Datasheet

Theory of operation STPMC1
24/77 Doc ID 15728 Rev 7
is 300 Hz signal. The ZCR signal is available on the MOP pin only when the STPMC1 works
as a peripheral with the configuration bit APL
=0.
9.6 Period and line voltage measurement (status bits: LIN , BFR ,
LOW , BFF )
From voltage channels, a base frequency signal LIN is obtained, which is high when the line
voltage is rising and it is low when the line voltage is falling, so that,
LIN signal represents
the sign of dv/dt. With further elaboration, the
ZCR signal is also produced.
A period meter, which is counting up pulses of f
MCLK
/8 reference signal, measures the
period of voltage channel base frequency and checks if the voltage signal frequency is in the
band going from f
MCLK
/(2
18
- 2
3
) ≈ f
MCLK
/2
18
to f
MCLK
/2
16
.
This is done, phase by phase, by means of the signal
LIN , which trailing edge is extracted
and it is used to reset the period meter.
If the counted number of f
MCLK
/8 pulses between two trailing edges of LIN is higher than the
2
18
equivalent pulses or if the counting is never stopped (no more LIN trailing edge), the
base frequency exceeds the lower limit and an error flag
BFR is set. This error flag is part of
the 8-bit status byte of each phase (see
Tabl e 32 ).
Figure 9.
ZCR signal
Table 13. Good frequency ranges for different clock source values
f
XTAL
f
MCLK
freq. min. = f
MCLK
/2
18
freq. max. = f
MCLK
/2
16
4.194 MHz 8.389 MHz 32.0 Hz 128.0 Hz
4.915 MHz 9.830 MHz 37.5 Hz 150.0 Hz
8.192 MHz 8.192 MHz 31.3 Hz 125.0 Hz
9.830 MHz 9.830 MHz 37.5 Hz 150.0 Hz