STFPC320 datasheet(25/78 Pages) STMICROELECTRONICS

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STFPC320

Real-time clock (RTC) operation

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6.4.1

Reading the real-time clock

The real-time clock (RTC) is read by specifying the address corresponding to the register of

the real-time clock and then initiating a Read command. The RTC registers can then be

read in a sequential read mode. Since the clock runs continuously and a read takes a finite

amount of time, there is the possibility that the clock could change during the course of a

read operation. In this device, the time is latched by the read command (falling edge of the

clock on the ACK bit prior to RTC data output) into a separate latch to avoid time changes

during the read operation. The clock continues to run. Alarms occurring during a read are

unaffected by the read operation.

6.4.2

Writing to the real-time clock

The time and date may be set by writing to the RTC registers. To avoid changing the current

time by an uncompleted write operation, the current time value is loaded into a separate

buffer at the falling edge of the clock on the ACK bit before the RTC data input bytes, the

clock continues to run. The new serial input data replaces the values in the buffer. This new

RTC value is loaded back into the RTC register by a stop bit at the end of a valid write

sequence. An invalid write operation aborts the time update procedure and the contents of

the buffer are discarded. After a valid write operation the RTC will reflect the newly loaded

data beginning with the next “one second” clock cycle after the stop bit is written. The RTC

continues to update the time while an RTC register write is in progress and the RTC

continues to run during any nonvolatile write sequences. A single byte may be written to the

RTC without affecting the other bytes.

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