ADC Application Example Usage Instructions

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Introduction

This example demonstrates how to use the RA8 series MCU ADC (Analog-to-Digital Converter) on the Titan Board, combined with the RT-Thread ADC driver framework for analog signal acquisition and processing.

Main functionalities include:

• Initializing the ADC hardware module

• Configuring ADC channels, sampling time, and trigger mode

• Reading analog signals through the RT-Thread ADC driver interface

• Supporting single-shot sampling, continuous sampling, and hardware-triggered sampling

RA8P1 ADC Features

1. ADC Overview

ADC (Analog-to-Digital Converter) is a device or module that converts continuous analog signals into discrete digital signals. It is a core component in modern digital control systems, signal processing, and measurement systems.

• Function: Converts continuous signals like voltage and current into digital values for processing by microcontrollers (MCU), DSPs, or FPGAs.

• Key Parameters:

  • Resolution: The number of digital bits output by the ADC, representing the number of distinguishable levels. The RA8P1 has 16-bit resolution, i.e., 2^16 = 65,536 distinct levels.

  • Sampling Rate: The number of ADC samples per second, affecting the frequency range of signals that can be captured.

  • Input Range: The range of analog voltages the ADC can process.

  • Accuracy: Indicates how close the ADC output is to the actual input signal, affected by noise, nonlinearity, and offset errors.

2. ADC Working Principle

The ADC operation can be divided into several stages:

1. Sample & Hold (S/H)

   • Captures and holds the analog signal at the sampling instant to ensure stability during conversion.

   • RA8 series ADCs support adjustable sample-and-hold time, optimizing sampling for high-impedance signals.

2. Quantization

   • Divides the analog signal into discrete levels, each corresponding to a digital code.

   • A 16-bit ADC divides the input voltage range into 65,536 levels, with quantization precision:ΔV = VREF / 65536.

3. Encoding

• Converts the quantized level into a binary code output.

• RA8P1 ADC supports right-aligned / left-aligned data output for flexible data handling.

3. ADC Type and RA8P1 Features

This MCU contains two units of Noise-Shaping SAR-type A/D converters (ADC16H) that is the hybrid architecture with

features of SAR-type and Delta-Sigma modulation-type. The A/D converter unit 0 (ADC0) can select up to 15 channels of

analog inputs. The A/D converter unit 1 (ADC1) can select up to 15 channels of analog inputs. The temperature sensor,

internal reference voltage, VBATT 1/6 voltage monitor output, and D/A converters can be A/D-converted by A/D converter

unit 0 or unit 1. A/D conversion data can be selected from 16-bit, 14-bit, 12-bit, and 10-bit length data formats.

The ADC16H has the following features:

● Resolution: Up to 16-bit

● Fast conversion: Up to 6.25 Msps (0.16 µs per channel) (when A/D conversion clock ADCLK = 50 MHz)

● Input channels: Up to 23 analog input channels

● Support single-ended input or differential inputs

● Self-calibration function

● Built-in channel-dedicated sample-and-hold circuit (S&H)

SAR ADC Working Flow:

1. Sample and hold the input voltage

2. Stepwise comparison with DAC output

3. Adjust the binary code according to the comparison result

4. Output the digital value

4. Key ADC Parameters

1. Resolution

• RA8P1 16-bit ADC: theoretical minimum detectable voltage ΔV = VREF / 65536

• Example: VREF = 3.3V, ΔV ≈ 0.00005 V ≈ 50 μV

2. Sampling Time

• Determines input voltage stability and ADC error.

• High-impedance signals require longer sampling time to avoid errors.

3. Linearity

• INL (Integral Nonlinearity): Cumulative error, deviation from ideal straight line

• DNL (Differential Nonlinearity): Step size error between adjacent codes

4. Noise and Accuracy

• System noise affects low-level resolution. The effective resolution in 16-bit mode is usually slightly below 16 bits (e.g., 15 effective bits).

5. Input Impedance

• High-impedance signals can be sampled directly; low-impedance or high-speed signals may require buffering.

5. Typical RA8P1 ADC Applications

• Industrial Measurement: Temperature, pressure, and flow sensor acquisition

• Motor Control: Current and voltage sampling for closed-loop control

• Signal Processing: Audio acquisition, vibration monitoring

• Data Logging: Multi-channel high-speed sampling for storage or transmission to a host

RT-Thread ADC Driver Framework

The RT-Thread ADC (Analog to Digital Converter) framework is a unified interface provided by the RT-Thread device driver layer to manage ADC hardware modules across various MCUs. This framework abstracts the underlying ADC hardware into a standardized device interface, allowing applications to obtain digital values from analog signals through a uniform API and achieve cross-platform ADC functionality.

1. Device Model

In RT-Thread, ADCs are managed as device objects (subclass of struct rt_device, type RT_Device_Class_ADC). Developers do not need to manipulate hardware registers directly. All channel enabling, sampling, and disabling operations can be done via standard RT-Thread device interfaces.

2. Operation Interfaces

Applications access ADC devices through RT-Thread’s I/O device management interfaces. Key APIs include:

• Find ADC device

rt_device_t rt_device_find(const char* name);

• Enable ADC channel

rt_err_t rt_adc_enable(rt_adc_device_t dev, rt_uint32_t channel);

• Read ADC channel value

rt_uint32_t rt_adc_read(rt_adc_device_t dev, rt_uint32_t channel);

• Disable ADC channel

rt_err_t rt_adc_disable(rt_adc_device_t dev, rt_uint32_t channel);

3. Framework Features

• Unified Interface: All ADC hardware modules are accessed through the same interface, simplifying application development.

• Cross-Platform Support: Applications can be ported across different MCU platforms without modifying ADC code.

• Flexible Channel Control: Supports independent enable/disable for multiple channels.

• High Extensibility: Can be combined with DMA, timers, and other modules for high-speed data acquisition.

• Accurate Sampling: Supports multiple resolutions and different reference voltage configurations.

Reference: RT-Thread ADC Device

Hardware Description

As shown in the following schematic diagram, there are 4 ADC channel interfaces on the Titan Board, namely channels 0, 1, 2 and 3 of adc0.

FSP Configuration

• Step 1: Open FSP and import the XML configuration file (or directly click the FSP link file in RT-Thread Studio).

• Step 2: Create a new r_adc stack, configure the ADC device, and select the channels to be used.

• Step 3: Save and click “Generate Project”. The generated code will be saved in hal_data.c.

RT-Thread Settings Configuration

• Enable ADC0:

Example Code Description

The ADC source code is located in /project/Titan_driver_adc/src/hal_entry.c. The macros used are defined as follows:

The specific function is to sample the analog voltage from ADC0 channel 1 every 1000ms and perform a conversion. The code is as follows:

static int adc_vol_sample()
{
    rt_adc_device_t adc_dev;
    rt_uint32_t value, vol;
    rt_err_t ret = RT_EOK;

    adc_dev = (rt_adc_device_t)rt_device_find(ADC_DEV_NAME);
    if (adc_dev == RT_NULL)
    {
        rt_kprintf("adc sample run failed! can't find %s device!\n", ADC_DEV_NAME);
        return RT_ERROR;
    }

    ret = rt_adc_enable(adc_dev, ADC_DEV_CHANNEL);

    while(1)
    {
        value = rt_adc_read(adc_dev, ADC_DEV_CHANNEL);
        rt_kprintf("the value is :%d \n", value);

        vol = value * REFER_VOLTAGE / CONVERT_BITS;
        rt_kprintf("the voltage is :%d.%02d \n", vol / 100, vol % 100);

        rt_thread_mdelay(1000);
    }

    ret = rt_adc_disable(adc_dev, ADC_DEV_CHANNEL);

    return ret;
}

In the example, the while loop calls adc_vol_sample every 1000ms.

Compilation & Download

• RT-Thread Studio: In RT-Thread Studio’s package manager, download the Titan Board resource package, create a new project, and compile it.

After compilation, connect the development board’s USB-DBG interface to the PC and download the firmware to the development board.

Run Effect

The effect when using ADC0 channel 1 to sample a 1.8V voltage is shown below: