FDS_6533_6534_004
71M6533/71M6534 Data Sheet
Since TC1 and TC2 are given in µV/°C and µV/°C2, respectively, the value of the VREF voltage
(1.195V) has to be taken into account when transitioning to PPM/°C and PPM/°C2. This means
that PPMC = 26.84*TC1/1.195 and PPMC2 = 1374*TC2/1.195).
3.5.2 Temperature Compensation for VREF
The bandgap temperature is used to digitally compensate the power outputs for the temperature depen-
dence of VREF, using the CE register GAIN_ADJ (internal temperature compensation). Since the band
gap amplifier is chopper-stabilized via the CHOP_E bits, the most significant long-term drift mechanism in
the voltage reference is removed.
In internal temperature compensation mode, the CE applies the following formula to determine the
GAIN_ADJ value. In this formula TEMP_X is the deviation from nominal or calibration temperature ex-
pressed in multiples of 0.1 °C:
TEMP _ X ⋅ PPMC TEMP _ X 2 ⋅ PPMC2
GAIN _ ADJ =16385+
+
214
223
3.5.3 System Temperature Compensation
In a production electricity meter, the 71M6533 and 71M6534 is not the only component contributing to
temperature dependency. A whole range of components (e.g. current transformers, resistor dividers,
power sources, filter capacitors) will contribute temperature effects.
Since the output of the on-chip temperature sensor is accessible to the MPU, temperature compensation
mechanisms with great flexibility are possible. MPU access to GAIN_ADJ permits a system-wide temper-
ature correction over the entire meter rather than local to the chip.
3.5.4 Temperature Compensation for the RTC
In order to obtain accurate readings from the RTC, the following calibration procedures are recommend-
ed:
1. At the time of meter calibration, the crystal oscillator is calibrated using the RTCA_ADJ register in I/O
RAM to be as close to 32768 Hz as possible. The recommended procedure is to connect a high-
precision frequency counter to the TMUXOUT pin and select 0x11 for TMUX[4:0]. This will generate a
4-second pulse at TMUXOUT that can be used to trim RTCA_ADJ to the best value.
2. When the meter is in service, the MPU takes frequent temperature readings. If the temperature cha-
racteristics of the crystal are known, the temperature readings can be used to modify the settings for
the I/O RAM registers PREG[16:0] and QREG[1:0] in order to keep the crystal frequency close to
32768 Hz.
3. After periods of operation under battery power, the temperature for the time the meter was not po-
wered can be estimated by averaging the temperatures before and after battery operation. Based on
this, the overall correction for the RTC time can be calculated and applied to the RTC after main
power returns to the meter.
3.6 Connecting LCDs
The 71M6533 and 71M6534 has an on-chip LCD controller capable of controlling static or multiplexed
LCDs. Figure 33 shows the basic connection for an LCD.
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