STPM11_08 datasheet(21/45 Pages) STMICROELECTRONICS

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STPM11/12/13/14

Theory of operation

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7.8

Error detection

In addition to the no-load condition and the line frequency band, the integration of power can

be suspended also due to detected error on the source signals.

There are two kinds of error detection circuits involved. The first checks all the

∑Δ signals

from the analog part if any are stacked at 1 or 0 within the 1/128 of fCLK period of

observation. In case of detected error the corresponding

∑Δ signal is replaced with an idle ∑

Δ signal, which represents a constant value 0.

Another error, condition occurs if the MOP, MON and LED pin outputs signals are different

from the internal signals that drive them. This can occur if some of this pin is forced to GND

or to some other imposed voltage value.

7.9

Tamper detection module (STPM13/14 only)

The STPM13/14 is able to measure the current in both live and neutral wires. This

mechanism has been adapted to implement an anti-tamper function. If this function is

selected (see Table 8:), the live and neutral wire currents are monitored. When a difference

between the two measurements is detected, the STPM13/14 enters the Tamper State.

When there is a very small difference between the two channels, the STPM13/14 is in

Normal state.

In particular, both channels are not constantly observed. A time multiplex mechanism is

used. During the observation time of the selected channel, its active energy is calculated.

The detection of a tamper condition occurs when the absolute value of the difference

between the two active energy values is greater than a certain percentage of the averaged

energy during the activated tamper module. This percentage value can be selected between

two different values (12.5% and 6.25%) according to the value of the configuration bit CRIT.

The tamper condition will be detected when the following formula is satisfied:

EnergyCH1 - EnergyCH2 > KCRIT (EnergyCH1 + EnergyCH2)/2; where KCRIT can be 12.5%

or 6.25%.

The detection threshold is much higher than the accuracy difference of the current

channels, which should be less than 0.1%. Some margin should be left for a possible

transition effect, due to accidental synchronism between the actual load current change and

the rhythm of taking the energy samples.

The tamper circuit works if the energies associated with the two current channels will be

both positive or both negative. If the two energies have different signs, the tamper remains

on constantly. However, the channel with the associated higher power is selected for the

final computation of energy.

In single wire mode, the Apparent energy rather than active is used for Tamper detection.

Detailed operational description

Normal state

The meter is initially set to normal state, i.e. tamper not detected. In such state, we expect

that the values of both load currents should not differ more than the accuracy difference of

the channels. For this reason, we can use an average value of currents of both channels for

the active energy calculation. The average is implemented with the multiplex ratio of 32:32

periods of line per channel. This means that for 32 periods of line voltage, i.e. 640ms at 50

Hz, the current of the primary channel is used for the calculation followed by another 32

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