In this lesson, you will learn how to configure an Analog-to-Digital Converter (ADC) to automatically cycle through multiple input pins, allowing you to monitor several sensors without manually restarting the conversion process.
The Problem with Single-Channel Polling
Up to this point, you have performed single-channel data acquisition. If you wanted to read three different sensors (like a temperature sensor, a battery monitor, and a light sensor), you would normally trigger a conversion, wait for completion, read the data, switch the input multiplexer to the next channel, and repeat. This "stop-and-go" software approach wastes CPU cycles and introduces latency between readings.
The Automated Solution: Scan Sequences
Most modern microcontrollers feature a hardware-based "Scan Sequence" or "Sequence Generator." Instead of manually switching channels, you define a sequence list in a specialized register array. For example, you might program the ADC to scan Channel 0, then Channel 1, then Channel 2 in a single pass.
When you trigger the hardware start bit, the ADC performs the following autonomously:
- Selects the first channel in the sequence.
- Performs the Sample and Hold conversion.
- Automatically switches the internal multiplexer to the next programmed channel.
- Repeats until the end of the sequence is reached.
Implementation Concept
Think of the scan sequence as a "to-do list" for your ADC hardware. You don't need to intervene until the entire sequence is complete.
// Concept of setting up a scan sequence
ADC_Sequence_Config(SEQ_LENGTH_3); // Define we want to read 3 pins
ADC_Set_Sequence(0, CH_TEMP); // Position 1: Temperature Sensor
ADC_Set_Sequence(1, CH_V_BAT); // Position 2: Battery Voltage
ADC_Set_Sequence(2, CH_LIGHT); // Position 3: Light Sensor
ADC_Start_Scan(); // Hardware handles the switching and conversion of all 3
By leveraging this hardware automation, the CPU is free to perform other tasks, such as managing your PWM motor control or updating system timers, while the ADC silently populates your data registers. Once the scan is complete, an interrupt can signal the CPU to fetch the full array of fresh sensor values, ensuring all your data points are synchronized in time.
Key Takeaway: Automating scan sequences offloads channel multiplexing from the CPU to the ADC hardware, allowing for seamless, high-efficiency monitoring of multiple analog signals.