Two important points must be remembered when using 
the PSEN for forcing entry into the bootloader:

   (a)   THE PSEN MUST NOT BE TIED PERMANENTLY TO 
         GROUND !  Neglecting this can cause the device 
         to become unresponsive as it is important 
         that the PSEN be free to toggle at power-on !

   (b)   THE PSEN IS NORMALLY AN OUTPUT SO USE A PULLDOWN 
         RESISTOR e.g. 1k ohm.

Neglecting this could possibly damage the device and will 
provoke superior current consumption than normal.