Overview

The XRC PRO is an advanced, open-source RC transmitter and receiver system designed to offer professional-level performance in a compact and customizable package. Built around the STM32F103C8T6 microcontroller and the NRF24L01 wireless transceiver module, the XRC PRO provides precise, real-time control for various RC applications like drones, cars, and boats. With a compact design, robust functionality, and user-friendly interface, the XRC PRO is an ideal tool for both hobbyists and professionals.

Key features

Tech Specs

Components

The XRC PRO is designed with readily available, off-the-shelf components, making it easy for others to replicate or modify. Below are the key components:

Transmitter

Receivers(PWM+PPM)

Receivers(PPM+SBUS)

Schematic and PCB Layout

The XRC PRO has multiple PCBs and it consists of three main PCBs: the Transmitter PCB, the 8-channel PWM+PPM Receiver PCB, and the PPM+SBUS Receiver PCB. Each PCB was carefully designed for space efficiency and optimal performance.

Transmitter PCBs

The Transmitter PCB is built around the STM32F103C8T6, NRF24L01 module, OLED display, and various input buttons (trim, menu, encoder). The schematic includes connections for power management (5v to 3.3v), data lines for the OLED, and button inputs for settings navigation etc.

Schematic

PCB Layout

Schematic and PCB Layout

2D and 3D Preview

Final Result

Menu-Keys

Schematic and PCB Layout

2D and 3D Preview

Final Result

Rotary Encoder+Switch

Schematic and PCB Layout

2D and 3D Preview

Final Result

Trim Buttons

2D and 3D Preview

Final Result

Type C Jack

Schematic and PCB Layout

2D and 3D Preview

Final Result

Receciver PCBs

8-channel PWM+PPM Receiver PCB

The 8-channel receiver supports both PWM and PPM outputs. It is based on the NRF24L01 and STM32F103C8T6 for signal decoding and generation.

Schematic

PCB Layout

Schematic and PCB Layout

2D and 3D Preview

Final Result

PPM+SBUS Receiver PCB

The PPM+SBUS receiver is another compact design focused on supporting more advanced control protocols. It also features the STM32F103C8T6 for signal processing and output.

Schematic

PCB Layout

Schematic and PCB Layout

2D and 3D Preview

Final Result

Visual and Physical Connections

I’ve designed a detailed, color-coded connection schematic for all inputs and outputs, making it easy for anyone to assemble the XRC PRO transmitter and receivers. Additionally, I’ve included high-quality these images in PDF format, which you can download below.

Transmitter

Receiver(PWM+PPM)

Receiver(PPM+SBUS)

Firmware Overview

The XRC PRO's firmware is developed in Keil uVision using the CMSIS framework for STM32. It is fully open-source, with modular code to allow easy customization and extension of the system's capabilities. The main focus of the firmware is on handling communication between the transmitter and receivers, as well as managing the various control inputs.

Here the whole files in KeiluVision:

Below are the functions broken down from the main execution code, organized into smaller functional blocks. These functions are intended to be modular and handle specific tasks like initialization, signal strength calculation, display management, and event handling:

1. setup()

This function initializes all the necessary hardware peripherals and sets up the system for operation.

void setup() {
    // Initialize the delay function for timekeeping
    delayInit();
    
    // Initialize USART for serial communication
    usart1Init();
    
    // Initialize timers for PWM and other timing-related functions
    timer2Init();
    timer3Init();
    
    // Initialize DMA for memory transfers
    dmaInit();
    
    // Initialize ADC for analog reading (e.g., battery level)
    adcInit();
    
    // Initialize NRF24L01 for wireless communication
    if (nrf24l01Init() == NRF_OK) {
        nrf24l01SetModeTX(); // Set NRF to transmit mode
    } else {
        // Handle NRF24L01 initialization error
        beepError();
    }

    // OLED display initialization and show start-up screen
    oledInit();
    oledShowLogo();
    
    // Perform throttle self-check and configure NRF24L01 power mode
    throttleSelfCheck();
    
    // Set up low-power mode if necessary
    lowPowerModeConfig();
}

2. loop()

The main loop that continuously runs during operation. It handles the display updates, signal strength checks, and key events.

void loop() {
    // Handle clock alarms and time display on the OLED
    handleClockAlarm();

    // Update OLED display with throttle values and battery percentage
    displayThrottleValues();
    displayBatteryLevel();

    // Update signal strength display
    updateSignalStrength();

    // Check for key events and menu navigation
    keyEventHandle();
    
    // Handle menu events, if any
    menuEventHandle();
    
    // Small delay to avoid constant polling
    delay(50); 
}

3. keyEventHandle()

Handles user input through key presses. This function updates throttle settings and processes navigation within the menu.

void keyEventHandle() {
    if (isKeyPressed(KEY_LEFT)) {
        // Adjust throttle channel for left-hand throttle
        adjustThrottleLeft();
    }
    
    if (isKeyPressed(KEY_RIGHT)) {
        // Adjust throttle channel for right-hand throttle
        adjustThrottleRight();
    }

    // Update OLED display with new throttle or menu setting
    oledRefresh();

    // Save user data or preferences (e.g., throttle setting)
    saveUserDataToFlash();
}

4. addSignalStrengthSample()

Adds a new sample to the signal strength buffer and maintains a moving average of signal strength.

void addSignalStrengthSample(int newSignalStrength) {
    // Add new signal strength to the sample buffer
    signalStrengthBuffer[sampleIndex] = newSignalStrength;
    
    // Update sample index for circular buffer
    sampleIndex = (sampleIndex + 1) % SIGNAL_STRENGTH_BUFFER_SIZE;
}

5. getAverageSignalStrength()

Calculates the moving average of the signal strength based on the collected samples.

int getAverageSignalStrength() {
    int sum = 0;
    for (int i = 0; i < SIGNAL_STRENGTH_BUFFER_SIZE; i++) {
        sum += signalStrengthBuffer[i];
    }
    return sum / SIGNAL_STRENGTH_BUFFER_SIZE;
}

6. displaySignalIcon()

Displays the signal strength icon on the OLED based on the signal strength percentage.

void displaySignalIcon(int signalStrengthPercent) {
    if (signalStrengthPercent >= 75) {
        oledDrawIcon(iconSignal100);
    } else if (signalStrengthPercent >= 50) {
        oledDrawIcon(iconSignal75);
    } else if (signalStrengthPercent >= 25) {
        oledDrawIcon(iconSignal50);
    } else if (signalStrengthPercent > 0) {
        oledDrawIcon(iconSignal25);
    } else {
        oledDrawIcon(iconSignalOff);
    }
}

void nrf24l01InitAndCheck() {
    // Initialize NRF24L01
    if (nrf24l01Init() == NRF_OK) {
        // Set NRF24L01 to transmit mode
        nrf24l01SetModeTX();
    } else {
        // Handle NRF24L01 error by beeping and showing error message
        beepError();
        oledShowErrorMessage("NRF24L01 ERROR");
    }
}

u8 sendDataPacket(void)
{
    u8 chPacket[32]; // Array to hold the data packet
    u16 t = 0;
    u8 sendIsOK; // Flag to check if sending is successful

    for (t = 0; t < 16; t++)
    {
        if (t == 0) // Add data header (0x00)
        {
            chPacket[2 * t] = 0x00;
            chPacket[2 * t + 1] = 0x00;
        }
        else if (t <= chNum) // Add PWM channel data
        {
            chPacket[2 * t] = (u8)(PWMvalue[t - 1] >> 8) & 0xFF; // High byte of 16-bit PWM value
            chPacket[2 * t + 1] = (u8)PWMvalue[t - 1] & 0xFF;    // Low byte of 16-bit PWM value
        }
        else // Fill unused bytes with 0xFF (padding)
        {
            chPacket[2 * t] = 0xFF;
            chPacket[2 * t + 1] = 0xFF;
        }
    }

    sendIsOK = NRF24L01_TxPacket(chPacket); // Transmit the packet
    return sendIsOK; // Return status of the transmission (success or failure)
}

int getSignalStrength(void)
{
    const int totalPackets = 40; // Total number of packets to send
    int successfulPackets = 0;   // Counter for successful transmissions

    if (setData.NRF_Mode == ON) // If the NRF mode is ON
    {
        NRF24L01_TX_Mode(setData.NRF_Power); // Set NRF to transmit mode
    }
    else
    {
        NRF24L01_LowPower_Mode(); // Otherwise, set it to low-power mode
    }

    // Loop to send multiple packets
    for (int i = 0; i < totalPackets; i++)
    {
        if (sendDataPacket() == TX_OK) // If the packet was sent successfully
        {
            successfulPackets++; // Increment success counter
        }
        delay_us(700); // Short delay between transmissions
    }

    // Calculate signal strength as a percentage of successful packets
    int signalStrength = (successfulPackets * 100) / totalPackets;
    return signalStrength;
}

10. updateSignalStrength()

Updates the signal strength display and checks if the receiver is connected. It also adds a new sample to the signal strength buffer.

void updateSignalStrength() {
    // Get current signal strength from NRF24L01
    int currentSignalStrength = nrf24l01GetSignalStrength();

    // Add the sample to the signal strength buffer
    addSignalStrengthSample(currentSignalStrength);

    // Calculate the average signal strength
    int avgSignalStrength = getAverageSignalStrength();

    // Display the corresponding signal strength icon
    displaySignalIcon(avgSignalStrength);

    // Check if receiver is connected
    if (avgSignalStrength > SIGNAL_STRENGTH_THRESHOLD) {
        receiverConnected();
    } else {
        receiverDisconnected();
    }
}

11. menuEventHandle()

Handles the user interface menu and responds to user input for changing settings.

void menuEventHandle() {
    // Check if a specific menu item is selected
    if (isMenuItemSelected(MENU_ITEM_PWM_ADJUST)) {
        adjustPWMSettings();
    }
    
    if (isMenuItemSelected(MENU_ITEM_CHANNEL_CALIBRATION)) {
        calibrateChannels();
    }

    // Refresh OLED display with updated menu or settings
    oledMenuRefresh();
}

12. Battery Voltage Display

Updates the battery level icon and percentage on the OLED based on the battery voltage.

void displayBatteryLevel() {
    float batteryVoltage = readBatteryVoltage();
    int batteryPercent = convertVoltageToPercentage(batteryVoltage);
    
    // Display the battery icon and percentage on the OLED
    oledDrawBatteryIcon(batteryPercent);

    if (batteryVoltage < BATTERY_WARNING_THRESHOLD) {
        // Beep to warn low battery
        beepWarning();
    }
}

13. Clock and Alarm Handling

Handles the clock and alarm functionality, beeping when an alarm is triggered.

void handleClockAlarm() {
    // Check if the clock alarm is active
    if (isAlarmActive()) {
        // Beep if the alarm time is reached
        beepAlarm();

        // Display alarm icon on the OLED
        oledDrawIcon(iconAlarm);
    }

    // Update the time on the OLED display
    displayCurrentTime();
}

These functions represent the core operations of the system, which handles initialization, user input, signal strength monitoring, display updates, and more. Each function focuses on a specific aspect of the overall operation, making the code more modular and easier to manage.

Firmware Uploading

To upload the firmware to the transmitter and receivers, follow the connection diagram below. It shows how I connected the CH340 USB-to-TTL adapter to both the transmitter and receivers.

Connections

Transmitter

There are two methods of uploading code into transmitter.First,you can directly upload the code via USB TYPE-C Interface.  Second,you can upload the code with the help of USB to TTL (ch340 driver).

Rceiver(PWM+PPM)

Receiver(PPM+SBUS)

Software

Download and install the STMicroelectronics Flash Loader Demonstrator software: Click to download

Steps

Follow:

1. Plug the USB serial converter into the PC.
2. Press the reset button on the STM32 board.
3. Select the COM port in the software.
4. Click 'Next,Next and Next.
5. Select Download to device.Click on 3 dots and select the hex file.
6. Select Global Erase.After that Click on Next
7. Within 15 seconds, the code will be uploaded to the device.
8. After finsihed.Click Close.