Hello. I'm using the Atmel AT89S8253 and I'm noticing something with the fuse and lock bits.

If I don't set any of the lock bits, I can disable the SerProgEn fuse with my Atmel AT89ISP programmer and it will disable that fuse. However, no lock bits are enabled so a parallel programmer should still be able to read the chip, which would not be good.

However, if I enable ANY of the program lock bits (even just Lock Bit 1 alone), I try to disable the SerProgEn fuse with the AT89ISP programmer and the programmer will not disable that fuse no matter what I do.

I've done several tests with this...disabling SerProgEn with no memory lock bits set, then re-enabling SerProgEn with a parallel "fuse reset circuit" that I fabbed up, then enabling any of the lock modes while disabling SerProgEn. Each time, I was successful in disabling SerProgEn as long as I had no memory lock bits set, but was unsuccessful in disabling SerProgEn if any of the memory lock modes were set.

Is there any way possible to set memory lock bits while also disabling SerProgEn with the Atmel AT89ISP cable and software?