Datasheet

STPM10 Theory of operation
Doc ID 17728 Rev 4 19/53
7.4 Zero-crossing detection
The STPM10 has a zero-crossing detector circuit on the voltage channel which can be used
by application for synchronization of some utility equipment in the event of zero-crossing of
the line voltage. This circuit produces the internal signal ZCR which has a rising edge every
time the line voltage crosses zero, and a negative edge every time the voltage reaches its
positive or negative peak. The ZCR signal is then at twice the line voltage frequency. The
ZCR signal is available on the ZCR pin.
7.5 Period and line voltage measurement
The period module measures the period of the base frequency of the voltage channel and
checks if the voltage signal frequency is within the f
CLK
/2
17
to f
CLK
/2
15
band. To do this, the
LIN signal is produced, which is low when the line voltage is rising, and 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 the f
CLK
/4 reference signal. The LIN signal is available
on the status bit register (see Table 1 0).
If the counted number of pulses between two trailing edges of LIN is higher than 2
15
, or if
the counting is never stopped (no LIN trailing edge) this means that the base frequency is
lower than f
CLK
/2
17
Hz and a BFR (base frequency range) error flag is set.
Figure 15. ZCR signal