71M6511H-IGT/F Maxim Integrated Products, 71M6511H-IGT/F Datasheet - Page 78

• Manufacturer Part Number: 71M6511H-IGT/F
• Description: IC ENERGY METER RESIDENT 64-LQFP
• Manufacturer: Maxim Integrated Products

Specifications of 71M6511H-IGT/F

• Processor Series: 6511x
• Core: 80515
• Data Bus Width: 8 bit
• Program Memory Type: Flash
• Program Memory Size: 64 KB
• Data Ram Size: 7 KB
• Interface Type: I2C, SSI, UART
• Maximum Clock Frequency: 5 MHz
• Number Of Programmable I/os: 12
• Number Of Timers: 2
• Operating Supply Voltage: 3 V to 3.6 V
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: LQFP-64
• 3rd Party Development Tools: PK51, CA51, A51, ULINK2
• Development Tools By Supplier: 71M6511H-DB
• Minimum Operating Temperature: - 40 C
• On-chip Adc: 22 bit Delta Sigma
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Temperature Compensation and Mains Frequency Stabilization for the RTC
The accuracy of the RTC depends on the stability of the external crystal. Crystals vary in terms of initial accuracy as well as in
terms of behavior over temperature. The flexibility provided by the MPU allows for compensation of the RTC using the sub-
strate temperature. To achieve this, the crystal has to be characterized over temperature and the three coefficients Y_CAL,
Y_CALC, and Y_CAL_C2 have to be calculated. Provided the IC substrate temperatures tracks the crystal temperature, the
coefficients can be used in the MPU firmware to trigger occasional corrections of the RTC seconds count, using the
RTC_DEC_SEC or RTC_INC_SEC registers in I/O RAM.
It is not recommended to measure crystal frequency directly due to the error introduced by the measurement probes. A
practical method to measure the crystal frequency (when installed on the PCB with the 71M6511) is to have a DIO pin toggle
every second, based on the RTC interrupt, with all other interrupts disabled. When this signal is measured with a precision
timer, the crystal frequency can be obtained from the measured time period t (in µs):
Example: Let us assume a crystal characterized by the measurements shown in Table 62. The values show that even at
nominal temperature (the temperature at which the chip was calibrated for energy), the deviation from the ideal crystal
frequency is 11.6 PPM, resulting in about one second inaccuracy per day, i.e. more than some standards allow.
As Figure 29 shows, even a constant compensation would not bring much improvement, since the temperature characteristics
of the crystal are a mix of constant, linear, and quadratic effects (in commercially available crystals, the constant and quadratic
effects are dominant).
The temperature characteristics of the crystal are obtained from the curve in Figure 29 by curve-fitting the PPM deviations. A
fairly close curve fit is achieved with the coefficients a = 10.89, b = 0.122, and c = –0.00714 (see Figure 30).
Page: 78 of 98
Temperature [°C]
Deviation from
Nominal
A Maxim Integrated Products Brand
f
=
32768
10
6
t
µs
Frequency [Hz]
Measured
32768.5
32768.4
32768.3
32768.2
32768.1
32767.9
32767.8
32767.7
32767.6
32767.5
32768
Figure 29: Crystal Frequency over Temperature
© 2005–2010 Teridian Semiconductor Corporation
+50
+25
-50
-25
-50
0
Table 62: Frequency over Temperature
Frequency [PPM]
Deviation from
Nominal
-25
32767.98
32768.28
32768.38
32768.08
32767.58
0
Single-Phase Energy Meter IC
71M6511/71M6511H
25
-12.817
11.597
DATA SHEET
8.545
2.441
-0.61
50
NOVEMBER 2010
V2.7