71M6521DE/71M6521FE
Energy Meter IC
DATASHEET
JANUARY 2008
1/32768Hz =
30.518µs
IB
VB
IA
VA
13/32768Hz = 397µs
per mux cycle
Figure 4: Samples from Multiplexer Cycle
The end of each multiplexer cycle is signaled to the MPU by the CE_BUSY interrupt. At the end of each multiplexer cycle,
status information, such as sag data and the digitized input signal, is available to the MPU.
833ms
20ms
XFER_BUSY
Interrupt to MPU
Figure 5: Accumulation Interval
Figure 5 shows the accumulation interval resulting from PRE_SAMPS = 42 and SUM_CYCLES = 50, consisting of 2100 samples
of 397µs each, followed by the XFER_BUSY interrupt. The sampling in this example is applied to a 50Hz signal.
There is no correlation between the line signal frequency and the choice of PRE_SAMPS or SUM_CYCLES (even though when
SUM_CYCLES = 42 one set of SUM_CYCLES happens to sample a period of 16.6ms). Furthermore, sampling does not have to
start when the line voltage crosses the zero line, and the length of the accumulation interval need not be an integer multiple of
the signal cycles.
It is important to note that the length of the accumulation interval, as determined by NACC, the product of SUM_CYCLES and
PRE_SAMPS, is not an exact multiple of 1000ms. For example, if SUM_CYCLES = 60, and PRE_SAMPS = 00 (42), the
resulting accumulation interval is:
NACC
fS
60⋅42
32768Hz
13
2520
τ =
=
=
= 999.75ms
2520.62Hz
This means that accurate time measurements should be based on the RTC, not the accumulation interval.
v1.0
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