isl12020m Intersil Corporation, isl12020m Datasheet - Page 18

• Manufacturer Part Number: isl12020m
• Description: Real Time Clock With Embedded Crystal, ?5ppm Accuracy
• Manufacturer: Intersil Corporation

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/°C
converted to a binary number of 01000110b.
The practical range of Actual ALPHA values is from -0.020 to
-0.060.
The ISL12020M has a preset ALPHA value corresponding to
the crystal in the module. This value is recalled on initial
power-up and should remain unchanged for best
compensation performance, although the user can
override this preset value if so desired.
The ALPHA register should only be changed while the TSE
(Temp Sense Enable) bit is “0”. Note that both the ALPHA
and the ALPHA Hot registers need to be programmed with
values for full range temperature compensation.
BETA Register (BETA)
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.
All changes to the IDTR, IATR, ALPHA and BETA registers
must be made with TSE = 0. After loading the new values,
TSE can be enabled and the new values are used. 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
V
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,
0Dh
ADDR
BAT
<2.7V.
TSE
7
2
BTSE BTSR
is first scaled (|2048*(-0.034)| = 70d) and then
6
5
BETA4 BETA3 BETA2 BETA1
18
TABLE 14.
4
3
2
1
ISL12020M
BETA0
0
BTSR = 1 is every 1.0 minute. Note that BTSE has to be
enabled in both cases. See Table 15.
The temperature measurement conversion time is the same
for battery mode as for V
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:
For the 10 minute temperature period the average current is:
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
two AT settings in Equation 3:
where:
AT(max) = F
AT(min) = F
The BETA VALUES result is indexed in the right hand
column and the resulting Beta factor (for the register) is in
the same row in the left column.
ΔI
ΔI
BETA VALUES
TABLE 15. FREQUENCY OF TEMPERATURE SENSING AND
BAT
BAT
=
=
BTSE
0.022s
----------------- -
0.022s
----------------- -
600s
60s
0
0
1
1
OUT
CORRECTION BIT
OUT
=
×
×
68μA
68μA
in ppm (at AT = 3FH).
(
in ppm (at AT = 00H) and
AT max
.
To use Table 16, the device is tested at
(
=
=
250nA
25nA
DD
) AT min
–
mode, approximately 22ms. The
BTSR
0
1
0
1
(
)
)/63
BATTERY MODE
TC PERIOD IN
10 Minutes
1 Minute
January 15, 2009
OFF
OFF
FN6667.2
(EQ. 1)
(EQ. 3)
(EQ. 2)