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ISL12020M
25
FN6667.5
December 13, 2011
function is only guaranteed over -40°C to +85°C. The TSE bit must
be set to “1” to enable temperature sensing.
NPPM Registers (NPPM)
The NPPM value is exactly 2x the net correction required to bring
the oscillator to 0ppm error. The value is the combination of
oscillator Initial Correction (IPPM) and crystal temperature
dependent correction (CPPM).
IPPM is used to compensate the oscillator offset at room
temperature and is controlled by the ITR0 and BETA registers,
which are fixed during factor test.
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
where
T is the reading of the ADC, result is 2 x temperature in degrees
Kelvin.
or
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.
The ISL12020M has a preset turnover temperature
corresponding to the crystal in the module. This value is recalled
on initial power-up and should never be changed for best
temperature compensation performance, although the user may
override this preset value if so desired.
Table 26 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.
TEMP
76
543
210
TK0L
TK07 TK06 TK05 TK04 TK03 TK02 TK01 TK00
TK0M
000
TK09 TK08
TABLE 25. TURNOVER TEMPERATURE
ADDR
7654321
0
2Ch
0
XT4XT3
XT2XT1
XT0
NPPM
IPPM
CPPM
+
=
NPPM
IPPM
ALPHA
T
T0
–
2
4096
----------------------------------------------------
+
=
(EQ. 5)
ALPHA
2048
=
T
2
298
XT0
+
=
(EQ. 6)
T
596
XT0
+
=
TABLE 26. XT0 VALUES
XT<4:0>
TURNOVER TEMPERATURE
01111
32.5
01110
32.0
01101
31.5
01100
31
01011
30.5
01010
30
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
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