71M6541D Datasheet, PDF (54/166 Pages) Maxim Integrated Products – 0.1% Accuracy Over 2000:1 Current Range Energy Meter ICs

71M6541D/F/G and 71M6542F/G Data Sheet
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.
LIMIT
63
STEMP _>>2
10+S
8+S -256 -64
-64
Look Up
RAM
ADDR
63 255 _6+S
Q
Σ
4*RTC_P+RTC_Q
7+S
19
19
0x40000
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)
-256
-255
-254
-253
…
-4
-3
-2
-1
Temp (^oC)
(Equation)
-61.71
-61.39
-61.06
-60.73
…
20.69
21.02
21.35
21.67
STEMP[10:0]>>2
(8+S)
(decimal)
Limiter Output
(6+S)
(decimal)
Temp (^oC)
(LU Table)
-64
-64
-61.06
…
…
…
-1
-1
21.35