DS17885 datasheet(15/38 Pages) DALLAS

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DS17885/DS17887

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UPDATE CYCLE

The serialized RTC executes an update cycle once per second regardless of the SET bit in Register B.

When the SET bit in Register B is set to 1, the user copy of the double-buffered time, calendar, alarm,

and elapsed time byte is frozen and does not update as the time increments. However, the time countdown

chain continues to update the internal copy of the buffer. This feature allows the time to maintain

accuracy independent of reading or writing the time, calendar, and alarm buffers and also guarantees that

time and calendar information is consistent. The update cycle also compares each alarm byte with the

corresponding time byte and issues an alarm if a match or if a “don’t care” code is present in all alarm

locations.

There are three methods that can handle access of the RTC that avoid any possibility of accessing

inconsistent time and calendar data. The first method uses the update-ended interrupt. If enabled, an

interrupt occurs after every update cycle that indicates that over 999ms is available to read valid time and

date information. If this interrupt is used, the IRQF bit in Register C should be cleared before leaving the

interrupt routine.

A second method uses the update-in-progress bit (UIP) in Register A to determine if the update cycle is in

progress. The UIP bit pulses once per second. After the UIP bit goes high, the update transfer occurs

244µs later. If a low is read on the UIP bit, the user has at least 244µs before the time/calendar data is

changed. Therefore, the user should avoid interrupt service routines that would cause the time needed to

read valid time/calendar data to exceed 244µs.

The third method uses a periodic interrupt to determine if an update cycle is in progress. The UIP bit in

occur at a rate of greater than tBUC allow valid time and date information to be reached at each occurrence

of the periodic interrupt. The reads should be complete within (tPI / 2 + tBUC) to ensure that data is not

read during the update cycle.

Figure 3. UPDATE-ENDED AND PERIODIC INTERRUPT RELATIONSHIP

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