STPM01FTR datasheet(20/60 Pages) STMICROELECTRONICS

20 / 60 page

Theory of operation

STPM01

20/60

Doc ID 10853 Rev 8

8.5

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 fCLK/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 fCLK /4 reference signal. The LIN signal is available

on the status bit register (see Table 15).

If the counted number of pulses between two trailing edges of LIN is higher than 215, or if

the counting is never stopped (no LIN trailing edge) this means that the base frequency is

lower than fCLK/2

17 Hz and a BFR (base frequency range) error flag is set.

If the number of pulses counted between two trailing edges of LIN is lower than 213, the

base frequency exceeds the limit (means it is higher than fCLK/2

15. In this case, the error

must be repeated three consecutive times in order to set the BFR error flag.

For example, with a 4.194304 MHz oscillator frequency and MDIV bit clear (or 8.192 MHz

with MDIV set), fCLK/4 is 1048.576 MHz. If the line frequency is 30 Hz, the counted fCLK/4

pulses between two LIN trailing edges are 34952, more than 215 (32768 pulses). The BFR

low frequency limit is then:

fCLK/2

17 = 4194304/131072 = 32 Hz.

With the same clock frequency, if the line frequency is 130Hz, the fCLK/4 pulses between two

LIN trailing edges are 8066, more than 213 (8192). The BFR high frequency limit is then:

fCLK/2

15 = 4194304/32768 = 128 Hz.

When the line frequency re-enters the nominal band, the BFR flag is automatically reset.

This BFR error flag is also assembled as part of the 8-bit status register (see Table 15).

Figure 15.

ZCR signal

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