Understanding Linux Through Designing a Four-Layer Board Terminal Device

Based on the current online materials, the author ported the mainline kernel Uboot and root by himself.Currently only supports SD card and SPI Nor booting, please do not refer to the current code for SPI Nand booting.

① UBoot

Quick Start

git clone https://gitee.com/fhcloud/uboot-v3scd uboot-v3smake v3s_pi_defconfigmake

The output file is in the root directory of the source code: u-boot-sunxi-with-spl.bin

② Linux

Quick Start

git clone https://gitee.com/fhcloud/linux-v3scd linux-v3smake v3s_pi_defconfigmake

The output kernel file is in arch/arm/boot/zImage

The output device tree should use arch/arm/boot/dts/sun8i-v3s-pi.dtb

③ Buildroot

Quick Start

git clone https://gitee.com/fhcloud/buildroot-v3s.gitcd buildroot-v3smake v3s_pi_defconfigmake

The output file is in output/images/rootfs.tar

Test image account: root, password: 123456

① Main Control Part

The main control of this project uses Allwinner V3S. It should be noted that R3/R5 resistors must use 1%, and there are two groups of voltages for EPHY.

Figure 1 Main Control Schematic

② ADC Keys

The board has four ADC keys, with a voltage span of 0.2V, implemented through voltage divider resistors.

Figure 2 ADC Keys

③ 2×16 PIN Expansion Interface

This uses pin headers to bring out 8 expansion interfaces, including one serial port/one I2C bus, which can be used to connect other devices.

Figure 3 Expansion Interface

④ Auxiliary Power Supply

The board has four power supply chips, producing 3.0V, 3.3V, 1.8V, 1.2V, among which

• 3.0V is used for PLL and AVCC analog power supply.
• 3.3V is responsible for chip IO and other peripheral power supply.
• 1.8V is used for memory power supply.
• 1.2V is used for V3S core main control power supply.

Three DC-DC converters have a maximum output current of 2A.

⑤ WIFI Module

The board has a 2.4G WIFI module, using the RTL8723BS module. The connection between V3S and the WIFI module uses SDIO interface, bringing out an IPEX interface for external antenna, R20-R27 are pull-up resistors required by SDIO and the chip, C50/C51/C70 are external filtering capacitors, the module uses 3.3V power supply.

Figure 5 WIFI Module Schematic

⑥ 10M/100M Ethernet Interface

The hundred megabit Ethernet requires two differential pairs, namely RX/TX differentials. The Ethernet PHY generally has automatic flip functionality, so RX/TX can be swapped. V3S Ethernet uses voltage drive, simply add a 100NF capacitor to ground at the center tap of the network transformer.

The internal packaged resistors and 2KV capacitors are used to discharge the motherboard’s static electricity, preventing high voltage from damaging the main control chip.

R28/R29 are LED current limiting resistors, SH1 SH2 are the shell, which can be directly grounded.

Figure 6 Ethernet Interface Schematic

⑦ 2.4-inch LCD Color Screen

The 2.4-inch color screen communicates with the main control via SPI.

• PE8 connects to the screen reset line.
• PE9 connects to DC.

Used to distinguish data/instructions.

LEDA is the backlight anode, LEDK connects to Q1 MOS, used for the main control to control the backlight switch.

The screen is soldered onto the PCB.

Figure 7 2.4-inch LCD Color Screen Schematic

⑧ CH340 Serial to USB

Using the CH340N chip to achieve serial to USB, for connecting the terminal to the computer.

R30/R31 is used to ensure the dual-headed TYPE-C cable provides normal power, D1 is TVS, for protecting the USB interface, since 3.3V power supply is used here, CH340N’s VCC and V3 need to be connected together, and a 100nf capacitor should be added.

Figure 8 CH340 Serial to USB

⑨ CH334R 1-to-4 USB HUB

Using the CH334R chip, to implement one to four USB Host ports.

Here, unified power supply is used, so VDD33 and V5 can be connected together.

• XI XO input 12Mhz crystal oscillator.

• DPU/DMU connects to the upstream USB port.

⑩ Headphone Interface

The headphone interface uses the PJ-342 interface, with audio and recording:

Figure 10 PJ-342 Headphone Interface

⑪ SD Card Interface

The SD card interface is used for inserting a MicroSD card.

R32-R36 are pull-up resistors for the SD card, here CLK does not need to be pulled up, otherwise it may affect SDIO communication.

Figure 11 SD Card Interface

The software solution mainly discusses these8 parts

——Linux kernel, 2.4-inch LCD color screen driver, USB, Ethernet, wireless card RTL8723BS, ADC keys, audio playback, libcurl.

① Linux Kernel

② 2.4-inch LCD Color Screen Driver

The LCD color screen connects to the main control via SPI.

• The software part uses TinyDRM, compared to the traditional fbtft, it no longer refreshes at a fixed frame rate.
• Uses DRM architecture, allowing for faster integration with new architecture programs.

Since the initialization code for screens varies by manufacturer, we only need to modify the code in the st7735r_pipe_enable function based on the original st7735r.c file.

The modified code can be referenced in the file below, the device tree configuration reference:

&spi0 {       status = "okay";       pinctrl-names = "default";       pinctrl-0 = <&spi0_pins>;

       display@0 {        compatible = "jianda,jd-t18003-t01";        reg = <0>;           spi-max-frequency = <95000000>;

        backlight = <&panel_backlight>;        dc-gpios = <&pio 4 9 0>; // PE9        reset-gpios = <&pio 4 8 0>; // PE8        rotation = <0>;    };};

In addition to SPI, a backlight node needs to be added, so that the backlight can be operated at the user level:

panel_backlight: panel-backlight {              compatible = "gpio-backlight";        gpios = <&pio 4 7 GPIO_ACTIVE_HIGH>; // PE7              default-on;              status = "okay";       };

③ USB

The device tree configuration reference:

&usb_otg {       dr_mode = "host"; // peripheral       status = "okay"};

&usbphy {       status = "okay"};

In addition to the referenced nodes, ochi/echi must be added, otherwise USB insertion will have no response:

soc {              ehci0: usb@01c1a000 {                     compatible = "allwinner,sun8i-v3s-ehci", "generic-ehci";                     reg = <0x01c1a000 0x100>;                     interrupts = <GIC_SPI 72 IRQ_TYPE_LEVEL_HIGH>;                     clocks = <&ccu CLK_BUS_EHCI0>, <&ccu CLK_BUS_OHCI0>;                     resets = <&ccu RST_BUS_EHCI0>, <&ccu RST_BUS_OHCI0>;                     status = "okay";              };

              ohci0: usb@01c1a400 {                     compatible = "allwinner,sun8i-v3s-ohci", "generic-ohci";                     reg = &lt;<|vq_15383|>                     <|vq_15384|>                     interrupts = &lt;GIC_SPI 73 IRQ_TYPE_LEVEL_HIGH&gt;;                     clocks = &lt;&amp;ccu CLK_BUS_EHCI0&gt;, &lt;&amp;ccu CLK_BUS_OHCI0&gt;, &lt;&amp;ccu CLK_USB_OHCI0&gt;;                     resets = &lt;&amp;ccu RST_BUS_EHCI0&gt;, &lt;&amp;ccu RST_BUS_OHCI0&gt;;                     status = "okay";              };       };

&emac {       allwinner,leds-active-low;       status = "okay"};

⑤ Wireless Card RTL8723BS

Under Linux, the SDIO architecture is similar to USB.

——After the device is inserted, if there is a corresponding driver, it will load automatically.

So in the device tree, we only need to configure the MMC1 interface, and then compile the corresponding driver ko, and load it into rootfs.

The device tree configuration reference is below:

&mmc1 {       broken-cd;       bus-width = <4>;       vmmc-supply = <&reg_vcc3v3>;       status = "okay"};

The compiled ko file is under the overlay directory of buildroot, you can refer to the materials below:

r8723bs.ko is the compiled kernel module, this module backports the driver from the 5.19 kernel, making it more stable compared to the 5.15 driver.

In addition to the ko file, you also need to load the network card firmware, refer to the directory below:

The rtl8723bs_nic.bin can be found on GitHub or other websites.

To connect to WiFi, refer to the script below:

[root@buildroot ~]# cat conn.shmodprobe r8723bs.kowpa_supplicant -B -i wlan0 -c /etc/wpa_supplicant.confudhcpc -i wlan0

Refer to the configuration file below:

ctrl_interface=/var/run/wpa_supplicantap_scan=1

network={        ssid="###############"        psk="###############"}

⑥ ADC Keys

The test file has been open-sourced, no need to elaborate, if you want to use it directly, please refer to the open-source URL at the end.

⑦ Audio Playback

Buildroot integrates alsa, the default sound card is muted, open the terminal, input alsamixer, first unmute:

In the current interface, select Headphone, then press M to unmute, and use the keyboard ↑ to adjust the volume to an appropriate level. The interface can refer to Figure 12:

For 【project source files + video demonstration files】, please “scan” or “copy the link” to enter the open-source page.

If you have any questions, you can comment and consult the original author under the open-source project!