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Lesson 258 of 9363. Microcontroller Fundamentals and Bare-Metal CFree lesson

Managing ADC Channel Selection and Conversion Sequences

Program the multiplexer and sequence registers to scan multiple physical analog input pins for sequential data acquisition.

In this lesson, you will learn how to configure the Analog-to-Digital Converter (ADC) to automatically sweep through multiple physical input pins to gather a collection of sensor data without manual CPU intervention.

The Concept of the Multiplexer (MUX)

An ADC is usually a single hardware engine that converts a voltage into a digital value. However, most microcontrollers have many physical pins capable of sensing analog signals. The Multiplexer (MUX) acts like a high-speed switch that connects the ADC engine to one pin at a time. To read multiple sensors, you must tell the MUX which channel to "look at" before starting a conversion.

Implementing Conversion Sequences

Rather than changing the MUX setting in your code every time you want a reading, many microcontrollers provide a Sequence Register. This allows you to define a "scan list." For example, you might define a sequence that reads Channel 0, then Channel 2, then Channel 5.

Once you enable the sequence and trigger the ADC, the hardware handles the switching for you:

  1. It connects to the first pin in your list.
  2. It performs the conversion and stores the result in a data register.
  3. It automatically switches to the next pin in the sequence.

Practical Implementation

To manage this, you configure the sequence length and populate the registers (often called ADC_SQR or similar) with the desired channel identifiers. Here is a conceptual look at how you might set up a three-channel scan:

// Define the number of conversions in the sequence
ADC->SEQUENCE_LENGTH = 3; 

// Map sequence positions to physical channels
ADC->SEQ_REG_1 = 0x00; // First: Channel 0
ADC->SEQ_REG_2 = 0x02; // Second: Channel 2
ADC->SEQ_REG_3 = 0x05; // Third: Channel 5

// Trigger the sequence to start automatic scanning
ADC->CONTROL |= START_SEQUENCE_BIT;

By setting up a sequence, you reduce the workload on your CPU. Instead of writing code to manually switch channels and wait for each conversion to complete, you simply trigger the hardware once and retrieve the data from the results array once the sequence completes. This ensures precise timing between your samples and frees up your main loop for other tasks.

Key Takeaway: By defining a scan sequence in the ADC hardware registers, you allow the multiplexer to cycle through multiple analog pins automatically, offloading the timing and switching logic from your main code.