Samples will be written directly to RAM, and EasyDMA must be configured accordingly.
The address pointer for the EasyDMA channel is set in SAMPLE.PTR register. If the destination address set in SAMPLE.PTR is not pointing to the Data RAM region, an EasyDMA transfer may result in a HardFault or RAM corruption. See Memory for more information about the different memory regions.
The DMA transfer supports Stereo (left and right 16-bit samples) and Mono (left only) data transfer as configured in the OPERATION field of the MODE register. The samples are stored little endian.
| MODE.OPERATION | Bits per sample | Result stored per RAM word | Physical RAM allocated (32-bit words) | Result boundary indexes in RAM | Note |
|---|---|---|---|---|---|
| Stereo | 32 (2x16) | L+R | ceil(SAMPLE.MAXCNT/2) | R0=[31:16]; L0=[15:0] | Default |
| Mono | 16 | 2xL | ceil(SAMPLE.MAXCNT/2) | L1=[31:16]; L0=[15:0] |
The destination buffer in RAM consists of one block, the size of which is set in SAMPLE.MAXCNT register. Format is number of bytes used by the samples that will be stored in the memory. The physical RAM allocated is always:
(RAM allocation, in bytes) = SAMPLE.MAXCNT * 2;(but the mapping of the samples depends on MODE.OPERATION.
If OPERATION=Stereo, RAM will contain a succession of left and right samples.
If OPERATION=Mono, RAM will contain a succession of left only samples.
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.