MAXQ3181 Maxim, MAXQ3181 Datasheet - Page 55

• Manufacturer Part Number: MAXQ3181
• Description: The MAXQ3181 is a dedicated electricity measurement front-end that collects and calculates polyphase voltage, current, active power and energy, and many other metering parameters of a polyphase load
• Manufacturer: Maxim

The purpose of a meter pulse is generally to advance a
mechanical counter when such a device is used as a
display. Meter pulses are also used during calibration
since time intervals can be measured with great preci-
sion.
The MAXQ3181 supports one meter pulse output. This
output can be configured for either active positive or
active negative pulses by means of the POPOL bit in
the OPMODE1 register. When triggered, the pulse goes
to its active state and remains there for a period of time
defined by the PLS1_WD register, and then returns to
the inactive state (unless triggered again).
The PLS1_WD register contains the time in ADC frame
periods that the pulses remain in the active state when
triggered. By default, this register contains decimal 156
(0x9C) giving, at the default frame rate, a pulse width of
50ms.
Among the quantities that can be accumulated by the
pulse subsystem are the arithmetic active energy (that
is, the accumulated positive real energy minus the
accumulated negative real energy) and the absolute
active energy (that is, accumulated positive real energy
plus accumulated negative real energy). Other quanti-
ties include RMS voltage and current, positive and neg-
ative real energy. Select the desired accumulation
value in the QNSEL field of the PLSCFG1 register.
Also in the pulse configuration registers you can select
which phases to include in the accumulation. Set any or
all the PHASEA, PHASEB, and PHASEC bits in the
PLSCFG1 register to include them in the accumulation.
On every DSP cycle, the MAXQ3181 adds the value in
the selected register (or set of registers) to the pulse
accumulator. If the value in the pulse accumulator
exceeds the value in the associated threshold register
(THR1), then a pulse is started and the value in the
threshold register is subtracted from the value in the
pulse accumulator.
A meter constant is the number of pulses that are gen-
erated during a standard measurement interval; for
example, a meter might specify a meter constant of
1600 pulses per kilowatt-hour. The THR1 register is
used to specify the meter constant according to the fol-
lowing formula:
Low-Power, Active Energy, Polyphase AFE
THR
______________________________________________________________________________________
=
K
M
×
I
FS
2
×
16
V
Generating Pulses
FS
Meter Constant
×
t
FR
Meter Pulse
In this formula, THR is the value to be written to the
threshold register, K
pulses per kilowatt hour), I
voltage and current, respectively, and t
period in units of hours, as in the previous calculation.
As an example, assume once again a full-scale voltage
value of 558.1V = 0.5581kV, a full-scale current value of
102.4A, a desired meter constant of 1600 pulses per
kilowatt hour, and a default frame time of 320μs (88.9 x
10
lated as:
Increasing the value of the threshold register reduces
the meter constant (that is, there are fewer pulses per
kilowatt-hour); reducing the threshold register increases
the meter constant (that is, there are more pulses per
kilowatt-hour.)
The MAXQ3181 contains an interrupt subsystem to
relieve the host processor of the burden of constantly
polling the device for status. Instead, under certain cir-
cumstances, the MAXQ3181 can activate an external
pin to alert the host processor that some condition
requiring host attention has occurred.
Interrupts are managed globally by the IRQ_MASK and
IRQ_FLAG registers. In general, when a bit becomes
set in the IRQ_FLAG register, an interrupt is generated
if the corresponding bit is set in the IRQ_MASK register.
Interrupts can be configured for the following conditions:
-9
PWRF: This flag indicates the V
MAXQ3181 has fallen below its nominal operating
threshold (about 2.85V). This can be taken as an
indication that power failure is imminent and that the
host processor should begin taking steps to ensure
an orderly shutdown.
CHSCH: This flag indicates that the CHKSUM regis-
ter changed its value.
EOVF: Energy overflow. This flag indicates that one or
more energy accumulators (X.EAPOS, X.EANEG, etc.)
have overflowed. In a traditional meter, the host
processor would poll the MAXQ3181 to determine
which of the energy accumulators have overflowed and
adjust its internal accounting registers accordingly.
OC: The RMS current value on one or more of the
phases over the most recent DSP cycle has exceed-
ed the value set in the OCLVL register.
65,536/(1600 x 102.4 x 0.5581 x 88.9 x 10
hours). The threshold register value can be calcu-
8,063,071 = 0x7B085F
M
is the desired meter constant (in
FS
and V
FS
are the full-scale
FR
DVDD
is the frame
Interrupts
-9
) =
to the
55