The initial value of %S51 is 1 (after a factory reset), meaning that the RTC has not been set. After the first successful write to the real-time clock (RTC) with valid data, %S51 will be set to 0.

One of the following conditions must be true in order for %S51 to be set to 1:

• Hardware RTC is not present
• Hardware RTC is not readable or writeable
• Hardware RTC has never been updated
• Hardware RTC contains an invalid date/time (ex. 31 February)
• Hardware RTC does not update correctly from second to second or is just frozen

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