Update serial_pm.py

In Home Assistant Particulate Matter result was inconsistent in time. It was because serial device is open once, so data from sensor was collected by system in device buffer and slowly read from it. So results was "extended" in time. So added here is cleaning input buffer before next read of PM value. Additionally during research was added check sum control for PLANTOWER1 config.

pmsensor/serial_pm.py

@@ -125,7 +125,8 @@ def __init__(self,
                                  baudrate=configuration[BAUD_RATE],
                                  parity=serial.PARITY_NONE,
                                  stopbits=serial.STOPBITS_ONE,
-                                 bytesize=serial.EIGHTBITS)
+                                 bytesize=serial.EIGHTBITS,
+                                 timeout=0.1)
 
         # Update date in using a background thread
         if self.scan_interval > 0:
@@ -163,6 +164,12 @@ def read_data(self):
         finished = False
         sbuf = bytearray()
         starttime = time.time()
+        checkCode = int(0);
+        expectedCheckCode = int()
+        #it is necessary to reset input buffer because data is cotinously received by the system and placed in the device buffer when serial is open.
+        #But "Home Assistant" code read it only from time to time so the data we read here would be placed in the past. 
+        #Better is to clean the buffer and read new data from "present" time.
+        self.ser.reset_input_buffer()
         while not finished:
             mytime = time.time()
             if mytime - starttime > self.timeout:
@@ -182,6 +189,19 @@ def read_data(self):
                         sbuf = sbuf[1:]
 
                 if len(sbuf) == self.record_length:
+                    #Check the control sum if it is known how to do it
+                    if self.config == PLANTOWER1:                    
+                        for c in sbuf[0:(self.record_length-2)]:
+                            checkCode += c
+                        expectedCheckCode = sbuf[30]*256 + sbuf[31]
+                        if checkCode != expectedCheckCode:
+			    #because of data inconsistency clean the buffer
+                            LOGGER.error("PM sensor data sum error %d, expected %d", checkCode, expectedCheckCode)
+                            sbuf = []
+                            checkCode = 0                      
+                            continue    
+
+                    #if it is ok then send it for interpretation
                     res = self.parse_buffer(sbuf)
                     LOGGER.debug("Finished reading data %s", sbuf)
                     finished = True