Analog-to-Digital Conversion (ADC)

Introduction

What Is an ADC?

An Analog-to-Digital Converter samples a continuous (analog) voltage and produces a discrete (digital) number. The TM4C123 microcontroller provides:

• Resolution: 12 bits (0-4095)
• Reference Voltage (Vref): Typically 3.3 V (can vary if externally configured)
• Multiple analog input channels (AN0-AN11) mapped to specific GPIO pins
• 4 Sample Sequencers (SS0-SS3) with different depths (SS3: 1 sample, SS2: 4, SS1: 4, SS0: 8)
• 2 ADC modules (ADC0, ADC1) sharing the same input pins.

ADC Input Channel Mapping

ADC Channel	AN0	AN1	AN2	AN3	AN4	AN5	AN6	AN7	AN8	AN9	AN10	AN11
Pin	PE3	PE2	PE1	PE0	PD3	PD2	PD1	PD0	PE5	PE4	PB4	PB5

Key Concepts

• Sampling: Capturing the analog value at a specific instant (sample rate must satisfy Nyquist criteria for varying signals)
• Quantization: Mapping the sampled voltage to one of 2^12 discrete codes
• Resolution (LSB size): Vref / 4096. For Vref = 3.3 V ⇒ ~0.000805 V per count
• Conversion Formula: Voltage = (Result * Vref) / 4095

Sequencers

Each Sample Sequencer:

• Collects one or more samples into a FIFO
• Can be triggered by: processor, timer, PWM, comparator, GPIO, always, etc.
• Is configured via:
  • EMUX (event trigger)
  • SSMUXn (channel selection)
  • SSCTLn (end/sample interrupt control)
  • IM (interrupt mask)
  • ACTSS (enable/disable)

Simplified Polling Flow

1. Enable clocks: ADC + GPIO (for analog pin)
2. Configure GPIO pin as analog input:
   • Disable digital (DEN)
   • Enable alternate function & analog mode (AFSEL/AMSEL)
3. Disable target sample sequencer
4. Set trigger source (processor for simplest)
5. Select input channel in SSMUXn
6. Configure SSCTLn (END0 | IE0)
7. Enable sample sequencer
8. Start conversion (PSSI)
9. Poll RIS or check FIFO not empty
10. Read SSFIFOx
11. Clear interrupt (ISC)
12. Convert to voltage

Interrupt-Driven Flow Differences

• Enable interrupt in ADC IM
• Enable NVIC interrupt line
• ISR reads FIFO, clears flag, processes data

Sample Voltage Conversion (C)

float voltage = (sample * 3.3f) / 4095.0f;

Choosing Polling vs Interrupt

• Polling: Simpler, CPU waits or loops
• Interrupt: Efficient for periodic or multi-channel sampling
• Timer-trigger + interrupt: Deterministic sampling rate

Mode	Pros	Cons	Use When
Polling	Simple, predictable order	CPU busy-wait	Single slow sensor
Interrupt	CPU free between samples	Slight overhead	Periodic sampling / display tasks
Timer + ADC	Stable sampling frequency	More setup	Signal analysis / filtering

Examples

Polling ADC

#include "TM4C123.h"
#include <stdio.h>

volatile unsigned int adc_value;

int main(void)
{

    /* Enable Clock to ADC0 and GPIO pins*/
    SYSCTL->RCGCGPIO |= (1 << 4); /* Enable Clock to GPIOE or PE3/AN0 */
    SYSCTL->RCGCADC |= (1 << 0);  /* AD0 clock enable*/

    /* initialize PE3 for AIN0 input  */
    GPIOE->AFSEL |= (1 << 3); /* enable alternate function */
    GPIOE->DEN &= ~(1 << 3);  /* disable digital function */
    GPIOE->AMSEL |= (1 << 3); /* enable analog function */

    /* initialize sample sequencer3 */
    ADC0->ACTSS &= ~(1 << 3);            /* disable SS3 during configuration */
    ADC0->EMUX &= ~0xF000;               /* software trigger conversion */
    ADC0->SSMUX3 = 0;                    /* get input from channel 0 */
    ADC0->SSCTL3 |= (1 << 1) | (1 << 2); /* take one sample at a time, set flag at 1st sample */
    ADC0->ACTSS |= (1 << 3);             /* enable ADC0 sequencer 3 */

    /*Iniitialize PF3 as a digital output pin */

    SYSCTL->RCGCGPIO |= 0x20; // turn on bus clock for GPIOF
    GPIOF->DIR |= 0x08;       // set GREEN pin as a digital output pin
    GPIOF->DEN |= 0x08;       // Enable PF3 pin as a digital pin

    while (1)
    {

        ADC0->PSSI |= (1 << 3); /* Enable SS3 conversion or start sampling data from AN0 */
        while ((ADC0->RIS & 8) == 0)
            ;                      /* Wait untill sample conversion completed*/
        adc_value = ADC0->SSFIFO3; /* read adc coversion result from SS3 FIFO*/
        ADC0->ISC = 8;             /* clear coversion clear flag bit*/

        if (adc_value >= 2048)
            GPIOF->DATA = 0x08; /* turn on green LED*/
        else if (adc_value < 2048)
            GPIOF->DATA = 0x00; /* turn off green LED*/
    }
}

Interrupt-Driven ADC

#include "TM4C123.h"
#include <stdio.h>

// Functions Declaration
void delayUs(int); // Delay in Micro Seconds
volatile unsigned int adc_value;

void ADC0SS3_Handler(void)
{
    adc_value = ADC0->SSFIFO3; /* read adc coversion result from SS3 FIFO*/

    if (adc_value >= 2048)
        GPIOF->DATA = 0x08; /* turn on green LED*/
    else if (adc_value < 2048)
        GPIOF->DATA = 0x00; /* turn off green LED*/
    ADC0->ISC = 8;          /* clear coversion clear flag bit*/
    ADC0->PSSI |= (1 << 3); /* Enable SS3 conversion or start sampling data from AN0 */
}

int main(void)
{

    /* Enable Clock to ADC0 and GPIO pins*/
    SYSCTL->RCGCGPIO |= (1 << 4); /* Enable Clock to GPIOE or PE3/AN0 */
    SYSCTL->RCGCADC |= (1 << 0);  /* AD0 clock enable*/

    /* initialize PE3 for AIN0 input  */
    GPIOE->AFSEL |= (1 << 3); /* enable alternate function */
    GPIOE->DEN &= ~(1 << 3);  /* disable digital function */
    GPIOE->AMSEL |= (1 << 3); /* enable analog function */

    /* initialize sample sequencer3 */
    ADC0->ACTSS &= ~(1 << 3);            /* disable SS3 during configuration */
    ADC0->EMUX &= ~0xF000;               /* software trigger conversion */
    ADC0->SSMUX3 = 0;                    /* get input from channel 0 */
    ADC0->SSCTL3 |= (1 << 1) | (1 << 2); /* take one sample at a time, set flag at 1st sample */
    ADC0->ACTSS |= (1 << 3);             /* enable ADC0 sequencer 3 */

    /*Iniitialize PF3 as a digital output pin */

    SYSCTL->RCGCGPIO |= 0x20; // turn on bus clock for GPIOF
    GPIOF->DIR |= 0x08;       // set GREEN pin as a digital output pin
    GPIOF->DEN |= 0x08;       // Enable PF3 pin as a digital pin

    /* Enable ADC Interrupt */
    ADC0->IM |= (1 << 3);        /* Unmask ADC0 sequence 3 interrupt*/
    NVIC->ISER[0] |= 0x00020000; /* enable IRQ17 for ADC0SS3*/
    ADC0->ACTSS |= (1 << 3);     /* enable ADC0 sequencer 3 */
    ADC0->PSSI |= (1 << 3);      /* Enable SS3 conversion or start sampling data from AN0 */

    while (1)
    {
    }
}

---

Tasks

Task 1: LED Control Based on ADC Value

Modify the polling example so that:

• RED LED ON only when ADC value > 1000
• GREEN LED ON only when ADC value <= 1000

Task 2: Display ADC Value and Voltage on LCD

Modify the interrupt-based example so both raw value and computed voltage are displayed on the LCD in the form:

ADC: 767
Voltage: 2.474