ADC (analog-to-digital read)

Embedded → Analog & Serial Peripherals (HAL)

The real world is analog — your MCU only understands bits

A potentiometer, thermistor, or mic produces a continuously-varying voltage, not a clean 0/1 a GPIO register can read. An ADC samples that voltage and reports an integer — the ADC count — representing where it fell within the measurable range. How faithfully that integer represents the real voltage comes down to two numbers: resolution and reference.

Resolution and reference are the whole story

Term	Meaning
Resolution	distinct output codes — 12-bit = 2^12 = 4096 counts, 10-bit = 1024. More bits = finer voltage steps
Reference (Vref)	voltage mapping to the max count — a reading of 0 = 0V, max count = Vref, everything between linearly interpolated. A noisy/drifting Vref scales every reading by the same error

Vin = ADC_count × (Vref / (2^N − 1))

// 12-bit ADC, Vref = 3.3V, count = 2048:
Vin = 2048 × (3.3 / 4095) ≈ 1.65V

The reading you get back is never the exact analog voltage — it's the nearest of 2^N discrete steps, an error called quantization error. A 12-bit ADC at 3.3V has a step size of ~0.8mV; a 10-bit ADC at the same Vref steps in ~3.2mV increments. This is the direct, unavoidable cost of trading resolution for a cheaper/faster converter.

Sampling time — the ADC needs a moment to charge

Most ADCs charge a small sample-and-hold capacitor to the input voltage before converting. If the source can't supply enough current to charge it within the sampling window, the reading comes out low/lagging — a classic high-impedance-source (e.g. a thermistor divider with large resistors) symptom. Fix with a longer sample-time setting or a buffer op-amp — not fixable in software after the fact.

Reading it — STM32 HAL, single-channel polling

ADC_HandleTypeDef hadc1;

HAL_ADC_Start(&hadc1);
HAL_ADC_PollForConversion(&hadc1, HAL_MAX_DELAY);
uint32_t raw = HAL_ADC_GetValue(&hadc1);   // 0 .. 4095 on a 12-bit ADC

float vref  = 3.3f;
float vin   = raw * (vref / 4095.0f);

printf("raw=%lu  vin=%.3fV\r\n", raw, vin);

Polling is fine for a one-off reading, but it blocks the CPU until the conversion finishes. Sampling a signal continuously at a fixed rate — a potentiometer at 1kHz, an audio input far faster — is exactly the case the next lesson (DMA) exists to solve: let the ADC free-run and have a peripheral move each finished sample into a buffer with zero CPU involvement per sample.

Try it — resolution, reference, and quantization error

Drag Vin and switch resolution/Vref — watch the ADC count and the quantization error it introduces:

Resolution

Vref

Vin: 1.650V

ADC_count = round(1.650 / 3.3 × 4095) = 2048
Recovered Vin = 2048 × (3.3 / 4095) = 1.6504V
Quantization error = 0.403mV (step size ≈ 0.806mV)

Practice

Open Wokwi and wire a potentiometer to an ADC input. Print the raw count and the computed voltage over UART every 200ms while you turn the pot end to end. Then swap the 12-bit resolution for the lowest resolution your target supports and confirm the reported voltage visibly gets "steppier" on the serial plotter.

Completion saves automatically after you read through to the end.

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