ISL12022M Datasheet, PDF (17/27 Pages) Intersil Corporation – Real Time Clock with Embedded Crystal, ±5ppm Accuracy

ISL12022M
ALPHA Register (ALPHA)
TABLE 13. ALPHA REGISTER
ADDR 7
6
5
4
3
2
1
0
0Ch D ALPHA6 ALPHA5 ALPHA4 ALPHA3 ALPHA2 ALPHA1 ALPHA0
The ALPHA variable is 8 bits and is defined as the temperature
coefficient of crystal from -40°C to T0, or the ALPHA Cold
(there is an Alpha Hot register that must be programmed as
well). It is normally given in units of ppm/°C², with a typical
value of -0.034. The ISL12022M device uses a scaled version
of the absolute value of this coefficient in order to get an integer
value. Therefore, ALPHA <7:0> is defined as the (|Actual
ALPHA Value| x 2048) and converted to binary. For example, a
crystal with Alpha of -0.034ppm/°C² is first scaled
(|2048*(-0.034)| = 70d) and then converted to a binary number
of 01000110b.
The practical range of Actual ALPHA values is from
-0.020 to -0.060.
The ISL12022M has a preset ALPHA value corresponding to
the crystal in the module. This value is recalled on initial
power-up and is preset in device production. It is READ
ONLY and cannot be overwritten by the user.
BETA Register (BETA)
TABLE 14.
ADDR 7 6
5
4
3
2
1
0
0Dh TSE BTSE BTSR BETA4 BETA3 BETA2 BETA1 BETA0
The BETA register has special Write properties. Only the
TSE, BTSE and BTSR bits can be written; the BETA bits
are READ-ONLY. A write to both bytes in this register
will only change the 3 MSB’s (TSE, BTSE, BTSR), and
the 5 LSB’s will remain the same as set at the factory.
TEMPERATURE SENSOR ENABLED BIT (TSE)
This bit enables the Temperature Sensing operation, including
the temperature sensor, A/D converter and FATR/FDTR
register adjustment. The default mode after power-up is
disabled:
(TSE = 0). To enable the operation, TSE should be set to 1.
(TSE = 1). When temp sense is disabled, the initial values for
IATR and IDTR registers are used for frequency control.
When TSE is set to 1, the temperature conversion cycle
begins and will end when two temperature conversions are
completed. The average of the two conversions is in the
TEMP registers.
TEMP SENSOR CONVERSION IN BATTERY MODE BIT
(BTSE)
This bit enables the Temperature Sensing and Correction in
battery mode. BTSE = 0 (default) no conversion, Temp
Sensing or Compensation in battery mode. BTSE = 1
indicates Temp Sensing and Compensation enabled in battery
mode. The BTSE is disabled when the battery voltage is lower
than 2.7V. No temperature compensation will take place with
VBAT<2.7V.
FREQUENCY OF TEMPERATURE SENSING AND
CORRECTION BIT (BTSR)
This bit controls the frequency of Temp Sensing and
Correction. BTSR = 0 default mode is every 10 minutes,
BTSR = 1 is every 1.0 minute. Note that BTSE has to be
enabled in both cases. See Table 15.
TABLE 15. FREQUENCY OF TEMPERATURE SENSING AND
CORRECTION BIT
BTSE
BTSR
TC PERIOD IN
BATTERY MODE
0
0
OFF
0
1
OFF
1
0
10 Minutes
1
1
1 Minute
The temperature measurement conversion time is the same
for battery mode as for V_DD mode, approximately 22ms. The
battery mode current will increase during this conversion time
to typically 68µA. The average increase in battery current is
much lower than this due to the small duty cycle of the
ON-time versus OFF-time for the conversion.
To figure the average increase in battery current, we take the
the change in current times the duty cycle. For the 1 minute
temperature period, the average current is expressed in
Equation 1:
ΔI_BAT =
0_----._--0_---2_---2_----s_-
60 s
×
68
μ
A
=
250 n A
(EQ. 1)
For the 10 minute temperature period the average current is
expressed in Equation 2:
ΔI_BAT
=
0_----._--0_---2_---2_----s_-
600 s
×
68 μ A =
25 n A
(EQ. 2)
If the application has a stable temperature environment that
doesn’t change quickly, the 10 minute option will work well
and the backup battery lifetime impact is minimized. If quick
temperature variations are expected (multiple cycles of more
than 10° within an hour), then the 1 minute option should be
considered and the slightly higher battery current figured into
overall battery life.
GAIN FACTOR OF AT BIT (BETA<4:0>)
Beta is specified to take care of the Cm variations of the
crystal. Most crystals specify Cm around 2.2fF. For example, if
Cm > 2.2fF, the actual AT steps may reduce from 1ppm/step
to approximately 0.80ppm/step. Beta is then used to adjust for
this variation and restore the step size to 1ppm/step.
BETA values are limited in the range from 01000 to 11111, as
shown in Table 16. To use Table 16, the device is tested at
two AT settings as follows:
17
FN6668.4
December 18, 2008