DAY65 IMX6ULL: ADC Light Sensor Detection and LCD Display Driver

IMX6ULL: ADC Light Sensor Detection and LCD Display Driver

1. ADC Module: Light Sensor Illumination Intensity Acquisition

1.1 ADC Core Concepts and IMX6ULL Features

ADC (Analog-to-Digital Converter) is the bridge between the analog world and digital systems. Key parameters determine acquisition accuracy and range:

Range: The analog voltage range the ADC can convert, determined by the reference voltage (Vref).
Resolution: The number of bits of the ADC, determining the precision of digital output (higher bits mean higher precision).

Core features of IMX6ULL's ADC module:

12-bit resolution, digital output range 0~4095.
Supports multi-channel analog input, default reference voltage 3.3V.
Requires hardware calibration to ensure conversion accuracy.
Uses Successive Approximation Register (SAR) ADC architecture, suitable for sensor acquisition scenarios.

1.2 Comparison of Main ADC Architectures (Selection Basis for Sensor Acquisition)

Different ADC architectures suit different scenarios. SAR is the preferred choice for embedded sensor acquisition. Comparison as follows:

Architecture Type	Resolution Range	Conversion Rate Range	Power Consumption	Cost	Typical Application Scenarios
SAR (Successive Approximation)	8~16 bits	10kSPS~10MSPS	Low	Medium	Sensor acquisition, battery detection, industrial control
Flash	6~10 bits	100MSPS~1GSPS	Very High	High	High-speed signal acquisition (video, RF)
Σ-Δ	16~24 bits	10SPS~100kSPS	Medium-Low	Medium-High	High-precision measurement (weighing, temperature calibration)
Pipeline	10~16 bits	10MSPS~100MSPS	Medium-High	High	High-speed, high-precision acquisition (radar, medical imaging)

1.3 SAR ADC Conversion Formula

The core of light sensor detection is converting the ADC's digital value to actual voltage. Formula:

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Actual Voltage (V) = ADC Raw Value × Reference Voltage / 2^ADC Bits

For IMX6ULL (reference voltage 3.3V, 12-bit resolution):

If ADC value is 2048, actual voltage = 2048 × 3.3 / 4096 = 1.65V.

1.4 IMX6ULL ADC Register Configuration (Light Sensor Scenario)

The light sensor uses a voltage divider circuit (light sensor + fixed resistor in series, ADC channel measures the voltage at the divider point). Core registers to configure:

(1) Clock Enable (CCM Register)

IMX6ULL's ADC clock is controlled by CCM (Clock Controller Module). Enable ADC module clock:

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// Enable ADC clock (CCM_CCGR6 register, BIT23~BIT24 set to 1)  
CCM->CCGR6 |= (3 << 23);

(2) Pin Multiplexing (IOMUXC Register)

Configure specified GPIO pin as ADC input (e.g., ADC1_CH0):

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// Configure pin as ADC function, disable pull-up/down  
IOMUXC_SetPinMux(IOMUXC_GPIO1_IO00_ADC1_IN0, 0);  
IOMUXC_SetPinConfig(IOMUXC_GPIO1_IO00_ADC1_IN0, 0x10B0);

(3) ADC Control Register (ADC_CR)

Configure sampling channel, sampling time, enable ADC:

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ADC1->CR = 0;  
ADC1->CR |= (1 << 0);          // Enable ADC  
ADC1->CR |= (0 << 2);          // Select channel 0 (ADC1_CH0)  
ADC1->CR |= (0x1F << 8);       // Set sampling time to maximum (improve accuracy)  
ADC1->CR |= (1 << 16);         // Single conversion mode

(4) ADC Data Read (SR+DR Register)

Wait for conversion completion, read digital value:

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// Wait for conversion (ADC_SR BIT1 set to 1)  
while(!(ADC1->SR & (1 << 1)));  
// Read ADC raw value (DR register lower 12 bits valid)  
uint16_t adc_val = ADC1->DR & 0xFFF;

2. LCD Module: Illumination Value Visualization Display

2.1 LCD Core Principles (TFT-LCD)

This project uses TFT-LCD (320×240 resolution, RGB565 color depth). Core principles:

Based on RGB interface, refreshes pixels line-by-line/column-by-column synchronized with pixel clock.
Each pixel's color is determined by RGB components. In RGB565 format, 16 bits represent one pixel (R:5 bits, G:6 bits, B:5 bits).
IMX6ULL has a built-in LCD controller, supporting direct TFT-LCD driving without additional driver chips.

2.2 IMX6ULL LCD Register Core Configuration

(1) Clock Configuration (CCM)

Enable LCD controller clock and configure pixel clock:

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// Enable LCD clock (CCM_CCGR5 register, BIT26~BIT27 set to 1)  
CCM->CCGR5 |= (3 << 26);  
// Configure LCD pixel clock (adjust based on screen parameters, e.g., 6MHz for 320×240 screen)  
CCM->CDCDR |= (0x1F << 0);

(2) LCD Controller Basic Configuration (LCDC_CTRL)

Set resolution, color depth, display mode:

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LCDC->CTRL = 0;  
LCDC->CTRL |= (1 << 0);          // Enable LCD controller  
LCDC->CTRL |= (0 << 1);          // RGB interface mode  
LCDC->CTRL |= (16 << 4);         // Color depth: RGB565 (16 bits)  
LCDC->CTRL |= (320 << 16);       // Horizontal resolution: 320  
LCDC->CTRL |= (240 << 26);       // Vertical resolution: 240

(3) Timing Configuration (LCDC_TIMING)

Configure HSYNC (horizontal sync), VSYNC (vertical sync) key timings (example for 320×240 screen):

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LCDC->TIMING = 0;  
LCDC->TIMING |= (10 << 0);       // HSYNC pulse width: 10 clocks  
LCDC->TIMING |= (20 << 8);       // HSYNC back porch: 20 clocks  
LCDC->TIMING |= (20 << 16);      // HSYNC front porch: 20 clocks  
LCDC->TIMING |= (2 << 24);       // VSYNC pulse width: 2 clocks  
LCDC->TIMING |= (2 << 28);       // VSYNC back porch: 2 clocks  
LCDC->TIMING |= (2 << 30);       // VSYNC front porch: 2 clocks

(4) Frame Buffer Configuration (LCDC_FB)

Set frame buffer address (requires contiguous memory for pixel data):

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// Frame buffer address (example: 0x80000000, must be memory-aligned)  
LCDC->FB0 = (uint32_t)frame_buffer;

2.3 LCD Display Function Encapsulation

Implement character/numeric display functions to show ADC voltage values at specified LCD positions:

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// Display string at (x,y)  
void lcd_show_string(uint16_t x, uint16_t y, char *str) {  
    // Pixel filling logic: Write to frame buffer based on character bitmap  
    // Omit bitmap parsing and frame buffer writing details (can use ASCII bitmap library)  
}  

// Display ADC voltage  
void lcd_show_adc_volt(uint16_t x, uint16_t y, float volt) {  
    char buf[32];  
    sprintf(buf, "Light Voltage: %.2f V", volt);  
    lcd_show_string(x, y, buf);  
}

3. Complete Practical Code (Core Process)

3.1 Overall Process

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Initialize system clock → Initialize ADC → Initialize LCD → Loop ADC acquisition → Convert to voltage → LCD display

3.2 Core Code Example

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#include "imx6ull.h"

// Frame buffer (320×240×2 bytes, RGB565)
uint16_t frame_buffer[320*240] __attribute__((aligned(32)));

// ADC initialization
void adc_init(void) {
    // 1. Enable ADC clock
    CCM->CCGR6 |= (3 << 23);
    // 2. Configure pin multiplexing as ADC1_CH0
    IOMUXC_SetPinMux(IOMUXC_GPIO1_IO00_ADC1_IN0, 0);
    IOMUXC_SetPinConfig(IOMUXC_GPIO1_IO00_ADC1_IN0, 0x10B0);
    // 3. Configure ADC control register
    ADC1->CR = (1 << 0) | (0 << 2) | (0x1F << 8) | (1 << 16);
}

// LCD initialization (simplified)
void lcd_init(void) {
    // 1. Enable LCD clock
    CCM->CCGR5 |= (3 << 26);
    // 2. Configure LCD pin multiplexing (omitted, requires hardware schematic configuration)
    // 3. Configure LCD controller
    LCDC->CTRL = (1 << 0) | (0 << 1) | (16 << 4) | (320 << 16) | (240 << 26);
    LCDC->TIMING = (10 << 0) | (20 << 8) | (20 << 16) | (2 << 24) | (2 << 28) | (2 << 30);
    // 4. Set frame buffer
    LCDC->FB0 = (uint32_t)frame_buffer;
    // 5. Clear screen (black)
    memset(frame_buffer, 0, 320*240*2);
}

// Read ADC value and convert to voltage
float adc_read_volt(void) {
    // Start conversion
    ADC1->CR |= (1 << 24);
    // Wait for conversion completion
    while(!(ADC1->SR & (1 << 1)));
    // Read raw value
    uint16_t adc_val = ADC1->DR & 0xFFF;
    // Convert to voltage (3.3V reference, 12-bit resolution)
    return (float)adc_val * 3.3 / 4096;
}

int main(void) {
    float volt;
    // Initialization
    adc_init();
    lcd_init();
    
    while(1) {
        // Sample voltage
        volt = adc_read_volt();
        // LCD display (coordinates: x=10, y=10)
        lcd_show_adc_volt(10, 10, volt);
        // Delay
        for(int i=0; i<1000000; i++);
    }
    return 0;
}

4. Testing and Verification

4.1 Hardware Wiring

Photoresistor voltage divider output → IMX6ULL ADC1_CH0 (GPIO1_IO00);
LCD RGB interface → IMX6ULL LCD controller corresponding pins;
Power supply: 5V for IMX6ULL core board, 3.3V for LCD screen.

4.2 Verification Results

After compiling and flashing the code to the IMX6ULL core board, the LCD screen will display "Light Voltage: X.XX V" in real-time. When the photoresistor is shaded or exposed to light, the voltage value changes with light intensity, verifying normal ADC acquisition and LCD display functionality.

5. Summary and Expansion

This article completes the core development of ADC photoresistor acquisition and LCD display on IMX6ULL, mastering:

The principle and register configuration of successive approximation ADC;
The driving principle of TFT-LCD and frame buffer operations;
The complete process of analog signal acquisition → digital conversion → visual display.

Expansion Directions:

Add threshold judgment: Display warning information on the LCD when light intensity falls below a threshold;
Add touchscreen interaction: Switch display modes (voltage/light percentage) via touchscreen;
Serial output: Output ADC values to the serial port for debugging with a host computer;
Low-power optimization: Adjust ADC sampling frequency to reduce system power consumption.