49.6.2 Pre-Filter

The pre-filtering circuitry is implemented as a SINC filter that can be reinforced by a predefined audio filter (see Audio Filter). When the audio filter is not enabled (PDMC_MR.SINC_OSR>0), the application software can perform a software processing on the output of the SINC filter to get the required spectral response of the overall filter.

The SINC filter has a programmable order (1 to 5) and OSR (8, 16, 32, 64, 128 and 256).

The oversampling rate of the SINC filter cannot be configured if the audio filter is enabled (PDMC_MR.SINC_OSR=0); then, the OSR results only from the configuration of PDMC_MR.OSR.

Note: For example, when the application requires a 32 Ksps sampling frequency, the OSR can be set to 64 and thus the GCLK frequency must be 32K x 64 x16 = 32.768 MHz. The microphone is driven by 2.048 MHz. For the same GCLK frequency, an OSR of 128 provides a 16 Ksps audio stream. Refer to the microphone reference manual for the maximum frequency on PDMC_CLK.

The SINC filter is configured by the following fields:

  • PDMC_MR.SINCORDER defines the order of the SINC filter from 1 to 5.
  • PDMC_MR.SINC_OSR defines the OSR of the SINC filter (when SINC_OSR is greater than 0).
  • If PDMC_MR.SINC_OSR=0, the audio filter is enabled and the OSR of the SINC filter cannot be configured. Thus the overall (SINC + audio filter) oversampling rate is configured in PDMC_MR.OSR.