Device Tree Configuration for ToF Cameras: i.MX8M Plus Hardware Integration

This guide provides comprehensive coverage of device tree configuration for Time of Flight (ToF) camera systems on i.MX8M Plus processors. Using real-world examples from a Variscite SOM implementation, we’ll explore how device tree entries translate to hardware functionality and how to customize them for your specific setup.

Understanding Device Tree for ToF Systems

Device tree describes hardware to the Linux kernel without requiring code changes. For ToF cameras, this includes:

• MIPI CSI interface configuration: Lane count, frequencies, timing
• I2C sensor configuration: Addresses, power supplies, clocks
• GPIO assignments: Laser control, safety systems, reset lines
• Power management: Regulators, startup sequences, dependencies
• Clock configuration: Sensor clocks, frequencies, sources

Real-World Example: Variscite i.MX8M Plus SOM

Let’s analyze an actual device tree configuration from a production system:

MIPI CSI Controller Configuration

&mipi_csi_0 {
    #address-cells = <1>;
    #size-cells = <0>;
    status = "okay";
 
    port@0 {
        reg = <0>;
        mipi_csi0_ep: endpoint {
            remote-endpoint = <&mlx7502x_mipi0_ep>;
            data-lanes = <4>;                      // 4-lane MIPI configuration
            csis-hs-settle = <13>;                 // High-speed settle time
            csis-clk-settle = <2>;                 // Clock settle time
            csis-wclk;                             // Enable word clock
        };
    };
};

Configuration Analysis:

• data-lanes = <4>: Configures 4-lane MIPI CSI

  • Why 4 lanes: ToF sensors generate high data rates (16-bit × multi-phase)
  • Alternative: 2-lane configuration (lower bandwidth, may limit frame rate)
  • Hardware dependency: Must match physical lane connections

• csis-hs-settle = <13>: High-speed settle time

  • Purpose: Timing for MIPI receiver to stabilize after lane transitions
  • Value derivation: Based on MIPI CSI-2 specification and sensor characteristics
  • Impact: Incorrect values cause sync errors or data corruption

• csis-clk-settle = <2>: Clock lane settle time

  • Purpose: Clock lane stabilization timing
  • Calculation: Derived from sensor clock frequency and MIPI timing requirements

ISI (Image Sensing Interface) Configuration

&isi_0 {
    status = "okay";
 
    cap_device {
        status = "okay";
    };
 
    m2m_device {
        status = "okay";
    };
};

ISI Components Explained:

• cap_device: Capture interface for direct sensor-to-memory

  • Function: Handles raw ToF data capture
  • Buffer management: Interfaces with V4L2 buffer framework
  • Format support: 16-bit grayscale, various resolutions

• m2m_device: Memory-to-memory processing

  • Function: Format conversion, scaling, processing
  • ToF usage: Convert 16-bit raw to 8-bit for JPEG
  • Performance: Hardware-accelerated operations

ToF Sensor I2C Configuration

&i2c3 {
    clock-frequency = <400000>;
    pinctrl-names = "default", "gpio";
    pinctrl-0 = <&pinctrl_i2c3>;
    pinctrl-1 = <&pinctrl_i2c3_gpio>;
    scl-gpios = <&gpio5 18 GPIO_ACTIVE_HIGH>;
    sda-gpios = <&gpio5 19 GPIO_ACTIVE_HIGH>;
    status = "okay";
 
    mlx7502x_mipi1: mlx7502x_mipi@57 {
        compatible = "melexis,mlx75027";
        reg = <0x57>;                              // I2C address
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_csi0>;
        clocks = <&mlx7502x_clk>;
 
        assigned-clocks = <&mlx7502x_clk>;
        assigned-clock-rates = <8000000>;          // 8MHz sensor clock
 
        // Power supplies
        vdda-supply = <&reg_tim_2v7>;
        vddif-supply = <&reg_tim_1v8>;
        vddd-supply = <&reg_tim_1v2>;
        vdmix-supply = <&reg_tim_1v2mix>;
 
        csi_id = <0>;
        mipi_csi;
        status = "okay";
 
        port {
            mlx7502x_mipi0_ep: endpoint {
                remote-endpoint = <&mipi_csi0_ep>;
                clock-lanes = <0>;
                data-lanes = <1 2 3 4>;                // 4-lane MIPI
                link-frequencies = /bits/ 64 < 960000000
                                                904000000
                                                800000000
                                                704000000
                                                600000000
                                                300000000 >;
                clock-noncontinuous;
            };
        };
    };
 
    eye-safety@48 {
        compatible = "eyesafety";
        reg = <0x48>;                                  // BPW 34 FS-Z safety controller
        pinctrl-0 = <&pinctrl_eyesafety_gpio>;
        rst-gpios = <&gpio1 11 GPIO_ACTIVE_LOW>;
        test-gpios = <&gpio5 3 GPIO_ACTIVE_HIGH>;
        interrupt-parent = <&gpio5>;
        interrupts = <28 IRQ_TYPE_LEVEL_HIGH>;
        fw-name = "roommate/eyesafety.hex";
        status = "okay";
    };
};

I2C Configuration Analysis:

• reg = <0x57>: I2C address of ToF sensor

  • Hardware dependency: Must match sensor’s configured address
  • Common addresses: 0x3d, 0x57 (depends on sensor configuration)
  • Verification: Use i2cdetect -y 3 to confirm

• clock-frequency = <400000>: I2C bus speed (400kHz)

  • Standard speeds: 100kHz (standard), 400kHz (fast), 1MHz (fast+)
  • ToF consideration: 400kHz adequate for sensor control
  • Trade-off: Higher speeds may cause signal integrity issues

Power Supply Configuration

// Clock configuration for ToF sensor
mlx7502x_clk: mlx7502x_clk {
    compatible = "fixed-clock";
    #clock-cells = <0>;
    clock-frequency = <8000000>;                       // 8MHz sensor clock
};
 
// Power regulators for ToF sensor
reg_tim_1v2mix: regulator-tim_1v2mix {
    compatible = "regulator-fixed";
    pinctrl-0 = <&pinctrl_tim_enable_reg>;
    pinctrl-names = "default";
    regulator-name = "tim_1v2mix";
    regulator-min-microvolt = <1200000>;
    regulator-max-microvolt = <1200000>;
    gpio = <&gpio4 24 GPIO_ACTIVE_HIGH>;
    startup-delay-us = <20000>;
    enable-active-high;
    regulator-always-on;
};
 
reg_tim_1v2: regulator-tim_1v2 {
    compatible = "regulator-fixed";
    pinctrl-0 = <&pinctrl_tim_reset_reg>;
    pinctrl-names = "default";
    regulator-name = "tim_1v2";
    regulator-min-microvolt = <1200000>;
    regulator-max-microvolt = <1200000>;
    vin-supply = <&reg_tim_1v2mix>;
    gpio = <&gpio4 23 GPIO_ACTIVE_HIGH>;
    enable-active-high;
    regulator-always-on;
};

Power Supply Dependencies:

• startup-delay-us = <20000>: 20ms startup delay

  • Purpose: Ensures stable power before dependent regulators enable
  • Critical for ToF: Sensors require stable power sequencing
  • Calculation: Based on regulator specifications and sensor requirements

• vin-supply = <&reg_tim_1v2mix>: Supply dependency

  • Purpose: Defines power-up sequence (1v2mix → 1v2)
  • Safety: Prevents voltage conflicts during startup
  • Hardware requirement: Must match actual power circuit design

Customizing Device Tree for Your Hardware

Adapting MIPI CSI Configuration

// Example: 2-lane MIPI configuration
&mipi_csi_0 {
    port@0 {
        mipi_csi0_ep: endpoint {
            data-lanes = <2>;                      // Change to 2-lane
            csis-hs-settle = <15>;                 // Adjust timing
            csis-clk-settle = <3>;                 // Adjust timing
        };
    };
};

**When to Use 2-Lane Configuration