Verify that serial port is ready for connection before uploading

platformio/builder/tools/pioupload.py

@@ -23,7 +23,7 @@ from shutil import copyfile
 from time import sleep
 
 from SCons.Node.Alias import Alias
-from serial import Serial
+from serial import Serial, SerialException
 
 from platformio import util
 
@@ -72,6 +72,11 @@ def WaitForNewSerialPort(env, before):
                 new_port = p['port']
                 break
 
+    try:
+        Serial(new_port)
+    except SerialException:
+        sleep(1)
+
     if not new_port:
         sys.stderr.write("Error: Couldn't find a board on the selected port. "
                          "Check that you have the correct port selected. "