ADE7878 datasheet(45/92 Pages) AD

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ADE7878

Rev. 0 | Page 45 of 92

WTHR[47:0]

1 DSP PULSE = 1LSB OF WATTHR[47:0]

ACTIVE POWER

ACCUMULATION

IN DSP

DSP

GENERATED

PULSES

Figure 62. Active Power Accumulation Inside DSP

Figure 62 explains this process. The WTHR[47:0] 48-bit signed

is common for all phase total active and fundamental powers.

Its value depends on how much energy is assigned to one LSB

of watt-hour registers. Supposing a derivative of wh [10n wh], n

as an integer, is desired as one LSB of the xWATTHR register.

Then WTHR is computed using the following expression:

PMAX

WTHR

3600

(25)

where:

PMAX = 33,516,139 = 0x1FF6A6B as the instantaneous power

computed when the ADC inputs are at full scale.

fS = 8 kHz, the frequency with which the DSP computes the

instantaneous power.

UFS, IFS 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[47:0] is

247 − 1. The minimum value is 0x0, but it is recommended to

write a number equal to or greater than PMAX. Never use

negative numbers.

The WTHR[47:0] is a 48-bit register. As previously stated, the

serial ports of the ADE7878 work on 32-, 16-, or 8-bit words.

As shown in Figure 63, the WTHR register is accessed as two

32-bit registers (WTHR1[31:0] and WTHR0[31:0]), each

having eight MSBs padded with 0s.

WTHR[47:0]

24 23

WTHR1[31:0]

WTHR0[31:0]

0000 0000

24 BIT SIGNED NUMBER

0000 0000

24 BIT SIGNED NUMBER

Figure 63. 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.

()

⎭

⎬

⎫

⎩

⎨

⎧

∑

∫

∞

→

Lim

Energy

(26)

where:

n is the discrete time sample number.

T is the sample period.

In the ADE7878, the total phase active powers are accumulated

in the AWATTHR[31:0], BWATTHR[31:0], and CWATTHR[31:0]

32-bit signed registers, and the fundamental phase active powers

are accumulated in the AFWATTHR[31:0], BFWATTHR[31:0],

and CFWATTHR[31:0] 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.

Bit 0 (AEHF) in the STATUS0[31:0] register is set when Bit 30

of one of the xWATTHR registers changes, signifying that 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

vSTATUS0[31:0] 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[31:0] register enables 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.

Setting Bit 6 (RSTREAD) of the LCYMODE[7:0] register

enables a read-with-reset for all watt-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 register

is 125 μs (8 kHz 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 = 33,516,139 =

0x1FF6A6B. If the WTHR[47:0] threshold is set at the PMAX

level, this means that the DSP generates a pulse that is added to

the watt-hour registers every 125 μs.

The maximum value that can be stored in the watt-hour

accumulation register before it overflows is 231 − 1 or

0x7FFFFFFF. The integration time is calculated as

Time = 0x7FFF,FFFF × 125 μs = 74 hr 33 min 55 sec

(27)

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