ADE7932 datasheet(60/120 Pages) AD

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ADE7978/ADE7933/ADE7932

Data Sheet

Rev. 0 | Page 60 of 120

The threshold is formed by concatenating the 8-bit unsigned

VARTHR register (Address 0xEA03) to 27 bits equal to 0. The

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

the total reactive and fundamental powers on all phases. Its value

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

var-hour registers. For example, if a derivative of a volt ampere

reactive hour, varh (10n varh, where n is an integer) is desired

as one LSB of the xVARHR register, the VARTHR register is

calculated using the following equation:

3600

PMAX

VARTHR

(38)

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 VARTHR 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 39:

Reactive Energy =

















∑

∫

∞

→

Lim

q(nT)

q(t)dt

(39)

where:

n is the discrete time sample number.

T is the sample period.

In the ADE7978, the total phase reactive powers are accumulated

in the 32-bit signed registers AVARHR, BVARHR, and CVARHR

(Address 0xE406 to Address 0xE408). The fundamental phase

reactive powers are accumulated in the 32-bit signed registers

AFVARHR, BFVARHR, and CFVARHR (Address 0xE409 to

Address 0xE40B). The reactive energy register contents can roll

over to full-scale negative (0x80000000) and continue to increase

in value when the reactive power is positive. Conversely, if the

reactive power is negative, the energy register underflows to full-

scale positive (0x7FFFFFFF) and continues to decrease in value.

The ADE7978 provides a status flag to indicate that one of the

xVARHR registers is half full. Bit 2 (REHF) in the STATUS0

xVARHR registers changes, signifying that one of these registers

is half full.

•

If the reactive power is positive, the var-hour register

becomes half full when it increments from 0x3FFFFFFF

to 0x40000000.

•

If the reactive power is negative, the var-hour register

becomes half full when it decrements from 0xC0000000

to 0xBFFFFFFF.

Similarly, Bit 3 (FREHF) in the STATUS0 register is set when

Bit 30 of one of the xFVARHR registers changes, signifying that

one of these registers is half full.

Setting Bits[3:2] in the MASK0 register enables the FREHF and

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

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

for the REHF interrupt or xFVARHR for the FREHF 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 var-hour

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 a 90° phase difference

between the voltage and current signals (the largest possible

reactive power), the average word value representing the reactive

power is PMAX = 26,991,271. If the VARTHR register threshold

is set to 3 (its minimum recommended value), the first stage

accumulator generates a pulse that is added to the var-hour

registers at intervals of

5683

024

PMAX

The maximum value that can be stored in the var-hour accumu-

lation 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

(40)

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