AN4041

Hardware description

Doc ID 022706 Rev 2

7/14

of the output data on the high level of the clock or on the low level, refer to the following

table.

On this board the microphones are used in stereo configuration, so the two digital outputs

are shorted (Mic_DATA12, refer to the schematic) and the two MEMS LR pins must be set

one to GND and the other one to VDD. On the board, the resistors R1 and R13 have already

set the LR pin of the microphones. In order to change the MEMS polarity, the user must

unsolder these resistors and manually set the LR pin using the solder jumper J1 and J5.

Note that the PCB section mounting the MEMS microphones is detachable. This option has

been implemented in order to let the user put the microphones in the desired position (i.e.

for display applications in which the microphones are placed at the edges of the display).

Microcontroller

The STM32F107RC microcontroller simultaneously works as a PDM interface and also

supports the USB audio streaming. The board hosts two external clocks, the 12.288 MHz

and 8 MHz. The first one serves to clock an internal timer for generating both MIC_CK and

SPI_CK. The 8 MHz is the microcontroller’s external clock connected to the OSC_IN

OSC_OUT pins.

The STM32 samples the microphone’s output data by using the synchronous serial port

(SPI1). The microphone signal MIC_DATA12 is a stereophonic signal and the SPI must be

able to sample both channels. Since MIC_DATA12 is synchronous with its clock, SPI_CK

must be doubled. Summarizing, the timer generates:

-MIC_CK = 3.072 MHz

-SPI_CK = 6.144 MHz

Finally, the USB interface allows streaming the audio on the USB peripheral making this

demonstration board compatible with any laptop or desktop. This streaming capability allows

using any acoustic tool for the sound acquisition done by STMicroelectronics’ MEMS

microphones. The demonstration board also has the possibility to manage the

allows the user to test the signal using audio testing equipment such as Audio Precision. All

these signals can be monitored on the APWLink connector.

LEDs D1-D5

The status of the board is indicated by the following LEDs:

●

D1 on: USB enumeration successful

●

D2 on: Firmware correctly downloaded

●

D3 off: Firmware correctly downloaded

●

D4: not soldered

●

D5 on: Board supplied

Table 2.

L/R channel selection

L/R

Clock low

Clock high

GND

Data valid

High impedance

VDD

High impedance

Data valid