Microphone

The microphone part we will be using is the I2S MEMS Microphone Breakout by Adafruit. In the following subsections, we will explain the important inputs/outputs of a MEMS microphone, the I2S input protocol for the microphone we will be using, and what is meant by "breakout".

Overview of MEMS microphone pins

For portable devices, digital MEMS microphone are the popular choice for audio capture. They are small, cheap, and easy to integrate into one's desired application. MEMS microphone usually have the following inputs:

• VDD: (usually) 3.3V to power the device.

• GND: ground.

• CLK: "clock" signal which is directly related to the sampling frequency. Typically the sampling frequency $f_s = f_{CLK}/64$, or in other words the input clock is $64 \times$ the desired audio sampling frequency.
• SEL: "select" signal to specify whether the microphone data should be responsible for the left or right channel data. For this reason, SEL can also be called LR on datasheets. Typically, SEL=0 for the left channel and SEL=1 for the right channel.

A MEMS microphone typically returns a PDM (Pulse-Density Modulation) signal. This is essentially a 1-bit oversampled (usually $64 \times$ normal audio sampling frequencies) signal that requires downsampling and filtering in order to obtain a multi-bit PCM (Pulse-Code Modulation) signal. PCM is the format typically used for storing and processing audio and the format we will want to use in our applications. You can read more about PDM and PCM here and here. Luckily, we do not need to perform this conversion if we use a MEMS microphone that outputs an I2S signal. These microphones contain a decimator and a low-pass filter in its circuitry and output the multi-bit audio in a serial manner via the I2S format. However, this convenience requires us to provide at least one additional input to an I2S MEMS microphone:

• WS: "word select" signal for specifying whether we expect the left (WS=0) or right channel (WS=1) data. Typically, the frequency of WS is $f_{WS} = f_{CLK}/64$, thus requiring the two signals (WS and CLK) to be synchronized.

As I2S microphones perform the downsampling and low-pass filtering internally to produce the multi-bit audio, we need a "smart" way of transmitting this audio rather than having multiple output lines for each bit. A separate line for each bit would require up to 24 output lines! The WS input is our solution as it denotes:

• which channel's data we are expecting (left or right);

• the start of a particular channel's transfer, typically on the next rising edge of the CLK signal after WS has changed values (which is why it is important that CLK and WS are properly synchronized).

With one input signal, we replace the need for several output lines! Both the PDM and the I2S MEMS microphones output a 1-bit signal, often called DOUT or DATA on datasheets. Moreover, it is common for left and right channel microphones to share the same DATA line to further reduce the number of connections.

Figure: Adafruit I2S MEMS microphone pins, p. 9 of datasheet.

I2S timing diagram example

Let's look at an example timing diagram from the IS2 microphone we will be using.

Figure: I2S MEMS microphone output timing diagram. The output data format is I2S, 24 bit, 2's compliment, MSB first. p. 7 of datasheet.

From the figure above, we can make several interesting observations:

1. After WS switches to LOW/0 for the left channel, we receive the first bit of information on the DATA line from the microphone whose SEL=0. The same is true for when WS switches to HIGH/1 for the right channel and the microphone whose SEL=1.

2. Each new bit is received at a rising edge and held for an entire period of CLK.

3. For this microphone: the first 18 bits after a rising or falling edge of the WS signal corresponds to actual audio data, starting with the Most-Significant Bit (MSB) and finishing with the Least-Significant Bit (LSB).

4. For this microphone: bits 19-24 are set to 0 so our data precision is essentially 18 bits. Nonetheless, this zero-padding is required as the output format chosen by the manufacturer is: I2S, 24 bit, 2's compliment, MSB first (p. 7 of datasheet).

5. For this microphone: bits 25-32 are set to tri-state, which is a high impedance mode that essentially removes an output port from the circuit in order to avoid a short circuit. As we may share the DATA line between the left and right channel, this is desired when one microphone is done transmitting data and while the other microphone is transmitting. See here for more information on tri-state.

I2S wiring example

Now that we are familiar with the various inputs and output of a MEMS microphone, let's see how we would wire up an I2S MEMS microphone, in particular the one we will be using.

Figure: I2S MEMS microphone wiring for stereo use. Note that in our exercises we will be using a mono, i.e. one channel, setup. p. 7 of datasheet.

From the figure above, we can observe how two I2S microphones would be connected for stereo use. Some important observations that can be made:

• The component called "IS2 Master" would be our microcontroller. The terms "master" and "slave" are quite common in electronics to describe the device which acts as the controller and the devices(s) that are being controlled. See here for more information on the terminology.

• The DATA lines for the two microphones are connected to each other and are supplied as a single input to the I2S Master.

• The SEL input for each microphone is set differently: SEL=VDD for the top microphone and SEL=GND for the bottom microphone. This is absolutely essential if two microphones will be sharing the same DATA line!

• The two microphones use the same BCLK (aka CLK) and WS signal. This is also necessary for microphones using the same DATA line for synchronization purposes.

Adafruit breakout

From the figure above, we can observe that a MEMS microphone requires several additional components (capacitors and resistors) in addition to the multiple inter-connecting wires. We could use a breadboard to wire up all the necessary components, but this is far from convenient when we start to incorporate more parts.

To simplify this task of wiring, it is common to use breakout boards. With a breakout board, we can bundle all the necessary components and simply provide connections for the signals/ports that need to interact with our microcontroller. In the case of our microphone, all the components (microphone, resistors, capacitors) can be soldered on a compact board and convenient access can be given to the following signals:

1. VDD and GND: provided by the microcontroller to power the microphone.

2. WS and BCLK: generated by the microcontroller for the I2S transfer.

3. SEL: wired by the user to either VDD or GND to configure the microphone appropriately.

4. DIN: input to the microcontroller from the microphone(s).

It is possible to design your own breakout boards using CAD tools for PCBs (Printed Circuit Boards). But for popular components like microphones, it is easy to find breakout boards that have already been designed. Adafruit is a great place to find such boards and other cool electronics for personal projects, along with very well explained user guides. We will be using Adafruit's I2S MEMS Microphone Breakout in our exercises.