BU9873 Datasheet, PDF (14/40 Pages) Rohm – I2C BUS Serial Interface RTC with High-precision Oscillation Adjustment

BU9873
Datasheet
1-7. Notes during RTC Data Transmission
To avoid invalid read and write, two features should be noted when accessing the RTC.
Hold function of clock carry-up
While read and write operation is executed (at the same time, RTC clock is still counting-up), this IC temporarily holds the
clock carry-up from start condition to stop condition, to prevent invalid read and write. If clock carry-up happens during this
period (read or write from start condition to stop condition), it will be adjusted within approx. 61μs after stop condition.
Automatic release function of access
When 0.5 to 1.0 second elapses after start condition, any access to the RTC will be automatically terminated, to release
the temporarily holding of clock carry-up, set Fh to the address pointer, and access from the CPU is forced to be stopped (as
long as stop condition is received, the same action will be made: automatic release function from the I²C bus interface).
Therefore, one access must be completed within 0.5 seconds. The automatic release function prevents delay in SCL clock,
even if SCL is stopped because of system sudden failure during read operation.
In addition, a second start condition (after the first start condition and ahead of the stop condition) is regarded as the
“repeated start condition”. Therefore, when 0.5 to 1.0 seconds elapses after the first start condition, access to the RTC will
also be released automatically.
If access is tried after automatic release function is activated, no acknowledge signal will be output for writing while FFh
will be output for reading.
The following points should be noted during accessing the RTC.
(1) No stop condition shall be generated until clock and calendar data read/write is started and completed
Bad example of time read
(Start condition) → (Read of seconds) → (Read of minutes) → (Stop condition) → (Start condition) → (Read of hours) →
(Stop condition)
Assuming read is started at 05:59:59 P.M. and while reading seconds and minutes the time advanced to 06:00:00 P.M.
During this time, second digit is hold so the read result is 05:59:59. Then the IC confirms stop condition and carries
second digit that is being hold and the time changes to 06:00:00 P.M. Thus, when the hour digit is read, it changes to be
6. The invalid results of 06:59:59 will be read.
(2) One cycle of read/write operation shall be completed within 0.5 seconds.
(3) Do not send start condition within 61μs from stop condition, because the clock carry-up that is hold during I²C access
will be adjusted within approx.61μs from stop condition.
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