ISL12020MIRZ-T7A Datasheet, PDF (25/34 Pages) Intersil Corporation – Low Power RTC with Battery Backed SRAM, Integrated ±5ppm Temperature Compensation and Auto Daylight Saving

ISL12020M
function is only guaranteed over -40°C to +85°C. The TSE bit must
be set to “1” to enable temperature sensing.
TABLE 24.
TEMP 7
6
5
4
3
2
1
0
TK0L TK07 TK06 TK05 TK04 TK03 TK02 TK01 TK00
TK0M 0
0
0
0
0
0
TK09 TK08
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
NPPM = IPPM + CPPM
NPPM
=
I
P
P
M
+
A_-----L_----P_----H_-----A_------_----_--T_{-----–-----}T_----0_----_--²_-
4096
where
(EQ. 5)
ALPHA =   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 25. TURNOVER TEMPERATURE
ADDR 7
6
5
4
3
2
1
0
2Ch
0
0
0 XT4 XT3 XT2 XT1 XT0
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 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
25
FN6667.5
December 13, 2011