While on this, the following has been going smooth for me :

1. Have a toggle switch at the rear of the instrument to invoke the download mode as described in AN461 pdf.

2.Make the Emergency Exit procedure descibed by Erik as a standard code atarting at 0x0000. And provide the push-to-on switch to invoke this INSIDE the instrument.

3. Use a reset controller like DS1232.

Gone are the days of Flash MCUs refusing to enter code download mode !

Raghu