Reading incoming serial data with windows using C - c

Question

I am using the code found here to receive data from an ATmega microcontroller to my PC. The code "USB2SERIAL_Read_W32.c" handles the Windows COM ports and receives serial data, which it then prints to the console.
I tried to modify the code to run continuously, i.e. to wait for data, print them when it receives characters, and then return to waiting for new data.
Here you can see the code, with my interventions.
#include <Windows.h>
#include <stdio.h>
void main(void)
{
HANDLE hComm; // Handle to the Serial port
char ComPortName[] = "\\\\.\\COM9"; // Name of the Serial port(May Change) to be opened,
BOOL Status; // Status of the various operations
DWORD dwEventMask; // Event mask to trigger
char TempChar; // Temperory Character
char SerialBuffer[256]; // Buffer Containing Rxed Data
DWORD NoBytesRead; // Bytes read by ReadFile()
int i = 0;
int counter=0;
printf("\n\n +==========================================+");
printf("\n | Serial Port Reception (Win32 API) |");
printf("\n +==========================================+\n");
/*---------------------------------- Opening the Serial Port -------------------------------------------*/
hComm = CreateFile( ComPortName, // Name of the Port to be Opened
GENERIC_READ | GENERIC_WRITE, // Read/Write Access
0, // No Sharing, ports cant be shared
NULL, // No Security
OPEN_EXISTING, // Open existing port only
0, // Non Overlapped I/O
NULL); // Null for Comm Devices
if (hComm == INVALID_HANDLE_VALUE)
printf("\n Error! - Port %s can't be opened\n", ComPortName);
else
printf("\n Port %s Opened\n ", ComPortName);
/*------------------------------- Setting the Parameters for the SerialPort ------------------------------*/
DCB dcbSerialParams = { 0 }; // Initializing DCB structure
dcbSerialParams.DCBlength = sizeof(dcbSerialParams);
Status = GetCommState(hComm, &dcbSerialParams); //retreives the current settings
if (Status == FALSE)
printf("\n Error! in GetCommState()");
dcbSerialParams.BaudRate = CBR_9600; // Setting BaudRate = 9600
dcbSerialParams.ByteSize = 8; // Setting ByteSize = 8
dcbSerialParams.StopBits = ONESTOPBIT; // Setting StopBits = 1
dcbSerialParams.Parity = NOPARITY; // Setting Parity = None
Status = SetCommState(hComm, &dcbSerialParams); //Configuring the port according to settings in DCB
if (Status == FALSE)
{
printf("\n Error! in Setting DCB Structure");
}
else //If Successfull display the contents of the DCB Structure
{
printf("\n\n Setting DCB Structure Successfull\n");
printf("\n Baudrate = %d", dcbSerialParams.BaudRate);
printf("\n ByteSize = %d", dcbSerialParams.ByteSize);
printf("\n StopBits = %d", dcbSerialParams.StopBits);
printf("\n Parity = %d", dcbSerialParams.Parity);
}
/*------------------------------------ Setting Timeouts --------------------------------------------------*/
COMMTIMEOUTS timeouts = { 0 };
timeouts.ReadIntervalTimeout = MAXDWORD;
timeouts.ReadTotalTimeoutConstant = 0;
timeouts.ReadTotalTimeoutMultiplier = 0;
timeouts.WriteTotalTimeoutConstant = 0;
timeouts.WriteTotalTimeoutMultiplier = 0;
if (SetCommTimeouts(hComm, &timeouts) == FALSE)
printf("\n\n Error! in Setting Time Outs");
else
printf("\n\n Setting Serial Port Timeouts Successfull");
/*------------------------------------ Setting Receive Mask ----------------------------------------------*/
dwEventMask=EV_RXCHAR;
Status = SetCommMask(hComm, dwEventMask); //Configure Windows to Monitor the serial device for Character Reception
if (Status == FALSE)
printf("\n\n Error! in Setting CommMask");
else
printf("\n\n Setting CommMask successfull");
/*------------------------------------ Setting WaitComm() Event ----------------------------------------*/
while(1){
int j =0;
i=0;
for(j=0;j<sizeof(SerialBuffer);j++){ //Clear SerialBuffer
SerialBuffer[j]=0;
}
printf("\n\n Waiting for Data Reception");
Status = WaitCommEvent(hComm, &dwEventMask, NULL); //Wait for the character to be received
/*-------------------------- Program will Wait here till a Character is received ------------------------*/
if (Status == FALSE){
printf("\n Error! in Setting WaitCommEvent()");
}
else //If WaitCommEvent()==True Read the RXed data using ReadFile();
{
printf("\n\n Characters Received");
do{
Status = ReadFile(hComm, &TempChar, sizeof(TempChar), &NoBytesRead, NULL);
if(Status==TRUE){
if(NoBytesRead>0){
SerialBuffer[i] = TempChar;
}
i++;
}
else
printf("\n Error! in Setting ReadFile()");
}while (NoBytesRead);
/*------------Printing the RXed String to Console----------------------*/
printf("\n\n ");
for (j = 0; j < strlen(SerialBuffer); j++)
printf("%c", SerialBuffer[j]);
}
//CloseHandle(hComm);//Closing the Serial Port
printf("\n +==========================================+\n");
}
}//End of Main()
The changes I did were to add the while(1) loop, clear the buffer at the start of each loop, commented out the closing of the port handler, changed the timeouts as mentioned in the "Timeouts" section in this microsoft tutorial and few more minor tweaks and cosmetics.
The problem is that for some reason, whenever I receive the string "1234", firstly the expected event is triggered, printing the received characters, and then another event is triggered for some reason, with no characters waiting, as you can see in the following picture:
To sum up, why is the unexpected event triggered and how can I solve it?
EDIT: I haven't managed to find the solution yet, however I am posting a workaround that I found (perhaps it can help someone to spot the initial problem). If I set the Comm Mask to zero, and reset it to EV_RXCHAR in each loop iteration, it seems to work as expected; That is, when I reach the WaitCommEvent line for the second time, it is not automatically triggered as before. The bad thing is that I cannot explain why it works now, and I believe that it should be working correctly in the first place as well. So any help would be greatly appreciated. Here is the code with the workaround, which sets the CommMask to zero and back to EV_RXCHAR:
#include <Windows.h>
#include <stdio.h>
void main(void){
printf("Hello!\n");
DWORD dwCommEvent;
DWORD dwRead;
DWORD lpEvtMask;
char chRead;
char ComPortName[] = "\\\\.\\COM9"; // Name of the Serial port(May Change) to be opened,
int i=0;
HANDLE hComm;
hComm = CreateFile( ComPortName,
GENERIC_READ | GENERIC_WRITE,
0,
0,
OPEN_EXISTING,
0,
0);
if (hComm == INVALID_HANDLE_VALUE)
printf("Error opening port.\n");
//////////////////////////////////////////////////////
DCB dcb;
FillMemory(&dcb, sizeof(dcb), 0);
if (!GetCommState(hComm, &dcb)) // get current DCB
printf("Error GetCommState.\n");
// Update DCB rate.
dcb.BaudRate = CBR_9600 ;
dcb.ByteSize = 8; // Setting ByteSize = 8
dcb.StopBits = ONESTOPBIT; // Setting StopBits = 1
dcb.Parity = NOPARITY; // Setting Parity = None
dcb.DCBlength = sizeof(dcb);
// Set new state.
if (!SetCommState(hComm, &dcb))
printf("Error SetCommState.\n");
// Error in SetCommState. Possibly a problem with the communications
// port handle or a problem with the DCB structure itself.
/////////////////////////////////////////////////////////////////////
COMMTIMEOUTS timeouts;
timeouts.ReadIntervalTimeout = MAXDWORD;
timeouts.ReadTotalTimeoutMultiplier = 0;
timeouts.ReadTotalTimeoutConstant = 0;
timeouts.WriteTotalTimeoutMultiplier = 0;
timeouts.WriteTotalTimeoutConstant = 0;
if (!SetCommTimeouts(hComm, &timeouts))
printf("Error timeouts.\n");
if(!PurgeComm(hComm,PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT))
printf("Error PurgeComm.\n");
////////////////////////////////////////
for ( ; ; ) {
if (!SetCommMask(hComm, 0))
printf("Error CommMask.\n");
if (!SetCommMask(hComm, EV_RXCHAR))
printf("Error CommMask.\n");
printf("Waiting for characters.. \n\n");
if (WaitCommEvent(hComm, &dwCommEvent, NULL)) {
do {
if (ReadFile(hComm, &chRead, 1, &dwRead, NULL)){
if(dwRead!=0)
printf("Character Received: %c\n",chRead);
}
else{
printf("ErrorReadFile.\n");
break;
}
}while (dwRead);
}
else{
printf("Error WaitCommEvent.\n");
break;
}
printf("=========================\n");
}
}