Hi Paul,

Are these chips able to recover the normal operation mode after a reset ?

If no, I had a similar behavior with 89c668 (note I havn't read the PSEN pin).
Sometimes, the bootloader vector (in Flash) was corrupted. So, the chip was unable to jump to the ISP bootloader.
Then the only way to recover ISP was parallel programming.

That's why I've implemented the Erik Malund's tips to come back in ISP mode from the application.

I had a reset circuitry done with RC cell and I've replaced by a microcontroller supervision (MAX....)
It seems it has solved the problem.

A reason why could be a read/write into the flash during a power supply disturbance.


Regards
Stephane