BU9873 Datasheet, PDF (17/40 Pages) Rohm – I2C BUS Serial Interface RTC with High-precision Oscillation Adjustment

BU9873
Datasheet
3-4. Automatic Judgment of Leap Year
Automatic judgment function of leap year is included in this IC. Leap year is defined as follows.
The year that can be divided by 4 is leap year.
The year that can be divided by 100 is ordinary year.
The year that can be divided by 400 is leap year.
For example, year 2000 is a leap year while year 2100 is ordinary year.
Because the year register of this IC only supports the last two digits, a year will be automatically recognized as a leap year if
it is a multiple of 4. Therefore, year 2100 or 2000 will be determined as leap year because the last two digits are “00”. This
result in automatic judgment of leap years only can be up to the year 2099 in this IC.
4. Oscillation Adjustment Function with Digital Method
This IC has built-in oscillation capacitance C_G and C_D, the oscillation circuit can be configured easily by connecting an
external crystal oscillator. However, due to some variations such as parasitic capacitance, it is hardly for RTC to oscillate at
32,768 Hz exactly.
Therefore, if you want to achieve high-precision clock, it is necessary to use the error correction method. By using this
feature, you can achieve high-precision clock with only ±1.5ppm mismatch at a specified temperature. Because the crystal
oscillator has temperature dependency, the clock mismatch will increase when the temperature changes.
The clock adjustment step is about 3ppm and the total range is ±189ppm.
As following, some application is possible:
(1) If the temperature sensor is integrated in system, by setting the clock adjustment function in accordance with the
variation of temperature, it is possible to realize high-precision clock that does not depend on the temperature.
(2) By storing seasonal temperature information to the system, and using the clock adjustment function with this
temperature information, the realization of high-precision clock is available throughout the year.
4-1. Function Description
In the IC, counting up to seconds is made once per 32,768 of clock pulse generated by the oscillator. If oscillation
frequency is not 32,768 Hz which does not match with the number of clock counts, the time error will happen. This function is
designated to compensate the clock mismatch.
The adjustment function adds 2 clock pulses every 20 seconds: 2/(32,768×20)=3.051ppm, which delays the clock by
approx. 3ppm. Likewise, decrementing 2 clock pulses advances the clock by 3ppm. Thus the clock may be adjusted to the
precision of ±1.5ppm. and the total range is ±189.2ppm (±124 steps) according to the internal 7-bit trim register. The time
trimming circuit adjusts one second count based on this register when second digit is 00, 20 or 40 seconds. Note that the
time trimming function only adjust clock timing and oscillation frequency and 32-kHz clock output is not adjusted.
Setting data to internal register (internal address 7h) activates the time trimming circuit. And bit F6 decides either
increasing or decreasing the clock pulse.
The clock counts will be increased as ((F5, F4, F3, F2, F1, F0)－1) ×2 when F6 is set to “0”.
The clock counts will be decreased as ((/F₆, /F₅, /F₄, /F₃, /F₂, /F₁, /F₀)＋1) ×2 when F6 is set to “1”.
Counts will not change when (F6, F5, F4, F3, F2, F1, F0) are set to (＊, 0, 0, 0, 0, 0, ＊)
For example, when 32.768 kHz crystal is used:
When (F6, F5, F4, F3, F2, F1, F0) are set to (0, 0, 0, 0, 1, 1, 1), counts will change as: 32,768＋(7－1) ×2=32,780 (clock will
be delayed) when second digit is 00, 20 or 40.
When (F6, F5, F4, F3, F2, F1, F0) are set to (0, 0, 0, 0, 0, 0, 1), counts will remain 32,768 without changing when second
digit is 00, 20 or 40.
When (F6, F5, F4, F3, F2, F1, F0) are set to (1, 1, 1, 1, 1, 1, 0), counts will change as: 32,768＋(－2) ×2=32,764 (clock will
be advanced) when second digit is 00, 20 or 40.
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