2

I'm setting up a robust protocol to read data (from accelerometer) from an Arduino board via serial port with Python. I am using Python 3.4 at the moment, but I could go back to a previous version.

Now, the code is working until I shake the accelerometer a lot. At which point I receive this error:

b'\xf08\xf8\x15\xf8\xf3|\xf0\x15N\xe8\xf7'
(14576, 5624, -3080, -3972, 19989, -2072)
b'<8\xb0\x08\xdc\x1cY\xed$Sb\xf1'
(14396, 2224, 7388, -4775, 21284, -3742)
b'\xac&\xbe\xedRA\xdc\xf2'
Traceback (most recent call last):
  File "C:\Python34\test_4_serial.py", line 57, in <module>
    pacchetti=unpack('hhhhhh', last_received)
struct.error: unpack requires a bytes object of length 12
>>>

I used this solution and adapted it to Python 3.4 (added b infront of the \n character):

from serial import *
from struct import *
from threading import Thread
__name__ =  '__main__'
last_received = ''


ser = Serial(
    port='com4',
    baudrate=115200,
    bytesize=EIGHTBITS,
    parity=PARITY_NONE,
    stopbits=STOPBITS_ONE,
    timeout=0.1,
    xonxoff=0,
    rtscts=0,
    interCharTimeout=None
)


ser_buffer=b''
while True:
    temp=ser.read(ser.inWaiting())
    if temp:
        ser_buffer=ser_buffer+temp                        
        if b'\n' in ser_buffer:

            lines = ser_buffer.split(b'\n') # Guaranteed to have at least 2 entries
            last_received = lines[-2]
            print(last_received)
            pacchetti=unpack('hhhhhh', last_received)
            print(pacchetti)
            ser_buffer = lines[-1]

The data are sent by the board in a simple format (Arduino Code):

        Serial.write((uint8_t)(ax & 0xFF));Serial.write((uint8_t)(ax >> 8)); 
        Serial.write((uint8_t)(ay & 0xFF));Serial.write((uint8_t)(ay >> 8)); 
        Serial.write((uint8_t)(az & 0xFF));Serial.write((uint8_t)(az >> 8)); 
        Serial.write((uint8_t)(gx & 0xFF));Serial.write((uint8_t)(gx >> 8)); 
        Serial.write((uint8_t)(gy & 0xFF));Serial.write((uint8_t)(gy >> 8)); 
        Serial.write((uint8_t)(gz & 0xFF));Serial.write((uint8_t)(gz >> 8)); 

        //Serial.write("\r");
        Serial.write("\n");

I assume that my protocol is not robust or I made some error in the parsing, but I can't figure it out. Thanks a lot for help, Michele

Community
  • 1
  • 1
Michele
  • 31
  • 1
  • 7

1 Answers1

0

Have a look at:

https://kentindell.wordpress.com/2015/02/18/micrcontroller-interconnect-network-min-version-1-0/

It's a robust protocol that uses byte stuffing to mark a frame start so if somehow the receiver gets out of sync it automatically resets at the right point. There's a Python implementation there too, and a "hello world" program that runs across the Arduino and PC.

Ken Tindell
  • 343
  • 4
  • 7