STPM01 datasheet(18/56 Pages) STMICROELECTRONICS

18 / 56 page

Theory of operation

STPM01

18/56

8.5

Period and line voltage measurement

The period module measures the period of base frequency of voltage channel and checks if

the voltage signal frequency is in the band from fCLK/2

17 to f

CLK/2

15. In order to do this, the

signal LIN is produced which is low when the line voltage is rising and it is high when the line

voltage is falling. This means that the LIN signal is the sign of dv/dt. With further elaboration,

the ZCR signal is also produced. On the trailing edge of LIN (Line Frequency) the period

counter starts counting up pulses of fCLK/4 reference signal. The LIN signal is available on

the status bit register (see table 12).

If the counted number of pulses between two trailing edges of LIN is higher than the fCLK/2

Hz equivalent pulses or if the counting is never stopped (no more LIN trailing edge) it means

that the base frequency is lower than fCLK/2

17 Hz and an error flag BFR (Base Frequency

Range) is set.

If the counted number of pulses between two trailing edges of LIN is higher than the fCLK/2

equivalent pulses, the base frequency exceeds the limit. In this case, such error must be

repeated three times in a row, in order to set the error flag BFR.

The BFR flag is also set if the register value of the RMS voltage drops below 64. BFR is

cleared when the register value goes above 128. The BFR, then, gives also information

about the presence of the line voltage inside the meter.

Figure 15.

ZCR Signal

STPM01 Datasheet(PDF) 18 Page - STMicroelectronics