MAX98357A

The IC on this breakout board contains a 3W class D amplifier for driving things like speakers. Unlike other common hobbyist class D amplifiers, this breakout uses an I2S interface rather than a PWM or Analog signal. The board also has several configurable gain settings and a low power shutdown mode.

Link to Buy: https://www.adafruit.com/products/3006

I2S

I2S is a communication bus standard that is very popular for audio devices.

The bus uses three lines: -Serial Data: for audio data -Word Select (WS): selects whether the audio data is for the left or right channel -Clock

The mbed is I2S capable and can operate in either master or slave mode. The also has both a TX and RX capabilities that can be implemented as different instances of the I2S class. This may be useful if the user has a system with both an ADC and DAC as the mbed can interface easily with both. The library written by Giles Barton-Owen is linked below with example code.

Getting the MAX98357A to work with the mbed

The breakout board contains 7 pins that plug into the breadboard and two that go to a speaker. Starting from the left the LRC pin needs to connect to the

Datasheet: https://cdn-shop.adafruit.com/product-files/3006/MAX98357A-MAX98357B.pdf

mbed <-> MAX

• p5(Serial) <-> Din
• p6(WS) <-> LRC
• p7(clock) <-> BCLK
• GND <-> GND
• Vout or Vu<-> Vin

The gain pin can be configured in five different configurations as follows.

Example Code

This code generates a 100Hz ramp wave test tone using the I2S library linked below. There are a few things to note about the library before using it the the breakout board. In the beginning of the cpp file there are a set of definitions that correspond to default modes for the mbed. Some of these must be changed. The mbed should be configured as the Master. It does not matter if the i2s object is configured in stereo or mono mode. If set to stereo the breakout board just averages the left and right channels. If the mbed is receiving data then the i2s.attach() function can be used for an FIFO interrupt.

#include "mbed.h"
#include "I2S.h"

I2S i2s(I2S_TRANSMIT, p5, p6, p7);
Ticker timer;

int i = 0;
int fs = 32000;
int freq = 100;

int buff[1];
int bufflen = 1;

void isr()
{
    buff[0] = i * (fs/freq);
    i2s.write(buff, bufflen);
    i ++;
    if(i == (fs/freq))
    {
        i = 0; 
    } 
}

int main() {
  i2s.stereomono(I2S_MONO);
  i2s.masterslave(I2S_MASTER);
  i2s.start();
  timer.attach(&isr, 1/32000.0);
  while(1)
  {}
}