DocID024597 Rev 5 713/1830
RM0351 Digital filter for sigma delta modulators (DFSDM)
756
Figure 168. First conversion for Manchester coding (Manchester synchronization)
External serial clock frequency measurement
The measuring of a channel serial clock input frequency provides a real data rate from an
external modulator, which is important for application purposes.
An external serial clock input frequency can be measured by a timer counting DFSDM
clocks (f
DFSDMCLK
) during one conversion duration. The counting starts at the first input data
clock after a conversion trigger (regular or injected) and finishes by last input data clock
before conversion ends (end of conversion flag is set). Each conversion duration (time
between first serial sample and last serial sample) is updated in counter CNVCNT[27:0] in
register DFSDM_FLTxCNVTIMR when the conversion finishes (JEOCF=1 or REOCF=1).
The user can then compute the data rate according to the digital filter settings (FORD,
FOSR, IOSR, FAST). The external serial frequency measurement is stopped only if the filter
is bypassed (FOSR=0, only integrator is active, CNVCNT[27:0]=0 in
DFSDM_FLTxCNVTIMR register).
In case of parallel data input (Section 24.4.6: Parallel data inputs) the measured frequency
is the average input data rate during one conversion.
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