Question

How does a microcontroller, which operates exclusively with discrete digital values (1s and 0s), measure a continuous, real-world analog signal like the temperature from a sensor?

Answer 1

The bridge between the analog real world and the digital world of a microcontroller is a crucial mixed-signal circuit called an Analog-to-Digital Converter (ADC). Virtually all modern microcontrollers have one or more ADCs built directly into the chip as a key peripheral.

The process involves three main concepts: Sampling, Quantization, and Encoding.

1. The Fundamental Problem: Analog vs. Digital

• Analog World: Physical quantities like temperature, pressure, light intensity, and sound are continuous. A temperature sensor (e.g., an LM35) outputs a voltage that can be any value within its range (e.g., 0.25V, 0.251V, 0.2511V...).
• Digital World: A microcontroller's CPU can only understand discrete values represented by binary bits (1s and 0s). It cannot process an infinitely variable analog signal directly.

2. The Solution: The Analog-to-Digital Converter (ADC)

The ADC's job is to convert the analog voltage from the sensor into a digital number that the microcontroller can read and process. Here’s how it works:

Step 1: Sampling
The ADC doesn't look at the analog signal continuously. Instead, it takes instantaneous "snapshots" of the voltage at regular, very fast intervals. This is called sampling. For a slow-changing signal like temperature, this is more than adequate to get an accurate reading.

Step 2: Quantization (The Core of the Conversion)
This is the most important step. The ADC maps the sampled analog voltage to a discrete integer value. To do this, it uses two key parameters:

• Reference Voltage (Vref): This is the maximum voltage the ADC can measure. The ADC divides this voltage range into a number of discrete steps.
• Resolution (in bits): This determines how many steps the voltage range is divided into. An N-bit ADC has 2^N steps.
  • An 8-bit ADC has 2⁸ = 256 steps (0 to 255).
  • A 10-bit ADC (common in Arduino/AVR) has 2¹⁰ = 1024 steps (0 to 1023).
  • A 12-bit ADC (common in STM32) has 2¹² = 4096 steps (0 to 4095).

The ADC compares the input analog voltage to these steps and finds the closest one.

Example:
Imagine a common microcontroller (like an Arduino Uno) with a 10-bit ADC and a Reference Voltage (Vref) of 5V.

• The ADC can represent the input voltage as a digital number between 0 and 1023.
• The smallest voltage change it can detect (the size of one step) is 5V / 1024 steps ≈ 4.88 mV/step.
• If the temperature sensor outputs 2.5V, the ADC will quantize it to the digital value:
  Digital Value = (Input Voltage / Vref) × (Number of Steps)
Digital Value = (2.5V / 5.0V) × 1024 = 512

Step 3: Encoding
The resulting integer (512 in our example) is then encoded into its binary representation (1000000000 in binary) and stored in a special memory location inside the microcontroller called an ADC Data Register.

3. The Microcontroller Application: Putting It All Together

Now the microcontroller's software takes over:

1. Configuration: The programmer writes code to configure the ADC peripheral—selecting the input pin, setting the reference voltage, and choosing the conversion speed.
2. Start Conversion: The code sends a command to the ADC to start a conversion.
3. Wait & Read: The ADC hardware performs the conversion (which takes a few microseconds). The microcontroller's code then reads the final binary value from the ADC Data Register.
4. Process the Data: The microcontroller now has the digital number (e.g., 512). This number is meaningless on its own. The code must use a formula to convert it back into a real-world unit. Using our example:
   Voltage = (Digital Value / 1024) × Vref
Voltage = (512 / 1024) × 5V = 2.5V
   If the sensor's datasheet says it produces 10mV per degree Celsius, the final calculation is:
   Temperature = Voltage / 0.010 V/°C = 2.5V / 0.010 = 250°C (This is just an example formula).

In summary: The ADC acts as a translator. It takes a continuous analog voltage, measures it against a known reference, assigns it the closest discrete digital number based on its resolution, and hands that number to the microcontroller's CPU. The CPU then uses software to turn that raw number into a meaningful measurement like temperature, allowing a purely digital device to sense and react to the analog world.