Make sure for the boot loader entry point...

0) Make sure that you correctly have a -5 (5 volt) part for operation at the 5 volts for your design as opposed to a -3 part.

1) Make sure you ground the PSEN pin through a 1K ohm resistor instead of directly to GND.

2) Make sure that the RS485 part on your board is removed or fully disconnected from the MCU's RxD and TxD lines.

3) Add a 1K pulldown to GND for the RESET pin.

4) Add a 1K pullup to DVDD for the EA pin.

5) When you cycle power to invoke the on-chip POR circuit to generate the reset condition make sure you leave the power off long enough for the DVDD rail to bleed down to below 4.5 volts.

6) Make good and sure that your DVDD and AVDD reach greater than 4.5 volts before the internal POR circuit's 128 millisecond delay. If the voltage has not reached 4.5 volts by 128 msec the MCU will not start up correctly due to attempting operation at an invalid VDD level. If your supply does not meet this requirement then you need to add an external means to drive RESET high till your VDD is above 4.5 volts.

Michael Karas