STPM12 datasheet(22/43 Pages) STMICROELECTRONICS

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Theory of operation

STPM11/12/13/14

22/43

7.12

Resetting the STPM1X

The STPM1X has no reset pin. The device is automatically reset by the POR circuit when

the VCC crosses the 2.5V value. When the reset occurs, all clocks and both DC buffers in the

analog part are kept off for about 30ms and all blocks of the digital part are held in a reset

state for about 125ms after a reset condition.

Resetting the STPM1X causes all the functional modules of STPM1X to be cleared

including the OTP shadow latches (see 7.15 for OTP shadow latches description)

7.13

Energy to frequency conversion

The STPM1X provides energy to frequency conversion both for calibration and energy

readout purposes. In fact, one convenient way to verify the meter calibration is to provide a

pulse train signal with 50% duty cycle whose frequency signal is proportional to the active

energy under steady load conditions. It is convenient to have high frequency pulses during

calibration phase and low frequency for readout purposes; STPM1X supports both cases.

Let's suppose to choose a certain number of pulses on the LED pin (high frequency) that will

corresponds to 1kWh. We will name this value as P.

The Active Energy frequency-based signal is available in the LED pin. The LED is driven

from internal signal AW (Active Energy) whose frequency is proportional to the active

energy. The desired P is achieved acting on the digital calibrators during the calibration

procedure.

The APL configuration bit changes the internal divider that provides the signal on the LED

pin according to

Table 11., setting APL=1 the number of pulses are reduced in order to

provide low frequency pulses for readout purposes. The division factor is set according to

KMOT configuration bits. In this case the pulses will have a fixed width of 31.25 ms.

Figure 18.

Different oscillator circuits (a); (b); (c)

STPM12/14 with external source

STPM12/14 with quartz

STPM11/13

STPM12 Datasheet(PDF) 22 Page - STMicroelectronics