FN6667 datasheet(25/34 Pages) RENESAS

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ISL12020M

FN6667 Rev 6.00

Page 25 of 34

January 9, 2015

The practical range for the temp sensor register output is from 446d

to 726d, or -50°C to +90°C. The temperature compensation

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 turn-over 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.

Note that NPPM can also be predicted from the FATR and FDTR

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 27 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 25.

TEMP

543

210

TK0L

TK07 TK06 TK05 TK04 TK03 TK02 TK01 TK00

TK0M

000

TK09 TK08

NPPM

IPPM CPPM

NPPM

IPPM

ALPHA

T T0

–





4096

----------------------------------------------------

(EQ. 5)

ALPHA

 2048



2 298



 XT0

(EQ. 6)

596 XT0

TABLE 26. TURNOVER TEMPERATURE

ADDR

6543210

2Ch

XT4XT3

XT2XT1

XT0

TABLE 27. XT0 VALUES

XT<4:0>

TURNOVER TEMPERATURE

01111

32.5

01110

32.0

01101

31.5

01100

01011

30.5

01010

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

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