BQ4850Y_14 Datasheet, PDF (5/17 Pages) Texas Instruments – RTC Module With 512Kx8 NVSRAM

Not Recommended For New Designs
bq4850Y
Calibrating the Clock
The bq4850Y real-time clock is driven by a quartz con-
trolled oscillator with a nominal frequency of 32,768 Hz.
The quartz crystal is contained within the bq4850Y
package along with the battery. The clock accuracy of
the bq4850Y module is tested to be within 20ppm or
about 1 minute per month at 25°C. The oscillation rates
of crystals change with temperature as Figure 3 shows.
To compensate for the frequency shift, the bq4850Y of-
fers onboard software clock calibration. The user can
adjust the calibration based on the typical operating
temperature of individual applications.
The software calibration bits are located in the control
register. Bits D0–D4 control the magnitude of correc-
tion, and bit D5 the direction (positive or negative) of
correction. Assuming that the oscillator is running at
exactly 32,786 Hz, each calibration step of D0–D4 ad-
justs the clock rate by +4.068 ppm (+10.7 seconds per
month) or -2.034 ppm (-5.35 seconds per month) depend-
ing on the value of the sign bit D5. When the sign bit is
1, positive adjustment occurs; a 0 activates negative ad-
justment. The total range of clock calibration is +5.5 or
-2.75 minutes per month.
Two methods can be used to ascertain how much cali-
bration a given bq4850Y may require in a system. The
first involves simply setting the clock, letting it run for a
month, and then comparing the time to an accurate
known reference like WWV radio broadcasts. Based on
the variation to the standard, the end user can adjust
the clock to match the system’s environment even after
the product is packaged in a non-serviceable enclosure.
The only requirement is a utility that allows the end
user to access the calibration bits in the control register.
The second approach uses a bq4850Y test mode. When
the frequency test mode enable bit FTE in the days reg-
0
-20
-40
-60
-80
-100
-120
-30 -20 -10 0 10 20 30 40 50 60 70
GR485001
Temperature ( C)
Figure 3. Frequency Error
ister is set to a 1, and the oscillator is running at exactly
32,768 Hz, the LSB of the seconds register toggles at
512 Hz. Any deviation from 512 Hz indicates the degree
and direction of oscillator frequency shift at the test
temperature. For example, a reading of 512.01024 Hz
indicates a (1E6∗0.01024)/512 or +20 ppm oscillator fre-
quency error, requiring ten steps of negative calibration
(10∗-2.034 or -20.34) or 001010 to be loaded into the cali-
bration byte for correction. To read the test frequency,
the bq4850Y must be selected and held in an extended
read of the seconds register, location 7FFF9, without
having the read bit set. The frequency appears on DQ0.
The FTE bit must be set using the write bit control. The
FTE bit must be reset to 0 for normal clock operation to
resume.
SLUS057A- January 2005
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