Datasheet

Theory of operation STPM10
20/53 Doc ID 17728 Rev 4
If the number of pulses counted between two trailing edges of LIN is lower than 2
13
, the
base frequency exceeds the limit (this means it is higher than f
CLK
/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), f
CLK
/4 is 1.048576 MHz. If the line frequency is 30 Hz, the counted f
CLK
/4
pulses between two LIN trailing edges are 34952, more than 2
15
(32768 pulses).
The BFR low frequency limit is then:
f
CLK
/2
17
= 4194304/131072 = 32 Hz
With the same clock frequency, if the line frequency is 130 Hz, the f
CLK
/4 pulses between
two LIN trailing edges are 8066, less than 2
13
(8192). The BFR high frequency limit is then:
f
CLK
/2
15
= 4194304/32768 = 128 Hz.
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, also gives information
about the presence of the line voltage within the meter.
When the BFR error is set, the computation of power is zero unless the FRS bit is set.
In fact, the effect of the BFR bit can be overridden by setting FRS configuration bit.
Figure 16. LIN and BFR signals