71M6543F/H and 71M6543G/GH Data Sheet
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[16:0] + RTC_Q[1:0].
Refer to 2.5.4.3 RTC Rate Control for information on the rate adjustments performed by registers
RTC_P[16:0] and RTC_Q[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[16:0]
and RTC_Q[1:0]. Note that the sum of the looked-up 8-bit 2’s complement value and 0x40000 form a 19-
bit value, which is equal to 4*RTC_P[16:0] + RTC_Q[1:0], as shown in Figure 13. 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.
Look Up
RAM
LIMIT
63
STEMP
10+S
>>2
ADDR
8+S
-256 -64
63
255
6+S
Q
4*RTC_P+RTC_Q
Σ
-64
7+S
19
19
0x40000
Figure 13: Automatic Temperature Compensation
The 128 NV RAM locations are organized in 2’s complement format. 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 71M6543 Temperature Sensor on
page 55 for the equations to calculate temperature in degrees °C from the STEMP[10:0] reading.
For proper operation, the MPU has to load the lookup table with values that reflect the crystal properties
with respect to temperature, which is typically done once during initialization. Since the lookup table is
not directly addressable, the MPU uses the following procedure to load the NV RAM table:
1. Set the LKPAUTOI bit (I/O RAM 0x2887[7]) to enable address auto-increment.
2. Write zero into the I/O RAM register LKPADDR[6:0] (I/O RAM 0x2887[6:0]).
3. Write the 8-bit datum into I/O RAM register LKPDAT (I/O RAM 0x2888).
4. Set the LKP_WR bit (I/O RAM 0x2889[0]) to write the 8-bit datum into NV_RAM
5. Wait for LKP_WR to clear (LKP_WR auto-clears when the data has been copied to NV RAM).
6. Repeat steps 3 through 5 until all data has been written to NV RAM.
The NV RAM table can also be read by writing a 1 into the LKP_RD bit (I/O RAM 0x2889[1]). The process
of reading from and writing to the NV RAM is accelerated by setting the LKPAUTOI bit (I/O RAM 0x2887[7]).
When LKPAUTOI is set, LKPADDR[6:0] (I/O RAM 0x2887[6:0]) auto-increments every time LKP_RD or
LKP_WR is pulsed. It is also possible to perform random access of the NV RAM by writing a 0 to the
LKPAUTOI bit and loading the desired address into LKPADDR[6:0].
If the oscillator temperature compensation feature is not being used, it is possible to use the NV
RAM storage area as ordinary battery-backed NV storage space using the procedure described
above to read and write NV RAM data. In this case, the OSC_COMP bit (I/O RAM 0x28A0[5]) is
reset to disable the automatic oscillator temperature compensation feature.
2.5.4.5 RTC Interrupts
The RTC generates interrupts each second and each minute. These interrupts are called RTC_1SEC
and RTC_1MIN. In addition, the RTC functions as an alarm clock by generating an interrupt when the
minutes and hours registers both equal their respective target counts as defined in . The alarm clock
interrupt is called RTC_T. All three interrupts appear in the MPU’s external interrupt 6. See Table 33 in the
interrupt section for the enable bits and flags for these interrupts.
The minute and hour target registers are listed in Table 45.
54
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