ST AN4041 APPLICATION NOTE

AN4041

APWlink+ microphone USB demonstration board

based on the MP34DT01, MP45DT02, or MP34DB01

Introduction

The purpose of this application note is to give a brief description of the APWlink+
microphone USB demonstration board from STMicroelectronics. This board hosts the
microcontroller STM32F107RC and either the MP45DT02 or MP34DT01 (top-port digital
microphones) and the MP34DB01 (bottom-port digital microphone). Figure 1 shows the
capability of the board to host these microphones.

These microphones are analog-to-digital transducers, in other words, they are able to sense
sound pressure and convert this signal to a digital signal using the PDM technique. The
STM32 microcontroller decodes the PDM signal coming from the microphones and streams
the audio via the USB.

This document will provide a brief description of the software that decodes the PDM signal
and also information about the hardware as well as simple steps to use the board.

Figure 1. APWlink+ demonstration board

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Contents AN4041

Contents

1 Software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Appendix A Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Appendix B Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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AN4041 List of figures

List of figures

Figure 1. APWlink+ demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. DSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 3. Microphone USB demonstration board (block diagram) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 4. Windows message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Computer management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Recording properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 7. Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 8. Board layout - top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 9. Board layout - bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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Software description AN4041

1 Software description

The digital signal processing consists of the simple flow depicted in the figure below.

Figure 2. DSP

The PDM signal from the microphone is filtered and decimated in order to obtain a sound
signal at the required frequency and resolution. The loaded firmware implements a filter
pipeline that has been designed as two filtering-decimation stages.

Assuming an input frequency of 3.072 MHz and a desired output frequency of 48 kHz, the
filter stages are configured to implement a decimation factor of 64. The final gain of the
pipeline is therefore G
signal.

For further information, please refer to application note AN3998 available on www.st.com.

= 7.8125. As a result, the output of the filter pipeline is a 16-bit PCM

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