📚 CMPE2250: Lesson — Analog-to-Digital Conversion on STM32G0

🎯 Learning Objectives

By the end of this lesson, students will be able to:

• Explain what an ADC does and why embedded systems need it.

• Describe the STM32G0 ADC architecture: resolution, sampling time, channels, and clocking.

• Configure the ADC in single‑conversion and continuous modes.

• Trigger conversions using software and timers.

• Read conversion results using polling, interrupts, and possibly DMA.

• Validate ADC behavior using a multimeter, waveform generator, and logic analyzer (for triggers).

1️⃣ Why ADC Matters in Embedded Systems

Embedded systems interact with the real world, which is inherently analog. Sensors such as:

• Temperature (NTC, thermistors)

• Light (LDR, photodiodes)

• Position (potentiometers)

• Pressure, force, sound

All output voltages that must be digitized before the MCU can process them.

The ADC bridges this gap by converting an input voltage $V_{in}$ into a digital number:

\[\mathrm{Digital\ Value}=\frac{V_{in}}{V_{ref}}\times (2^N-1)\]

Where N is the resolution (12 bits on STM32G0).

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2️⃣ STM32G0 ADC Architecture (G0B1RE‑specific)

Key Features

• 12‑bit ADC

• Up to 19 channels (GPIO + internal: Vrefint, temperature sensor)

• Configurable sampling time (affects accuracy vs. speed)

• Hardware oversampling

• Single, continuous, scan, and discontinuous modes

• DMA support

• Hardware triggers (TIM1, TIM3, TIM6, EXTI, etc.)

Conceptual Diagram (Mermaid)

flowchart LR
    A[Analog Input Pin] --> B[Sample & Hold]
    B --> C[SAR Conversion Engine]
    C --> D[12-bit Result Register ADC_DR]
    D -->|optional| E[DMA Controller]
    D -->|optional| F[Interrupt Handler]

3️⃣ Clocking and Calibration

ADC Clock Sources

• System clock (SYSCLK)

• PLL outputs

• Asynchronous clock

For beginners, use the synchronous clock derived from the system clock.

Calibration

The STM32G0 ADC must be calibrated before first use:

• Enable ADC clock.

• Ensure ADC is disabled.

• Start calibration.

• Wait for calibration to finish.

• Enable ADC.

This is a common pitfall — conversions silently fail if calibration is skipped.

4️⃣ Registers We Must Know

Register	Purpose
ADC_ISR	Status flags: EOC (end of conversion), EOS (end of sequence), OVR, ADRDY
ADC_IER	Interrupt enables for EOC, EOS, OVR, ADRDY
ADC_CR	Main control: ADEN, ADDIS, ADSTART, ADCAL
ADC_CFGR1	Resolution, data alignment, continuous mode, DMA enable, trigger config
ADC_SMPR	Sampling time selection
ADC_CHSELR	Channel selection (bitmask of active channels)
ADC_DR	Data register containing the 12‑bit conversion result

5️⃣ Minimal Working Example (Single Conversion, Polling)

• Minimal bare-metal flow to read a single channel (e.g., PA0 = ADC_IN0):

    // Enable clock
    RCC->APBENR2 |= RCC_APBENR2_ADCEN;

    // Calibration
    ADC1->CR &= ~ADC_CR_ADEN;
    ADC1->CR |= ADC_CR_ADCAL;
    while (ADC1->CR & ADC_CR_ADCAL);

    // Enable ADC
    ADC1->CR |= ADC_CR_ADEN;
    while (!(ADC1->ISR & ADC_ISR_ADRDY));

    // Select channel (e.g., PA0 = ADC_IN0)
    ADC1->CHSELR = ADC_CHSELR_CHSEL0;

    // Start conversion
    ADC1->CR |= ADC_CR_ADSTART;

    // Wait for end of conversion
    while (!(ADC1->ISR & ADC_ISR_EOC));

    // Read result
    uint16_t value = ADC1->DR;

6️⃣ Adding Continuous Mode + DMA (for smoother signals)

Why DMA?

• Avoids CPU polling

• Enables high‑rate sampling

• Perfect for labs involving waveform capture or filtering

Workflow

• Configure ADC in continuous mode.

• Enable DMA in ADC_CFGR1.

• Configure DMA channel (peripheral → memory).

• Start ADC.

• DMA fills a circular buffer.

7️⃣ Hardware Triggering (Timer‑Driven Sampling)

ADC sampling frequency is not “automatic” — it must be controlled.

Example: TIM3 → ADC Trigger

• Configure TIM3 to generate TRGO at a fixed rate.

• Set ADC external trigger source to TIM3_TRGO.

• ADC samples at exactly the timer frequency. This is a great lab for teaching deterministic sampling.

8️⃣ Validation Strategy

Students should verify:

• ADC reading matches expected voltage Use a multimeter on the analog pin.

• Sampling frequency is correct

• Use a scope on the timer trigger pin.

• DMA buffer updates continuously

• Print buffer indices or use a debugger watch window.

• Noise behavior

• Show how oversampling reduces jitter.