ADE7932 datasheet(55/120 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932

Rev. 0 | Page 55 of 120

The second stage consists of accumulating the pulses generated

at the first stage into internal 32-bit accumulation registers. The

contents of these registers are transferred to the watthour registers,

xWATTHR and xFWATTHR, when these registers are accessed.

Figure 77 shows this process.

THRESHOLD

1 PULSE = 1LSB OF WATTHR[31:0]

FIRST STAGE OF

ACTIVE POWER

ACCUMULATION

PULSES

GENERATED

AFTER FIRST

STAGE

Figure 77. Active Power Accumulation Inside the DSP

The threshold is formed by concatenating the 8-bit unsigned

WTHR register (Address 0xEA02) to 27 bits equal to 0. The

WTHR register is configured by the user and is the same for

total active and fundamental powers on all phases. Its value

depends on how much energy is assigned to one LSB of the

watthour registers. For example, if a derivative of Wh (10n Wh,

where n is an integer) is desired as one LSB of the xWATTHR

3600

PMAX

WTHR

(31)

where:

PMAX = 26,991,271 = 0x19BDAA7, the instantaneous power

computed when the ADC inputs are at full scale.

fS = 1.024 MHz, the frequency at which every instantaneous

power computed by the DSP at 8 kHz is accumulated.

VFS and IFS are the rms values of the phase voltages and currents

when the ADC inputs are at full scale.

The WTHR register is an 8-bit unsigned number, so its maxi-

mum value is 28 − 1. Its default value is 0x3. Avoid using values

lower than 3, that is, 2 or 1; never use the value 0 because the

threshold must be a non-zero value.

This discrete time accumulation or summation is equivalent

to integration in continuous time, as shown in Equation 32.

( )













∑

∫

∞

→

Lim

Energy

(32)

where:

n is the discrete time sample number.

T is the sample period.

In the ADE7978, the total phase active powers are accumulated

in the 32-bit signed AWATTHR, BWATTHR, and CWATTHR

registers (Address 0xE400 to Address 0xE402).

The fundamental phase active powers are accumulated in the

32-bit signed AFWATTHR, BFWATTHR, and CFWATTHR

registers (Address 0xE403 to Address 0xE405). The active energy

and continue to increase 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 ADE7978 provides a status flag to indicate that one of the

xWATTHR registers is half full. Bit 0 (AEHF) in the STATUS0

xWATTHR registers changes, signifying that one of these

registers is half full.

•

If the active power is positive, the watthour register

becomes half full when it increments from 0x3FFFFFFF

to 0x40000000.

•

If the active power is negative, the watthour register

becomes half full when it decrements from 0xC0000000

to 0xBFFFFFFF.

Similarly, Bit 1 (FAEHF) in the STATUS0 register is set when

Bit 30 of one of the xFWATTHR registers changes, signifying

that one of these registers is half full.

Setting Bits[1:0] in the MASK0 register enables the FAEHF and

AEHF interrupts. When enabled, the IRQ0 pin goes low and the

status bit is set to 1 when one of the energy registers (xWATTHR

for the AEHF interrupt, or xFWATTHR for the FAEHF interrupt),

becomes half full. The status bit is cleared and the IRQ0 pin is

returned high by writing a 1 to the appropriate bit in the STATUS0

Setting Bit 6 (RSTREAD) in the LCYCMODE register

(Address 0xE702) enables a read with reset for all watthour

accumulation registers, that is, the registers are reset to 0 after

a read operation.

INTEGRATION TIME UNDER STEADY LOAD

The discrete time sample period (T) for the accumulation

registers is 976.5625 ns (1.024 MHz 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 =

26,991,271. If the WTHR register threshold is set to 3 (its mini-

mum recommended value), the first stage accumulator generates

a pulse that is added to the watthour registers at intervals of

5683

024

PMAX

The maximum value that can be stored in the watthour accu-

mulation register before it overflows is 231 − 1 or 0x7FFFFFFF.

The integration time is calculated as

Time = 0x7FFFFFFF × 14.5683 μs = 8 hr, 41 min, 25 sec

(33)

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