Product Specs

Table Of Contents
43 PDM Pulse density modulation interface
Page
439
For a given value of SAMPLE.MAXCNT, the buffer in RAM can contain half the stereo sampling time as
compared to the mono sampling time.
The PDM acquisition can be started by the START task, after the SAMPLE.PTR and SAMPLE.MAXCNT
registers have been written. When starting the module, it will take some time for the filters to start outputting
valid data. Transients from the PDM microphone itself may also occur. The first few samples (typically
around 50) might hence contain invalid values or transients. It is therefore advised to discard the first few
samples after a PDM start.
As soon as the STARTED event is received, the firmware can write the next SAMPLE.PTR value (this
register is double-buffered), to ensure continuous operation.
When the buffer in RAM is filled with samples, an END event is triggered. The firmware can start processing
the data in the buffer. Meanwhile, the PDM module starts acquiring data into the new buffer pointed to by
SAMPLE.PTR, and sends a new STARTED event, so that the firmware can update SAMPLE.PTR to the
next buffer address.
43.5 Hardware example
Connect the microphone clock to CLK, and data to DIN.
CLK
DIN
Figure 125: Example of a single PDM microphone, wired as left
CLK
DIN
Figure 126: Example of a single PDM microphone, wired as right
Note that in a single-microphone (mono) configuration, depending on the microphone’s implementation,
either the left or the right channel (sampled at falling or rising CLK edge respectively) will contain reliable
data. If two microphones are used, one of them has to be set as left, the other as right (L/R pin tied high or
to GND on the respective microphone). It is strongly recommended to use two microphones of exactly the
same brand and type so that their timings in left and right operation match.
CLK
DIN
Figure 127: Example of two PDM microphones
43.6 Pin configuration
The CLK and DIN signals associated to the PDM module are mapped to physical pins according to the
configuration specified in the PSEL.CLK and PSEL.DIN registers respectively. If the CONNECT field in
any PSEL register is set to Disconnected, the associated PDM module signal will not be connected to the
required physical pins, and will not operate properly.
Vdd
CLK
L/R
DATA
nRFxxxxx
CLK
DIN
Vdd
CLK
L/R
DATA
nRFxxxxx
CLK
DIN
Vdd
Vdd
CLK
L/R
DATA
CLK
L/R
DATA
nRFxxxxx
CLK
DIN