Problem with Software Serial

## Reproduce Steps
 1. I use the example given in the sketch SoftwareSerialExample.ino
 2. I set the correct baud rate.
## Error & Expected Results
Software Serial works properly after loading code, but it always works wrong (reads wrong characters) after reset, wdt reset or Re-supply power.
## Environment

 - Arduino BSP Version: Linkit 7697 0.10.21
 - OS(Windows/macOS/Linux): Win 10
 
