MAXQ3183 Maxim, MAXQ3183 Datasheet - Page 59

• Manufacturer Part Number: MAXQ3183
• Description: The MAXQ3183 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, power, energy, and many other metering and power-quality parameters of a polyphase load
• Manufacturer: Maxim

is taking place, i.e., both tasks are executed simultane-
ously sharing CPU time. It is essential that the DSP
postprocessing calculations be completed before the
next DSP trigger to avoid losing accumulated data.
The master should allow enough processing time by
adjusting the R_ADCRATE register. Default settings
provide plenty of CPU time for both tasks.
The MAXQ3183 accumulates raw sums and calculates
line-cycle integrals for each voltage-current pair sepa-
rately. The individual power accumulators are:
• PA = V
• PB = I
• PC = V
The PA and PC accumulators always operate in a sin-
gle mode: (V
the PC accumulator. Alternately, the operating mode of
the PB accumulator is defined by setting the CON-
CFG[1:0] bits in the OPMODE1 register.
All filters have a certain settling time before accurate
energy readings can be accumulated. To avoid accu-
mulation of invalid data from filters that are still settling,
an energy accumulation timeout period can be set in
the ACC_TIMO register. When ACC_TIMO > 0, comput-
ed energy is not accumulated for ACC_TIMO of DSP
cycles. The MAXQ3183 will decrement the ACC_TIMO
register every DSP cycle until it becomes 0. When
ACC_TIMO reaches 0 value, energy accumulation
begins (or resumes, if ACC_TIMO was set to nonzero
value by the master). Pulse outputs are also disabled
when ACC_TIMO > 0. The default value of ACC_TIMO
is 0x05.
To avoid “meter creep,” no energy accumulation should
take place when measured current is less than a cer-
tain threshold. The NOLOAD register can be pro-
grammed to enable and configure this feature. If the
measured X.IRMS value for a phase (A, B, or C) falls
below the NOLOAD threshold, the energy accumulators
for this phase are not incremented. Setting NOLOAD =
0 disables this feature. Full scale is represented by
0x10000.
(V
A
- V
B
C
A
C
)
x V
x I
x I
A
Energy Accumulation Start Delay
A
C
B
x I
Low-Power, Multifunction, Polyphase AFE
or -I
A
______________________________________________________________________________________
) for the PA accumulator, (V
B
x V
C
or -I
with Harmonics and Tamper Detect
B
No-Load Feature
x (V
A
+ V
C
C
) or -I
x I
C
) for
B
x
So far in this discussion, the values being calculated
and managed in the MAXQ3183 have been based on
fundamental units meaningful to the device itself: volt-
age as a binary fraction of full-scale voltage; current as
a binary fraction of full-scale current, and time as a non-
integer multiple of the ADC frame time.
But a practical electricity meter must report its results in
standard units, such as volts, amperes, and watts. The
MAXQ3183 contains a mechanism to convert the inter-
nal units (“meter units”) to real world units (“display
units”). This conversion is performed in the conversion
constant (CC) registers.
For some of these values (voltage, current) the calcula-
tion is simple: multiply by the conversion constant. For
other values (power, energy) the calculation is more
complex. In any case, the value in the CC register
affects only the conversion from a meter unit to a dis-
play unit; calibration is handled separately in the gain
adjustment registers for each recorded value.
The results of all on-demand calculations are reported
as 8-byte (64-bit) values of which no more than 6 bytes
(48 bits) are significant. Eight bytes are used as a com-
mon length; however, fewer bytes can be requested for
those registers known to have smaller maximum values.
For example, the power factor virtual register has a
maximum value that is expressed in only 3 bytes; con-
sequently, the register can be requested with a length
of 4 bytes without loss of data.
These registers (V.A, V.B, V.C, I.A, I.B, I.C) are calculat-
ed by simply multiplying the calculated RMS value
(A.VRMS, B.VRMS, C.VRMS, A.IRMS, B.IRMS, C.IRMS)
by the contents of the VOLT_CC or AMP_CC register.
Since the RMS voltage and RMS current are given in
32-bit registers and the conversion coefficients are
given in 16-bit registers, the result of the product is 48
bits.
Regardless of the internal units used, VOLT_CC and
AMP_CC can be tailored so that the LSB of the virtual
register can be any value. For example, if one wished
to have a 32-bit value representing milliamps, one
could multiply by a value that scaled the register such
that the LSB was 2
16 bits. The result is milliamps with 32 bits of precision;
thus, the maximum current that could be represented
would be 4,294,967,296mA, or just over 4MA.
-16
mA. Then, discard the low-order
On Demand Calculations
RMS Volts, RMS Amps
59