Fixing 1 in 256 edge-case that can lead to continuous no-output.

Adafruit_PM25AQI.cpp

@@ -91,15 +91,31 @@ bool Adafruit_PM25AQI::read(PM25_AQI_Data *data) {
         return false;
       }
     }
-    if (serial_dev->peek() != 0x42) {
-      serial_dev->read();
+    if (serial_dev->read() != 0x42) { // read assuming you got the first byte
       return false;
     }
-    // Now read all 32 bytes
-    if (serial_dev->available() < 32) {
+    // now catching 1 in 256 edge-case where last byte of checksum is 0x42 too,
+    // being mistaken for first start byte, leading to byte shift and checksum
+    // mismatch
+    if (serial_dev->peek() ==
+        0x42) {           // peek if next byte might match first start byte
+      serial_dev->read(); // if yes last assumption was wrong, read again
+                          // assuming you now got the start
+    }
+    if (serial_dev->read() != 0x4d) { // read & check the second byte
       return false;
     }
-    serial_dev->readBytes(buffer, 32);
+    // Now the first two bytes are guaranteed to be 0x42 0x4d but have already
+    // been read
+    buffer[0] = 0x42;
+    buffer[1] = 0x4d;
+    // Now read remaining 30 bytes
+    if (serial_dev->available() < 30) {
+      return false;
+    }
+    for (uint8_t i = 2; i < 32; i++) {
+      buffer[i] = serial_dev->read();
+    }
   } else {
     return false;
   }