71M6541F datasheet(54/166 Pages) MAXIM

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71M6541D/F/G and 71M6542F/G Data Sheet

Rev 5

Referring to Figure 17, the table lookup method uses the 10-bits plus sign-bit value in STEMP[10:0] right-

shifted by two bits to obtain an 8-bit plus sign value (i.e., NV RAM Address = STEMP/4). A limiter ensures

that the resulting look-up address is in the 6-bit plus sign range of -64 to +63 (decimal). The 8-bit NV RAM

content pointed to by the address is added as a 2’s complement value to 0x40000, the nominal value of

4*RTC_P + RTC_Q.

Refer to 2.5.4.3 RTC Rate Control for information on the rate adjustments performed by registers

RTC_P[16:0] (I/O RAM 0x289B[2:0], 0x289C, 0x289D[7:2]) and RTC_Q[1:0] (I/O RAM 0x2891[1:0]. The 8-bit

values loaded in to NV RAM must be scaled correctly to produce rate adjustments that are consistent

with the equations given in 2.5.4.3 RTC Rate Control for RTC_P and RTC_Q. Note that the sum of the

8-bit 2’s complement value looked-up and 0x40000 form a 19-bit value, which is equal to

4*RTC_P+RTC_Q, as shown in Figure 17. The output of the Temperature Compensation is automatically

loaded into the RTC_P[16:0] and RTC_Q[1:0] locations after each look-up and summation operation.

0x40000

10+S

STEMP

>>2

-64

255

-256

LIMIT

Look Up

RAM

ADDR

6+S

8+S

7+S

4*RTC_P+RTC_Q

Figure 17: Automatic Temperature Compensation

The 128 NV RAM locations are organized in 2’s complement format as shown in Table 44. As mentioned

above, the STEMP[10:0] digital temperature values are scaled such that the corresponding NV RAM

addresses are equal to STEMP[10:0]/4 (limited in the range of -64 to +63). See 2.5.5 71M654x Temperature

Sensor on page 56 for the equations to calculate temperature in degrees °C from the STEMP[10:0] reading.

The temperature equation is used to calculate the two temperature columns in Table 44 (the second

column and the rightmost column). The second column uses the full 11-bit values of STEMP[10:0], while

the values in the rightmost column are calculated using the post-limiter (6+S) values multiplied by 4.

Since each look-up table address step corresponds to a 4 x 0.325 °C temperature step, two is added to

the post-limiter 6+S value after multiplying by 4 to calculate the temperature values in the rightmost

column. This method ensures that the compensation data is loaded into the look-up table in a manner

that minimizes quantization error. Table 44 shows the numerical values corresponding to each node in

Figure 17. The values of STEMP[10:0] outside the -256 to +255 range are not shown in this table. The

limiter output is confined to the range of -64 to +63, which is directly the desired address of the 128-byte

look-up table. The rightmost column gives the nominal temperature corresponding to each address cell in

the 128-byte compensation table

Table 44: NV RAM Temperature Table Structure

STEMP[10:0]

(10+S)

(decimal)

Temp (

oC)

(Equation)

STEMP[10:0]>>2

(8+S)

(decimal)

Limiter Output

(6+S)

(decimal)

Temp (

oC)

(LU Table)

-256

-61.71

-64

-61.06

-255

-61.39

-254

-61.06

-253

-60.73

…

-4

20.69

-1

21.35

-3

21.02

-2

21.35

-1

21.67

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