Why Must EECON2 Be Initialized with 0x55 and 0xAA in PIC18F EEPROM Programming?

[Magna Visus]

Hello all,

Those who usually program microcontrollers in assembly, should be well familiar with the internal EEPROM (Series PIC18F).

In particular there's a register EECON2, that needs to be initialized ALWAYS with 0x55 then 0xAA before writing;
Why is that? What's the main reason behind these proper values?

Thanks.

 

[TurtleMeister]

It's to help prevent runaway code execution from overwriting the eeprom.

 

[Magna Visus]

Thanks!

But why use 0x55 and 0xAA why not some other random hex numbers?

 

[AlephZero]

Because that is how the chip was designed to work. There isn't the answer to a question about life, the universe, and everything here.

Of course it's probably not a coincidence that two values are 01010101 and 10101010 in binary. And EECON2 is not a physical register either. http://ww1.microchip.com/downloads/en/DeviceDoc/31007a.pdf section 7.4

 

[Magna Visus]

That's what I ment, the values were perfectly opposite (I mean NOT(01010101) is 10101010).
Anyways thanks for the answer and link