Datasheet

ADE7854/ADE7858/ADE7868/ADE7878 Data Sheet
Rev. G | Page 50 of 100
HPFDIS
[23:0]
HPF
HPFDIS
[23:0]
DIGITAL
INTEGRATOR
REVAPA BIT I N
STATUS0[31:0]
HPF
VA
IA
AWGAIN AWATTOS
APHCAL
ACCUMULATOR
2
4
WTHR[47:0]
AWATT
AWATTHR[31:0]
32-BIT
REGISTER
LPF2
08510-044
DIGITAL SIGNAL PROCESSOR
AVGAI N
AIGAIN
Figure 67. Total Active Energy Accumulation
The ADE7854/ADE7858/ADE7868/ADE7878 achieve the
integration of the active power signal in two stages (see Figure 67).
The process is identical for both total and fundamental active
powers. The first stage is accomplished inside the DSP: every
125 μs (8 kHz frequency) the instantaneous phase total or funda-
mental active power is accumulated into an internal register.
When a threshold is reached, a pulse is generated at the processor
port, and the threshold is subtracted from the internal register.
The sign of the energy in this moment is considered the sign of
the active power (see Sign of Active Power Calculation section
for details). The second stage is done outside the DSP and consists
of accumulating the pulses generated by the processor into internal
32-bit accumulation registers. The content of these registers is
transferred to watt-hour registers, xWATTHR and xFWATTHR,
when these registers are accessed.
WTHR[47:0]
1 DSP PULSE = 1LSB OF WATTHR[31:0]
ACTIVE POWER
A
CCUMULATION
IN DSP
DSP
GENERATED
PULSES
08510-045
Figure 68. Active Power Accumulation Inside the DSP
Figure 68 explains this process. The WTHR 48-bit signed register
contains the threshold. It is introduced by the user and is common
for all phase total and fundamental active powers. Its value
depends on how much energy is assigned to one LSB of watt-
hour registers. When a derivative of active energy (wh) of [10
n
wh], where n is an integer, is desired as one LSB of the
xWATTHR register, then, the xWATTHR register can be
computed using the following equation:
FSFS
n
S
IV
fPMAX
WTHR



103600
(25)
where:
PMAX = 33,516,139 = 0x1FF6A6B as the instantaneous power
computed when the ADC inputs are at full scale.
f
S
= 8 kHz, the frequency with which the DSP computes the
instantaneous power.
V
FS
, I
FS
are the rms values of phase voltages and currents when
the ADC inputs are at full scale.
The maximum value that can be written on WTHR is 2
47
− 1.
The minimum value is 0x0, but it is recommended to write a
number equal to or greater than PMAX. Never use negative
numbers.
WTHR is a 48-bit register. As stated in the Current Waveform
Gain Registers section, the serial ports of the ADE7854/ADE7858/
ADE7868/ADE7878work on 32-, 16-, or 8-bit words. As shown
in Figure 69, the WTHR register is accessed as two 32-bit
registers (WTHR1 and WTHR0), each having eight MSBs
padded with 0s.
WTHR[47:0]
47 24
31 24 23 0 31 24 23 0
23 0
WTHR1[31:0] WTHR0[31:0]
0000 0000 24 BIT SIGNED NUMBER 0000 0000 24 BIT SIGNED NUMBER
08510-046
Figure 69. WTHR[47:0] Communicated As Two 32-Bit Registers
This discrete time accumulation or summation is equivalent to
integration in continuous time following the description in
Equation 26.














0
0T
Lim
n
TnTpdttpEnergy (26)
where:
n is the discrete time sample number.
T is the sample period.
In the ADE7854/ADE7858/ADE7868/ADE7878, 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