Maxim Integrated 71M6534 Energy Meter IC Family Software - Manual (Page 84)

71M653X Software User’s Guide
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© Copyright 2005-2008 TERIDIAN Semiconductor Corporation
5.14.2 Firmware Application: Selected Tasks
5.14.2.1 Sag Detection
A sag is an undervoltage condition that persists for more than one period. A shorter undervoltage condition is called a
dip (see Figure Figure 5-1). The occurrence of sags can announce an impending loss of power. Since accumulated
energy values etc. in the meter will have to be saved to non-volatile memory in the case of loss of power, a sag can be
used to initiate data saving operations. Some applications may instead save or count the sag event for the purpose of
recording power quality data.
dip
sag
Figure 5-1: Sag and Dip Conditions
Sag detection is performed by the CE, based on the CE DRAM registers SAG_THR and SAG_CNT. SAG_THR defines
the threshold which the input voltage has to be continuously below, and SAG_CNT defines the number of samples
required to trigger the sag bit (see Figure 5-2).
16.67ms
SAG_THR
SAG_CNT
84 samples
16.67ms 16.67ms
SAG_THR
SAG_CNT
84 samples
Figure 5-2: Sag Event
When the CE detects a sag that meets the sag conditions specified in SAG_THR and SAG_CNT on one of the input
voltage channels, it will reflect this in the corresponding bit (SAG for single-phase, or SAG_A, SAG_B, SAG_C for poly-
phase) of the CE STATUS Word. See the CE Interface section in the 653X Data Sheet for details.
The demo code saves the power registers to the EEPROM when a sag is detected. It also has a timer to avoid
multiple saves because of grid-switching from a recloser or noisy power when the grid starts up. See the 5.4.2.3 about
the CE_BUSY interrupt for more information.
See Application Note AN651X_044 for more information.
5.14.2.2 Temperature Measurement
The temperature output of the on-chip temperature sensor (TEMP_RAW) is provided by the CE in CE DRAM location
0x7B. The relative chip temperature deltaT (MPU location 0x20) is derived by subtracting the raw temperature from the
nominal temperature (TEMP_NOM) and multiplying it with a constant factor. Thus, once the raw temperature obtained
at a known environmental temperature is stored in TEMP_NOM, deltaT will always reflect the deviation from nominal
temperature. The scaling is in tenths of Centigrades, i.e. a reading of 75 means that the measured temperature is
7.5°C higher than the reference temperature.