Why $55,$AA are used to reset watchdog?

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Why $55,$AA are used to reset watchdog?

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citron
Contributor I

Hello, I am using MC9S12, and realizing watchdog in COP of this chip. In this datasheet of this chip, they must need to put '$55' and '$AA' to some specific register.
I wonder the meaning of '$55' and '$AA'. In ohter words, I wonder why these characters($55,$AA) are used, and what machanism does the watchdog timer reset (by that charaters).

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RadekS
NXP Employee
NXP Employee

Hi Haneul,

It is because 0x55 and 0xAA are very nice numbers in binary format.

0b01010101 = 0x55

0b10101010 = 0xAA

So, every write into ARMCOP register should negate every individual bit.

 

The access to ARMCOP register is watched by special logic and any write different from 0x55 or 0xAA will cause immediately COP reset (only when COP is enabled). Write 0x55/0xAA will reset COP counter.

I hope it helps you.

Have a great day,
Radek

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