o which

is passed to the ReadFile() function and alerts if the transfer is complete or not (this is always initialized to {0}

before the hEvent is assigned). Creating an event with the CreateEvent(NULL, FALSE, FALSE, NULL) function

sets the hEvent property of o to prepare it to be passed to the ReadFile() function (more information on

CreateEvent()

is located at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/

createevent.asp).

Next, the ReadFile() function is called with the handle, data, length of the data, and variable to store the amount of

data that was written (more information on the ReadFile() function is located at http://msdn.microsoft.com/library/

default.asp?url=/library/en-us/fileio/base/readfile.asp). If this function returns successfully then a non-zero value is

returned. If the function fails, then it will return 0. The if statement will determine if the write succeeded and if it

didn't, the last error is retrieved to see if there really was an error or the write just wasn't finished. If

ERROR_IO_PENDING is returned then object o is then waited on until either the write finishes or fails (if

something other than ERROR_IO_PENDING is returned by the GetLastError() function, then there is the possibility

of surprise removal; see section 8. "Application Design Notes" for comments on surprise removal). When the wait

is over, the result is obtained so that the amount of bytes read is updated. The success variable will then be

assigned with the appropriate value, and the handle of o.hEvent is closed. Finally, the function returns the success

of the read, which will be true if the read successfully completed.

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5. Closing the COM Port

After all communication is finished, then the COM port should then be closed. First, the COM port should be set

back to its initial state, and then the handle to the COM port should be closed and set to an invalid handle. Example

code is shown below:

SetCommState(hMasterCOM, &dcbMasterInitState);

Delay(60);

CloseHandle(hMasterCOM);

hMasterCOM = INVALID_HANDLE_VALUE;

The SetCommState() function works the same as described in "3.3. Setting up a DCB Structure to Set the New

COM State" on page 2. A delay of 60 ms is used to make sure the settings have time to be set. Finally the device is

closed using the CloseHandle() function. This function just takes in the handle of the COM port. After this function

is called, it is important to set the variable to an INVALID_HANDLE_VALUE.

6. Sample program to Demonstrate Serial Communications

Included in the AN197SW.zip is a directory named "CP210xSerialTest" which contains the source code and

executables for a Visual Studio project that makes use of all the serial communication functions described in

section 3., section 4., and section 5. The program is a basic dialog based application that accepts two COM port

numbers, and then will send a test array of 64 bytes of data back and forth between them.

7. Discovering CP210x COM Port

To use the described functionality with a COM port, the number of the COM port needs to be known. In order to find

out the COM port number of a CP210x device, the VID, PID, and serial number are used to lookup a registry key.

This key is different between Windows XP/2000/Server 2003/Vista and Windows 98. Here are the keys that will

need to be looked up:

WinXP/2000/Server 2003/Vista (Driver Version 5.0 and higher):

HKLM\System\CurrentControlSet\Enum\USB\Vid_xxxx&Pid_yyyy\zzzz\DeviceParameters\PortName

WinXP/2000/Server 2003/Vista (Driver Version 4.40 and lower):

HKLM\System\CurrentControlSet\Enum\USB\Vid_xxxx&Pid_yyyy&Mi_00\zzzz_00\DeviceParameters\PortName

(where xxxx is the VID, yyyy is the PID, and zzzz is the serial number)

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7.1. Windows XP/2000/Server 2003/Vista

To find the port number in Windows XP/2000/Server 2003/Vista, the corresponding key listed above needs to be

opened. This is done using several registry calls using Windows API functions. The function is passed the VID,

PID, and serial number, and the return value is either the port number that the CP210x is located on, or a –1 if

there is a failure. The function below will find the CP210x port number in Windows XP/2000/Server 2003/Vista (this

function can be copied straight into application code that needs to discover the COM port number):

int GetPortNumXP2000Vista(WORD vid, WORD pid, char* ser)

{

//Variables used for Registry access

HKEY tmpKey, tmpSubKey, tmpPortKey;

CString portKeyString;

DWORD valtype;

char* portString;

DWORD length = 100;

portString = new char[101];

//Set portnum to -1, so if there is an error we will

//know by returning a negative port value

int portNum = -1;

// Open keys to get to the key where the port number is located. This key is:

//

HKLM\System\CurrentControlSet\Enum\USB\Vid_xxxx&Pid_yyyy&Mi_00\zzzz_00\DeviceParameters\PortN

ame

if (ERROR_SUCCESS == RegOpenKeyEx(HKEY_LOCAL_MACHINE, "SYSTEM\\CurrentControlSet\\", 0,

KEY_READ, &tmpKey))

{

if (ERROR_SUCCESS == RegOpenKey(tmpKey, "Enum\\USB\\", &tmpSubKey))

{

//Loop through and replace spaces for WinXP2000Vista

int i = 0;

while (ser[i] != '\0')

{

if (ser[i] == 0x20)

ser[i] = '_';

i++;

}

//The portkey string should look like this

//"Vid_XXXX&Pid_XXXX&MI_00\\XXXX_00" where the XXXX's are Vid, Pid and serial string

- version less than 5.0

//"Vid_XXXX&Pid_XXXX\\XXXX" where the XXXX's are Vid, Pid and serial string -

version greater than or equal to 5.0

portKeyString.Format("Vid_%04x&Pid_%04x&Mi_00\\%s_00\\Device Parameters\\", vid,

pid, ser);

//If the portkey string is in the registry, then go ahead and open the portname

if (ERROR_SUCCESS == RegOpenKeyEx(tmpSubKey, portKeyString, 0, KEY_READ,

&tmpPortKey))

{

if (ERROR_SUCCESS == RegQueryValueEx(tmpPortKey, "PortName", NULL, &valtype,

(unsigned char *)portString, &length))

{

// When we obtain this key, it will be in string format of

// "COMXX" where XX is the port. Simply make the first three

// elements of the string 0, and call the atoi function to obtain

// the number of the port.

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portString[0] = '0';

portString[1] = '0';

portString[2] = '0';

portNum = atoi(portString);

}

//Make sure to close all open keys for cleanup

RegCloseKey(tmpPortKey);

}

else

{

portKeyString.Format("Vid_%04x&Pid_%04x\\%s\\Device Parameters\\", vid, pid,

ser);

//If the portkey string is in the registry, then go ahead and open the portname

if (ERROR_SUCCESS == RegOpenKeyEx(tmpSubKey, portKeyString, 0, KEY_READ,

&tmpPortKey))

{

if (ERROR_SUCCESS == RegQueryValueEx(tmpPortKey, "PortName", NULL, &valtype,

(unsigned char *)portString, &length))

{

// When we obtain this key, it will be in string format of

// "COMXX" where XX is the port. Simply make the first three

// elements of the string 0, and call the atoi function to obtain

// the number of the port.

portString[0] = '0';

portString[1] = '0';

portString[2] = '0';

portNum = atoi(portString);

}

//Make sure to close all open keys for cleanup

RegCloseKey(tmpPortKey);

}

RegCloseKey(tmpSubKey);

RegCloseKey(tmpKey);

}

RegCloseKey(HKEY_LOCAL_MACHINE);

delete portString;

// Return the number of the port the device is connected too

return portNum;

}

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7.2. Windows 98 SE

Alternatively, to find the port number in Windows 98 SE the corresponding key listed above needs to be opened.

This is also done using several registry calls using Windows API functions. This key is not directly entered using

the VID, PID, and serial number. Instead, each time a new device appears it is enumerated and stored in the

registry. So two lookups have to be performed: one to determine if the current key in question contains the VID, PID

and serial number needed, and another to actually determine the port number if the VID, PID, and serial number

match from the previous lookup. The function is passed the VID, PID, and serial number, and the return value is

either the port number that the CP210x is located on, or a –1 if there is a failure. Here is the function to find the

CP210x port number in Windows 98 (this function can be copied straight into application code that needs to

discover the COM port number):

int GetPortNum98(WORD vid, WORD pid, char* ser)

{

// Variables used for Registry access

HKEY tmpKey;

CString portKeyString, serKeyString;

intserialIndex;

DWORD valtype;

char* portString;

DWORD length = 100;

portString = new char[101];

// Set portnum to -1, so if there is an error we will

// know by returning a negative port value

int portNum = -1;

// We will search through the keys by index, starting at 0

// Use this index in the key "Enum\SLABCR\guid\XXXX"

serialIndex = 0;

serKeyString.Format("Enum\\SLABCR\\guid\\%.4d", serialIndex);

// Loop through and convert the serial string to all upper case and replace spaces

// for Win98

int i = 0;

while (ser[i] != '\0')

{

if (ser[i] == 0x20)

ser[i] = '_';

else if ((ser[i] >= 0x61) && (ser[i] <= 0x7A))

ser[i] -= 32;

i++;

}

// The portkey string should look like this when we query the CLowerDeviceID

// "USB\VID_XXXX&PID_XXXX&MI_00\\XXXX_00" where the XXXX's are Vid, Pid and serial

// string

portKeyString.Format("USB\\VID_%04X&PID_%04X&MI_00\\%s_00", vid, pid, ser);

// Open keys to get to the key where the port number is located. This key is:

// HKLM\Enum\SLABCR\guid\XXXX\Portname is the key and we will loop through each key

// in the registry

while (ERROR_SUCCESS == RegOpenKey(HKEY_LOCAL_MACHINE, serKeyString, &tmpKey))

{

// Determine the status of the call

DWORD status = RegQueryValueEx(tmpKey, "CRLowerDeviceID", NULL, &valtype,

(unsigned char *)portString, &length);

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// If we need to make the call again, then decrement the serial index so we can

// accessit the next time through the loop

if (ERROR_MORE_DATA == status)

serialIndex--;

// Otherwise, the call is a success, and we can compare the portString from the

// key to the port string that we want, if they match then retrieve the port

else if (ERROR_SUCCESS == status)

{

if (portString == portKeyString)

{

if (ERROR_SUCCESS == RegQueryValueEx(tmpKey, "Portname", NULL, &valtype,

(unsigned char *)portString, &length))

{

// When we obtain this key, it will be in string format of

// "COMXX" where XX is the port. Simply make the first three

// elements of the string 0, and call the atoi function to obtain

// the number of the port.

portString[0] = '0';

portString[1] = '0';

portString[2] = '0';

portNum = atoi(portString);

}

// Increment the serial index

serialIndex++;

serKeyString.Format("Enum\\SLABCR\\guid\\%.4d", serialIndex);

// Make sure to close all open keys for cleanup

RegCloseKey(tmpKey);

}

RegCloseKey(HKEY_LOCAL_MACHINE);

// Return the number of the port the device is connected too

return portNum;

}

7.3. Sample Program to Retrieve a CP210x COM Port

Included in the AN197SW.zip is a directory named "CP210xPortNumExample" that contains the source code and

executables for a Visual Studio project that makes use of these functions. In the program, the number of CP210x

devices are found and then for each one a port number is retrieved for it and listed in the edit box. (This program

also makes use of the CP210xManufacturing.DLL to find the VID, PID, and serial number on a device. The use of

this DLL is defined in more detail in "AN144: CP210x Device Customization Guide".)

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8. Application Design Notes

The functions used in section 3., section 4., and section 5. are Windows COMM API functions. The examples

provided are just samples of the recommended way of dealing with serial communication. For more specific

information on these functions, see the MSDN website at: http://msdn.microsoft.com/library/default.asp.

It should also be noted that the SetCommState() function does not save the settings between opening and closing

the COM port. As stated before, it is good practice to get the current settings after the COM port is opened, and

then restore them before it is closed.

All of the functions here will return an error code. It is a good idea to nest these functions in order to catch errors if

they occur by using the GetLastError() function. This will also solve any surprise removal problems by allowing the

discovery of an invalid handle to be found and dealt with. The example application (CP210xSerialTest) has several

cases that will detect surprise removal. In this example, there are checks on every function to make sure that the

return code is true. If it is not, then it will display where the error occurred in the output window. As long as correct

and supported settings are passed to the functions they should execute normally. Most failures can occur from

having an INVALID_HANDLE_VALUE, however, the handles must be set to this value after a surprise removal

occurs.

Because regular COM ports will always be visible, then data can always be written to them successfully, even if

there is no way to read it. However, because the CP210x is a virtual COM port, if the device is removed, then the

handle that it uses becomes invalid when trying to write to it. If for some reason the CP210x device is unplugged

the write will fail and ERROR_OPERATION_ABORTED will be returned by GetLastError(). When this happens, the

handle needs to be closed and then set to INVALID_HANDLE_VALUE. Alternatively, a regular COM port can

always be read from, but if there is no data then it will time out. When using the CP210x as the virtual COM port

and it is removed before a read occurs, then the read will fail and ERROR_ACCESS_DENIED will be returned by

GetLastError(). Again when this happens, the handle needs to be closed and then set to

INVALID_HANDLE_VALUE.

9. References

MSDN - use this to search for specific Windows API functions

http://msdn.microsoft.com/

Serial Communication Reference

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnfiles/html/msdn_serial.asp

Communication Resource Reference

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/devio/base/communications_resources.asp

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DOCUMENT CHANGE LIST

Revision 0.3 to Revision 0.4

Updated "7.3. Sample Program to Retrieve a

CP210x COM Port" on page 10.

Revision 0.4 to Revision 0.5

Added CP2103 to Relevant Devices on page 1.

Revision 0.5 to Revision 0.6

Updated XP/2000 references to include Server

2003/Vista.

Updated WinXP/2000 key listing in section "7.

Discovering CP210x COM Port" on page 6.

Updated code in section "7.1. Windows XP/2000/

Server 2003/Vista" on page 7.

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NOTES:

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Rev. 0.6


型号：	AN197
厂家：	SILICON
描述：	SERIAL COMMUNICATIONS GUIDE FOR THE CP210X 串行通信指南的CP210X 通信
文件：	总14页 (文件大小：234K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

AN197 [SILICON]

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