Table of Contents

liteSOM and LCD display

Example available on GitHub. Check liteboard LCD + touchscreen.

<uml w=100%> title LCD display and i.MX6UL\nblock diagram

node “i.MX6UL” { 

package "LCD controller" {
  [LCD data]
  [LCD sync]
}
[Backlight]
[Touchscreen]

}

[LCD data] –> RGB888 RGB888 –> [LCD panel]

interface “VSYNC, HSYNC\nCLK, EN” as sync

[LCD sync] –> sync sync –> [LCD panel]

[Backlight] –> PWM PWM –> [LCD panel]

[Touchscreen] ←- ADC ADC ←- [LCD panel] </uml>

i.MX6UL has several built-in peripherals which can be used to build complete graphical interface:

  • eLCDIF - Enhanced LCD Interface,
  • TSC - Touch Screen Controller,
  • PWM - Pulse Width Modulation.

Below you can see how to configure liteboard to support lcd module.

On lcd we are using RVT7.0A800480TNWR00 TFT display produced by Riverdi. Below you can find short details about this device:

  • type: TFT/Transmissive/Normally white,
  • size: 7 inch,
  • resolution: 800 x 600,
  • backlight: 21 LEDs,
  • interface: 24-bit RGB,
  • touchscreen: resistive.

eLCDIF - Enhanced LCD Interface

The eLCDIF can be used to drive a wide range of display devices. It supports:

Basic configuration for eLCDIF is defined in imx6ul.dtsi file. 

lcdif: lcdif@021c8000 {
    compatible = "fsl,imx6ul-lcdif", "fsl,imx28-lcdif";
    reg = <0x021c8000 0x4000>;
    interrupts = <GIC_SPI 5 IRQ_TYPE_LEVEL_HIGH>;
    clocks = <&clks IMX6UL_CLK_LCDIF_PIX>,
             <&clks IMX6UL_CLK_LCDIF_APB>,
             <&clks IMX6UL_CLK_DUMMY>;
    clock-names = "pix", "axi", "disp_axi";
    status = "disabled";
};

which is used by mxsfb driver.

In the mxsfb.txt file you can find documentation with information how to configure eLCDIF via device tree file.

In addition please check display-timing.txt file with information how to configure display timings. 

&iomuxc {
  pinctrl_lcdif_ctrl: lcdifctrlgrp {
    fsl,pins = <
      MX6UL_PAD_LCD_CLK__LCDIF_CLK       0x79
      MX6UL_PAD_LCD_ENABLE__LCDIF_ENABLE 0x79
      MX6UL_PAD_LCD_HSYNC__LCDIF_HSYNC   0x79
      MX6UL_PAD_LCD_VSYNC__LCDIF_VSYNC   0x79
    >;
  };

  pinctrl_lcdif_dat: lcdifdatgrp {
    fsl,pins = <
      MX6UL_PAD_LCD_DATA00__LCDIF_DATA00  0x79
      MX6UL_PAD_LCD_DATA01__LCDIF_DATA01  0x79
      [...]
      MX6UL_PAD_LCD_DATA23__LCDIF_DATA23  0x79
    >;
  };
};


&lcdif {
  pinctrl-names = "default";
  pinctrl-0 = <&pinctrl_lcdif_dat
               &pinctrl_lcdif_ctrl>;
  display = <&display0>;
  status = "okay";

  display0: display {
    bits-per-pixel = <32>;
    bus-width = <24>;

    display-timings {
      native-mode = <&timing0>;
      timing0: timing0 {
        clock-frequency = <30000000>;
        hactive = <800>;
        vactive = <480>;
        hfront-porch = <210>;
        hsync-len = <2>;
        hback-porch = <46>;
        vfront-porch = <22>;
        vsync-len = <2>;
        vback-porch = <23>;
        hsync-active = <1>;
        vsync-active = <1>;
        de-active = <1>;
        pixelclk-active = <0>;
      };
    };
  };
};

In our case we have to connect eLCDIF with LCD display via 24-bits (RGB888) data bus.

In addition eLCDIF should generate vertical and horizontal synchronization signals, data enable strobe and main clock signal for display controller.

Nodes pinctrl_lcdif_ctrl and pinctrl_lcdif_dat are used to configure IOMUX Controller.

Node lcdif contains complete configuration for used LCD display. Most of configuration values are self-explaining.

To help you to understand timing configuration please check following image. 

+----------+---------------------------------+----------+-------+
|          |        ↑                        |          |       |
|          |        | 46 lines               |          |       |
|          |        ↓                        |          |       |
+----------###################################----------+-------+
|          #        ↑                        #          |       |
|    46    #        |                        #   210    |   2   |
|  pixels  #        |       800 pixels       #  pixels  | pixels|
|<-------->#<-------+----------------------->#<-------->|<----->|
|          #        |                        #          |       |
|          #        | 480 pixels             #          |       |
|          #        ↓                        #          |       |
+----------###################################----------+-------+
|          |        ↑                        |          |       |
|          |        | 22 lines               |          |       |
|          |        ↓                        |          |       |
+----------+---------------------------------+----------+-------+
|          |        ↑                        |          |       |
|          |        | 2 lines                |          |       |
|          |        ↓                        |          |       |
+----------+---------------------------------+----------+-------+

(based on: Documentation/devicetree/bindings/video/display-timing.txt)

In addition to device tree please remember to enable mxsfb driver (via CONFIG_FB_MXS) located in 

-> Device Drivers
  -> Graphics support
    -> Frame buffer Devices
      -> MXS LCD framebuffer support

TSC - Touch Screen Controller

i.MX6UL touchscreen controller can be used with 4-wire and 5-wire resistive touchscreens. Built-in A/D converter has 8-bit/10-bit/12-bit resolution. Parameters like pre-charge or de-glitch are fully configurable.

From software point of view TSC is controlled via imx6ul-tsc driver (see imx6ul_tsc.c). In imx6ul_tsc.txt file you can find information how to configure device tree for this peripheral. 

&iomuxc {
  pinctrl_tsc: tscgrp {
    fsl,pins = <
      MX6UL_PAD_GPIO1_IO01__GPIO1_IO01  0xb0
      MX6UL_PAD_GPIO1_IO02__GPIO1_IO02  0xb0
      MX6UL_PAD_GPIO1_IO03__GPIO1_IO03  0xb0
      MX6UL_PAD_GPIO1_IO04__GPIO1_IO04  0xb0
    >;
  };
};

&tsc {
  pinctrl-names = "default";
  pinctrl-0 = <&pinctrl_tsc>;
  status = "okay";
  xnur-gpio = <&gpio1 4 GPIO_ACTIVE_LOW>;
  measure_delay_time = <0xffff>;
  pre_charge_time = <0xfff>;
};

Node pinctrl_tsc is of course IOMUX Controller configuration.

tsc node is used to configure imx6ul-tsc driver. In most cases it's using default settings from imx6ul-tsc documentation.

xnur-gpio parameter is adjusted to the configuration required by liteboard design.

To compile imx6ul_tsc.c driver please select CONFIG_TOUCHSCREEN_IMX6UL_TSC option located in 

-> Device Drivers
  -> Input device support
    -> Generic input layer (needed for keyboard, mouse, ...)
      -> Touchscreens
        -> Freescale i.MX6UL touchscreen controller

Backlight

<uml> package “liteboard” { [PWM backlight driver] → [PWM] } package “Display” { [PWM] → [Backlight] } </uml> 

/ {
  backlight {
    compatible = "pwm-backlight";
    pwms = <&pwm2 0 5000000>;
    brightness-levels = <0 4 8 16 32 64 128 255>;
    default-brightness-level = <6>;
    status = "okay";
  };
};

&iomuxc {
  pinctrl_pwm2: pwm2 {
    fsl,pins = <
      MX6UL_PAD_GPIO1_IO09__PWM2_OUT   0x110b0
    >;
  };
};

&pwm2 {
  pinctrl-names = "default";
  pinctrl-0 = <&pinctrl_pwm2>;
  status = "okay";
  clocks = <&clks IMX6UL_CLK_PWM2>,
           <&clks IMX6UL_CLK_PWM2>;
};

On liteboard you can find dedicated LCD backlight boot converter (see TPS61042) controlled by litesom via PWM2 output pin.

Backlight can be controlled directly via PWM driver but additional PWM backlight driver will simplify backlight configuration.

Node backlight is documented in pwm-backlight.txt file.

Backlight configured as presented here will:

  • generate PWM with 200Hz frequency (period 5000000 ns),
  • use PWM2 peripheral as backlight controller,
  • define eight allowed backlight levels: 0%, 2%, 3%, 6%, 13%, 25%, 50%, 100%,
  • use 50% as default backlight level.

Node pinctrl_pwm2 is information for IOMUX controller how to configure PWM2 output. On liteboard we are using GPIO1__IO09.

Node pwm2 is configured as documented in pwm.txt.