Getting started with the digital MEMS microphones expansion board

Hardware description

UM1900

In this scenario, microphone acquisition functions thus: a precise clock signal is generated by I²S peripheral while SPI is configured in slave mode and is fed by the same timing signal generated by I²S. This clock is then halved by a timer and input to the microphones: the

SPI and I²S peripherals operate at twice the microphone frequency, so that they can read data on both the rising and falling edge of the microphone clock, thus reading the bits of two microphones each. A software demuxing step is required to separate the signal from

the two microphones and allows further processing like PDM to PCM conversion.

Figure 4:

"General acquisition strategy block diagram"

shows a simplified diagram of the acquisition

process described in this paragraph. For further information about MEMS microphone and

PDM to PCM decimation, please refer www.st.com and AN3998.

For single microphone acquisition, the correct microphone timer is generated directly by I²S and one single microphone data line is read by the same peripheral. For an example application of microphone acquisition, decimation and streaming based on X-NUCLEO-

CCA02M1 board, please refer the board firmware package (order code: X-CUBE-

MEMSMIC1).

Figure 4: General acquisition strategy block diagram

3.3

3.3.1

Solder bridges configurations

Various board configurations are possible, depending on the use cases. MEMS microphones can be connected (or disconnected) to morpho pins, and thus to MCU peripherals, using ad hoc solder bridges. Clock routing can als be changed according to specific needs. This section helps the user understand the role of each solder bridge and analyzes some of the more common use cases.

Solder bridges roles

In

Table 1: "Solder bridge descriptions"

, the solder bridge dunctions are summarized with

respect to the audio acquisition strategies described in the previous section.

Table 1: Solder bridge descriptions

Function Solder bridge

Connects USB D- pin to OTG_FS_DM pin on the MCU

Connects USB D+ pin to OTG_FS_DP pin on the MCU

Routes onboard oscillator output to OSC_IN MCU pin

Connect microphone clock to MCU timer output channel

Routes I²S clock to SPI clock

SB1

SB2

SB6

SB7

SB8

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