I want to parse GNSS data from NMEA sentence using STM32F0 and Truestudio - gps

Question

I am facing difficulty while parsing GPS data from NMEA sentence. I am using Quectel L89 GNSS module (baud rate: 115200) and Truestudio IDE. I had used CubeMX for making skeletal structure of program.
I had tried using both HAL_UART_RxCpltCallback with HAL_UART_Receive_IT and HAL_UART_Receive_IT independently. I am able to read data sometimes and sometimes not. The thing is that raw data starts from $GPRMC AND ends with $PSTMCPU while reading data from ST sometimes rec_buffer starts from $GPRMC(ok condition) and sometimes from $GNVTG(wrong condition).
How will I ensure that rec_buffer always starts from $GPRMC
I am calling this in while(1):
HAL_UART_Receive_IT(&huart2, rec_buff, 2400); // from GPS UART
I also tried this:
void HAL_UART_RxCpltCallback( UART_HandleTypeDef * huart )
{
if (huart->Instance == USART2) //GPS UART
{
HAL_UART_Transmit(&huart1, &rec_buff[0], 1, 10);
HAL_UART_Receive_IT(&huart2, rrc_buff, 2400);
}
}
L89 RAW DATA:
$GPRMC,094640.000,A,2838.86700,N,07711.56483,E,0.4,99.3,050119,,,A*5E
$GPGGA,094640.000,2838.86700,N,07711.56483,E,1,08,1.0,226.91,M,-35.9,M,,*42
$GNGNS,094640.000,2838.86700,N,07711.56483,E,ANNNNN,08,1.0,0226.9,-35.9,,*78
$GPVTG,99.3,T,,M,0.4,N,0.7,K,A*3D
$GPGST,094640.000,46.0,26.0,17.7,0.2,25.7,18.2,19.6*68
$GPGBS,094640.000,25.7,18.2,19.6,,,,*4B
$GNGSA,A,3,23,09,03,16,26,22,27,07,,,,,1.9,1.0,1.6*28
$GNGSA,A,3,,,,,,,,,,,,,1.9,1.0,1.6*22
$GPGSV,3,1,11,16,70,076,27,03,58,227,30,23,52,331,36,26,44,046,18*78
$GPGSV,3,2,11,22,44,198,21,09,24,316,34,27,19,137,19,07,16,266,27*70
$GPGSV,3,3,11,31,14,058,,14,10,112,,08,09,164,20,,,,*45
$PSTMPRES,14.7,-4.3,3.4,4.7,8.1,-34.9,-10.0,14.5,-8.6,,,,,,,,,,,,,,,,*05
$PSTMVRES,0.3,-0.0,0.0,-0.0,0.3,0.0,-0.5,-0.5,0.1,,,,,,,,,,,,,,,,*0B
$PSTMTG,2034,553618.0000,8,128199118,10,-46760.0000,002a,1025,0,0,10,2034,553618.0000,10,2034,553618.0000,6*5A
$PSTMTS,1,23,87178102.875,-44634.02,01,36,107871,1,9650745.91,13059080.41,21268551.81,-1601.01,2104.54,-615.14,-60384.66,6.69,0,0.00,0.00,0,0,0,0*01
$PSTMTS,1,09,88801609.812,-43889.80,01,34,102805,1,16028885.38,3193585.53,20890300.53,-1492.86,2210.83,804.75,142511.90,11.66,0,0.00,0.00,0,0,0,0*07
$PSTMTS,1,03,86575601.125,-48258.84,01,30,78613,1,13348561.56,22507400.16,4720188.47,-3.99,659.00,-3098.78,52480.77,6.38,0,0.00,0.00,0,0,0,0*2D
$PSTMTS,1,16,85874269.688,-47283.16,01,27,75521,1,-1886095.16,22466622.38,13519219.09,-747.65,-1602.10,2568.93,-3251.94,5.72,0,0.00,0.00,0,0,0,0*2A
$PSTMTS,1,26,3381954.688,-49146.08,00,18,53036,1,-8037289.25,15873771.62,19660297.47,-1174.67,-2217.50,1326.00,26300.98,7.53,0,0.00,0.00,0,0,0,0*3C
$PSTMTS,1,22,3609068.188,-49215.92,00,21,27316,1,10235851.56,24348322.31,-2780494.62,-516.89,-162.93,-3068.35,-184537.24,7.77,0,0.00,0.00,0,0,0,0*08
$PSTMTS,1,27,5418358.750,-44043.68,00,19,77495,1,-10438473.00,23018353.00,-7765070.44,-768.74,651.32,3023.75,-13120.58,14.66,0,0.00,0.00,0,0,0,0*23
$PSTMTS,1,07,90050798.438,-44830.90,01,27,76584,1,25565687.72,6701073.97,4735275.25,-590.77,171.17,3061.98,17221.07,16.12,0,0.00,0.00,0,0,0,0*04
$PSTMTS,1,08,454836.875,-43869.36,00,20,3383,0,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0.00,0,0.00,0.00,0,0,0,0*2B
$PSTMNOTCHSTATUS,3786017,0,2449,0,2,5420680,0,3824,0,2*57
$PSTMADCDATA,943,903,,,,,,*48
$PSTMANTENNASTATUS,0*4D
$PSTMSBAS,0,0,,,,*19
$PSTMCPU,48.64,-1,49*41
Any help/ suggestion will be appreciated.
Thanks clifford for your suggestions. I need to parse data from RMC, VTG, GGA AND GSA. I am unable to change the baud rate and also being unable to stop proprietary sentences.

Answer 1

There is nothing "wrong" about the output from your GNSS; it is normal for such a device to output a number of standard and proprietary sentences. It is your responsibility to either configure the module to output only the sentences you need or parse the sentences and discard those you don't want.
Send the following command to the GNSS to disable all configurable sentences but the RMC sentence:
$PSMTSETPAR,1201,0X00000040*75
You need to either send this configuration every time on start-up, or save the configuration in the non-volatile memory by:
$PSMTSAVEPAR*58
It is possible that the module will output other sentences that are not configurable (I am not familiare with the specific module); you may still need to perform parsing and filtering on the host. That is as simple as parsing the talker ID and format specifier and discarding sentences you are not interested in. There are nay number of ways to do that; your question shows no such attempt.
Your code is too "low-level" as it is - the IEC61162-1 (NMEA 0183) sentence format is line oriented, but HAL_UART_Receive_IT() will just grab a chunk of data without any knowledge of the protocol and entirely asynchronous to the sentence structure - you could as easily start and end mid-sentence. Moreover HAL_UART_RxCpltCallback() occurs in the interrupt context - that is not the place to be receiving large chunks of data and parsing it. You first need some higher-level serial I/O code that will buffer the incoming data into a queue and support line-oriented input.
You then need to process each line - discarding those that are of no interest. In that sense it is too broad a question and not specifically related to GNSS data parsing - that is not really your problem here, it is as I say lower-level than than - you need ot build a suitable serial I/O infrastructure.
Another issue is that of you wait for 2400 characters to be received, you will get the data in chunks, and only the most recently received will be valid. If real-time reception of current position is required; you cannot do it that way.
I am no ST HAL expert and would not choose to use it; it is poorly documented. What it seems you should do in HAL_UART_RxCpltCallback is place the received data into a FIFO buffer that will be processed by some other thread (i.e. not in the interrupt context); because of variable length of NMEA sentences, you will need to do that on a single character basis. A perhaps simpler method is to poll the HALL receive buffer with a zero timeout and accumulate characters until a line is available. You can then inspect that line to see if it is an RMC sentence and then process that. For example, the following allows non-blocking processing of serial input. If no other functions need be performed, the timeout can be extended to wait indefinitely for each character:
// Strictly NMEA0183 requires only 81 byte sentence buffer, but
// L89 appears to output non-compliant proprietary sentences.
static char line_buffer[1024];
int line_buffer_index = 0;
for (;;)
{
HAL_StatusTypeDef rx_status = HAL_OK ;
while( rx_status == HAL_OK )
{
// Receive a character if available (zero timeout)
char ch = 0;
HAL_StatusTypeDef rx_status = HAL_UART_Receive( &huart2, ch, 1, 0 ) ;
// If character received and not the LF following the previous CR
if( rx_status == HAL_OK && ch != `\n ` )
{
// If the end of the sentence...
if( ch == `\r ` )
{
// terminate the line
line_buffer[line_buffer_index] = `\0` ;
// Start a new line
line_buffer_index = 0 ;
// Check if line is an RMC from a GNSS
if( line_buffer_index > 6 &&
memcmp( line_buffer, "$G", 2) == 0 &&
memcmp( &line_buffer[3], "RMC", 3) == 0 )
{
// Process RMC
processRMC( line_buffer ) ;
}
}
else
{
// add character to line buffer
line_buffer[line_buffer_index] = ch ;
line_buffer_index++ ;
if( line_buffer_index > sizeof(line_buffer) - 1 )
{
// Line too long, discard all
line_buffer_index = 0 ;
}
}
}
}
// do other work here if necessary
}