Based on comments in:

http://answers.px4.ethz.ch/question/1337/px4io-receiver-connection-problem/?answer=1346#post-id-1346

increase the longest PPM pulse we recognize out to 550µs.

src/drivers/stm32/drv_hrt.c

@@ -330,7 +330,7 @@ static void		hrt_call_invoke(void);
 /*
  * PPM decoder tuning parameters
  */
-# define PPM_MAX_PULSE_WIDTH	500		/* maximum width of a pulse */
+# define PPM_MAX_PULSE_WIDTH	550		/* maximum width of a valid pulse */
 # define PPM_MIN_CHANNEL_VALUE	800		/* shortest valid channel signal */
 # define PPM_MAX_CHANNEL_VALUE	2200		/* longest valid channel signal */
 # define PPM_MIN_START		2500		/* shortest valid start gap */