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Data Sheet
ADE7880
Energy = ∫ p ( t ) dt = Lim ? ∑ p ( nT ) × T ?
The ADE7880 achieves the integrationoftheactivepowersignal
in two stages (see Figure 74). The process is identical for both
total and fundamental active powers. The first stage accumulates
? ∞ ?
T → 0 ? n = 0 ?
(27)
the instantaneous phase total or fundamental active power at
1.024MHz, although they are computed by the DSP at 8 kHz
rate. Every time a threshold is reached, a pulse is generated and
the threshold is subtracted from the internal register.
The sign of the energy in this moment is considered the sign
section for details). The second stage consists of accumulating
the pulses generated at the first stage into internal 32-bit accu-
mulation registers. The content of these registers is transferred
to watt-hour registers, xWATTHR and xFWATTHR, when
these registers are accessed.
THRESHOLD
FIRST STAGE OF
ACTIVE POWER
ACCUMULATION
PULSES
GENERATED
AFTER FIRST
STAGE
1 PULSE = 1LSB OF WATTHR[31:0]
Figure 75. Active Power Accumulation Inside the DSP
concatenating the WTHR 8-bit unsigned register to 27 bits
equal to 0. It is introduced by the user and is common for total
and fundamental active powers on all phases. Its value depends
on how much energy is assigned to one LSB of watt-hour regis-
ters. Supposing a derivative of Wh [10 n Wh], n as an integer, is
desired as one LSB of the xWATTHR register, WTHR is
computed using the following equation:
where:
n is the discrete time sample number.
T is the sample period.
In the ADE7880 , the total phase active powers are accumulated in
the AWATTHR, BWATTHR, and CWATTHR 32-bit signed
registers, and the fundamental phase active powers are
accumulated in AFWATTHR, BFWATTHR, and CFWATTHR
32-bit signed registers. The active energy register content can
roll over to full-scale negative (0x80000000) and continue
increasing in value when the active power is positive. Conversely, if
the active power is negative, the energy register underflows to
full-scale positive (0x7FFFFFFF) and continues decreasing in value.
The ADE7880 provides a status flag to signal when one of the
xWATTHR and xFWATTHR registers is half full. Bit 0 (AEHF)
in the STATUS0 register is set when Bit 30 of one of the
xWATTHR registers changes, signifying one of these registers is
half full. If the active power is positive, the watt-hour register
becomes half full when it increments from 0x3FFF FFFF to 0x4000
0000. If the active power is negative, the watt-hour register
becomes half full when it decrements from 0xC000 0000 to
0xBFFF FFFF. Similarly, Bit 1 (FAEHF) in STATUS0 register, is
set when Bit 30 of one of the xFWATTHR registers changes,
signifying one of these registers is half full.
Setting Bits[1:0] in the MASK0 register enable the FAEHF and
AEHF interrupts, respectively. If enabled, the IRQ0 pin is set
low and the status bit is set to 1 whenever one of the energy
registers, xWATTHR (for the AEHF interrupt) or xFWATTHR
(for the FAEHF interrupt), become half full. The status bit is
cleared and the IRQ0 pin is set to logic high by writing to the
STATUS0 register with the corresponding bit set to 1.
WTHR =
PMAX × f S × 3600 × 10 n
U FS × I FS × 2 27
(26)
Setting Bit 6 (RSTREAD) of the LCYCMODE register enables a
read-with-reset for all watt-hour accumulation registers, that is,
the registers are reset to 0 after a read operation.
where:
PMAX = 27,059,678 = 0x19CE5DE as the instantaneous power
computed when the ADC inputs are at full scale.
f S = 1.024 MHz, the frequency at which every instantaneous
power computed by the DSP at 8 kHz is accumulated.
U FS , I FS are the rms values of phase voltages and currents when
the ADC inputs are at full scale.
WTHR register is an 8-bit unsigned number, so its maximum
Integration Time Under Steady Load
The discrete time sample period (T) for the accumulation register
is 976.5625 ns (1.024MHz frequency). With full-scale sinusoidal
signals on the analog inputs and the watt gain registers set to
0x00000, the average word value from each LPF2 is PMAX =
27,059,678 = 0x19CE5DE. If the WTHR register threshold is set
at 3, its minimum recommended value, the first stage accumulator
generates a pulse that is added to watt-hour registers every
value is 2 8 ? 1. Its default value is 0x3. Values lower than 3, that
is 2 or 1 should be avoided and 0 should never be used as the
threshold must be a non-zero value.
3 × 2 27
PMAX × 1 . 024 × 10 6
= 14 . 531 μ sec
This discrete time accumulation or summation is equivalent to
integration in continuous time following the description in
Equation 27.
The maximum value that can be stored in the watt-hour
accumulation register before it overflows is 2 31 ? 1 or
0x7FFFFFFF. The integration time is calculated as
Rev. A | Page 47 of 104
Time = 0x7FFF,FFFF × 14.531 μs = 8 hr 40 min 6 sec
(28)
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