The alarm interrupt of the DS12887 is set by the three ALARM registers in the device. The three ALARM bytes can be used in two ways. One is to set the ALARM registers to the correct time, and the interrupt is generated when the data in the time register matches the time in ALARM registers. In this mode, the interrupt is generated once per day. The second use condition is to insert a “don’t care” state in one or more of the three ALARM bytes. The don’t care code is any hexadecimal value from C0 to FF. The don’t care codes in all three ALARM bytes create an interrupt every second. It is likely that the content of the ALARM register is set to "don't care."