STPM10 datasheet(18/51 Pages) STMICROELECTRONICS

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Theory of operation

STPM10

18/51

Doc ID 17728 Rev 3

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 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 10).

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 (this 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 1.048576 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

Figure 16.

LIN and BFR signals

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