ISL12022 Datasheet, PDF (22/28 Pages) Intersil Corporation – Real Time Clock with On Chip ±5ppm Temp Compensation

ISL12022
The CPPM compensates the oscillator frequency fluctuation
over temperature. It is determined by the temperature (T),
crystal curvature parameter (ALPHA), and crystal turnover
temperature (XT0). T is the result of the temp sensor/ADC
conversion, whose decimal result is 2x the actual
temperature in Kelvin. ALPHA is from either the ALPHA
(cold) or ALPHAH (hot) register depending on T, and XT0 is
from the XT0 register.
NPPM is governed by Equation 5:
NPPM = IPPM(ITR0,BETA) + ALPHA x (T-T0)²
NPPM = IPPM + CPPM
NPPM
=
I
P
P
M
+
-^A----^L----^P----^H-----^A------^•----⁽--^T-----–-----^T----⁰----⁾--²-
4096
where:
(EQ. 5)
ALPHA = α • 2048
T is the reading of the ADC, result is 2 x temperature in
degrees Kelvin.
T = (2 • 298) + XT0
or T = 596 + XT0
(EQ. 6)
Note that NPPM can also be predicted from the FATR and
FDTR register by the relationship (all values in decimal):
NPPM = 2*(BETA*FATR - (FDTR-16))
XT0 Registers (XT0)
TURNOVER TEMPERATURE (XT<3:0>)
The apex of the Alpha curve occurs at a point called the
turnover temperature, or XT0. Crystals normally have a
turnover temperature between +20°C and +30°C, with most
occurring near +25°C.
TABLE 23. TURNOVER TEMPERATURE
ADDR 7
6
5
4
3
2
1
0
2Ch
0
0
0 XT4 XT3 XT2 XT1 XT0
The ISL12022 allows setting the turnover temperature so
that temperature compensation can more exactly fit the
curve of a crystal. Table 24 shows the values available, with
a range from +17.5°C to +32.5°C in +0.5°C increments. The
default value is 00000b or +25°C.
TABLE 24. XT0 VALUES
XT<4:0>
TURNOVER TEMPERATURE
01111
32.5
01110
32.0
01101
31.5
01100
31
01011
30.5
01010
30
TABLE 24. XT0 VALUES (Continued)
XT<4:0>
TURNOVER TEMPERATURE
01001
29.5
01000
29.0
00111
28.5
00110
28.0
00101
27.5
00100
27.0
00011
26.5
00010
26.0
00001
25.5
00000
25.0
10000
25.0
10001
24.5
10010
24.0
10011
23.5
10100
23.0
10101
22.5
10110
22.0
10111
21.5
11000
21.0
11001
20.5
11010
20.0
11011
19.5
11100
19.0
11101
18.5
11110
18.0
11111
17.5
ALPHA Hot Register (ALPHAH)
TABLE 25. ALPHAH REGISTER
ADDR 7
6
5
4
3
2
1
0
2Dh D ALP_H6 ALP_H5 ALP_H4 ALP_H3 ALP_H2 ALP_H1 ALP_H0
The Alpha Hot variable is 7 bits and is defined as the
temperature coefficient of Crystal from the T0 value to +85°C.
(both Alpha Hot and Alpha Cold must be programmed to
provide full temperature compensation). It is normally given in
units of ppm/°C²,with a typical value of -0.034. Like the Alpha
Cold version, a scaled version of the absolute value of this
coefficient is used in order to get an integer value. Therefore,
AlphaH<7:0> is defined as the (|Actual AlphaH Value| x 2048)
and converted to binary. For example, a crystal with AlphaH of
-0.034ppm/°C² is first scaled (|2048*(-0.034)| = 70d) and then
converted to a binary number of 0100110b.
22
FN6659.2
June 23, 2009