Rainbow Electronics 71M6542G - Manual (Page 54)

54 © 2008–2011 Teridian Semiconductor Corporation v1.1
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.
Σ
0x40000
19
10+S
STEMP
>>2
63
-64
-64 63 255 -256
LIMIT Look Up
RAM
ADDR
6+S
8+S
Q
7+S
4*RTC_P+RTC_Q
19
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 0.327 °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 ( o C)
(Equation)
STEMP[10:0]>>2
(8+S)
(decimal)
Limiter Output
(6+S)
(decimal)
Temp ( o C)
(LU Table)
-256 -61.71
-64 -64 -61.06
-255 -61.39
-254 -61.06
-253 -60.73
… … … … …
-4 20.69
-1 -1 21.35
-3 21.02
-2 21.35
-1 21.67