Reading the Microphone Signal

Overview

The microphone on the BOOSTXL-EDUMKII is connected to PE5 (AIN8) on the TM4C1294XL LaunchPad. This analog signal represents the audio amplitude (AC waveform), which you can sample using the ADC0 peripheral.

In this section, you will learn how to:

• Configure ADC0 for single-ended conversion
• Read the 12-bit digital value from the microphone
• Estimate the sound level (volume) using a simple RMS (Root Mean Square) method

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Hardware Setup

Signal	Pin	Function	Peripheral
Mic Output	PE5	AIN8	ADC0, Sequencer 3

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Step-by-Step ADC Configuration

1. Enable Peripherals

Enable both the ADC0 and GPIOE modules before using them:

SysCtlPeripheralEnable(SYSCTL_PERIPH_ADC0);
SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOE);

2. Configure the Pin as Analog Input

PE5 must be configured as an ADC input instead of digital GPIO:

GPIOPinTypeADC(GPIO_PORTE_BASE, GPIO_PIN_5);

3. Configure ADC Sequencer

We use Sequencer 3 because it’s designed for single-sample conversions:

ADCSequenceConfigure(ADC0_BASE, 3, ADC_TRIGGER_PROCESSOR, 0);
ADCSequenceStepConfigure(ADC0_BASE, 3, 0, ADC_CTL_CH8 | ADC_CTL_IE | ADC_CTL_END);
ADCSequenceEnable(ADC0_BASE, 3);
ADCIntClear(ADC0_BASE, 3);

4. Read a Single Sample

Trigger the ADC conversion, wait until it completes, then read the 12-bit value (0–4095):

uint16_t Mic_Read(void)
{
    uint32_t value;
    ADCProcessorTrigger(ADC0_BASE, 3);
    while (!ADCIntStatus(ADC0_BASE, 3, false)); // wait until ready
    ADCIntClear(ADC0_BASE, 3);
    ADCSequenceDataGet(ADC0_BASE, 3, &value);
    return (uint16_t)value;
}

5. Compute Signal Level (RMS Method)

The microphone output fluctuates around a DC midpoint (~0.5 V).
To measure loudness, we take multiple samples, remove the DC offset, and compute the RMS value:

float Mic_Level(void)
{
    const int N = 128;   // number of samples
    float sum = 0.0f;

    for (int i = 0; i < N; i++) {
        uint16_t s = Mic_Read();          // raw ADC value
        float v = (float)s / 4095.0f;     // normalize to 0–1
        float ac = v - 0.5f;              // remove DC offset
        sum += ac * ac;                   // accumulate squared amplitude
    }

    float rms = sqrtf(sum / N);           // root mean square
    return rms;                           // 0.0 (silent) → ~0.5 (loud)
}

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Tips

Practical Notes

• Sample rate: Reading continuously in a loop gives roughly a few kHz sampling rate — enough for average volume detection.
• Filtering: The microphone output is already AC-coupled on the BoosterPack, so DC removal in software (subtracting 0.5) works well.
• Performance: Avoid floating-point math inside interrupts; use it in the main loop or background tasks.

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✅ Summary

Function	Purpose
Mic_Init()	Enables ADC and configures PE5 as input
Mic_Read()	Triggers one ADC conversion and returns a 12-bit value
Mic_Level()	Estimates sound level using RMS of 128 samples

Once you have the normalized sound level (0.0–1.0), you can easily map it to a bar height, LED brightness, or any other visual feedback in your system.