STM32MP135 Dev Board Buildroot

A Buildroot external tree for a custom PCB based on an MYiR MYC-YF135-256N SoM.

TODO: Add PCB Design Files

Harwdare

• STM32MP135DAF7 inside the MYC-YF135-256N SoM from MYiR Electonics.
  • 1 GHz
  • 256MB RAM
  • 256MB NAND (not used in this example)
• Silicon Labs WF200D WiFi chipset.
• Ti PCM3060 audio codec.
• 40-pin LCD, and 6-pin CTP connector compatible with Orient Display 480x272 TFT LCD panel.

Current State

• Uses Linux kernel v6.1 (from STM)
• Uses U-Boot 2023.10 (from STM)
• Uses TF-A v2.1 (from STM)
• Uses Optee-Os v4.0.0 (from STM)
• Tested with Buildroot 2023.05
• Uses mainline STM32 dtsi where possible, however some of the dt-bindings for the STM32MP135 have not made it to the mainline u-boot and Linux source yet. Fortunately STM provides customised TF-A, Optee, U-Boot, and Linux.
• Includes a file system overlay with some configuration, but boot/extlinux.conf is the only file which is actually required.
• Some other files in the overlay folder are just to override defaults or test features but are not strictly required.

Usage

Use like any external buildroot tree:

• Clone this repository, as well as Buildroot.
• In the Buildroot directory, create a config file based on this external tree:

BR2_EXTERNAL=../STM32MP135_Dev_Board_Buildroot make stm32mp135_dev_board_defconfig

• Review or modify the system in make menuconfig.
• Build the image using make.
• NOTE: There is an issue with UBoot the first time you try to build, see below.
• Use dd, Rufus, or your software of choice to image an SD card with the output/images/sdcard.img file.

Issues

• A patch is included to modify the conf.mk file in output/build/optee-os-custom/core/arch/arm/plat-stm32mp1 to support 256MB RAM. Further information on the process is described by STM here. If you're getting errors about things being in the wrong memory location it could be a related issue.

WARNING: The load address in optee header 0xce000000 - 0xce081310 is not in reserved area: 0xce200000 - 0xd0000000.
ERROR: OPTEE header parse error.

• NOTE: That it seems as though the NAU88c22 chip will only support I2C bus speeds up to 100khz. Although it will respont to ACK requests at a higher speed, it does not reset properly at this speed.

Bugs and TODO

• Some sort of soft start or larger capacitor is required across VSYS on BQ256xx chip (https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1021465/faq-why-vsys-does-not-start-up-normally-when-there-is-a-loading-on-the-sys-and-the-battery-is-not-present-for-bq25601-bq25601d-bq25606-bq25611d-bq25616-bq25616j-bq25618-bq25619-bq25600-bq25600d-bq25600c).
• Extra capacitor across VSYS to help with ripple
• Add a choke, or something, to help switchmode noise getting back up the power input.
• Add DC blocking capacitors to speaker outputs
• MMC CD line is incorrect
• A better wifi chipset
• LCD backlight voltage too damn high; change FB resistors (150k, 6.8k seems like a good number).
• Check if touch panel is even working.
• Charge and Power LEDs are wrong polarity.

The Overlay Folder

There are a few config files in the overlay file for either quality of life, or to make things actually work. Most are optional depending, or may even conflict with your desired settings.

• /boot/extlinux.conf - Required to boot linux. U-Boot will look for this.
• /etc/init.d/S40xorg - A dummy file to overwrite a default which is creatred when Xorg compiles.
• /etc/opkg/distfeeds.conf - Contains OpenWRT package feed. Need to force it to search the correct architecture when using opkg.
• /etc/X11/xorg.conf - Sometimes the default are bad, sometimes they aren't.
• /etc/directfbrc - DirectFB config file because it doesn't seem to use FBDev by default, even though it says it does.
• /lib/firmwmare/wfx - Firmware files for some wifi cards. SiliconLabs GitHub.
• /root - Just some extra files for testing and playing with.

Trips and Tricks

Some things that caught me out, or helped me out, along the way:

• I had trouble figuring out how to convince ARM Trusted Firmwawre to build a FIP package from within Buildroot. For reference, or if you want, or need, to create one manually you can do so from within the buildroot/output/build/arm-trusted-firmware/build/stm32mp1/debug directory:
  

fiptool create --tos-fw bl32.bin --fw-config fdts/stm32mp135_dev_board-tf-a-fw-config.dtb --hw-config u-boot.dtb --nt-fw u-boot.bin --tos-fw-config fdts/stm32mp135_dev_board-tf-a-bl32.dtb fip.bin

Where the u-boot.bin, and u-boot.dtb files are found in the buildroot/output/build/uboot directory.

• Remember that the FIP package contains the second stage bootloader, and loads the U-Boot DTB file. So if you modify the device tree you will need to manually rebuild U-Boot, and ARM Trusted Firmware, and probably the Linux kernel as well depending on your changes.

1. make optee-os-rebuild
2. make linux-rebuild
3. make uboot-rebuild
4. make arm-trusted-firmware-rebuild
5. Finally; make to generate the image.

• I had to install libssl-dev for the OpenSSL Headers in order to build ATF.

• The WiFi was getting buffer errors when the SPI bus was running at 42MHz. It seems to be fine at 24MHz, it might be able to run a bit quicker but I haven't tried yet.

• The /etc/X11/xinit/xinitrc file, by default, opens several xterm windows and starts a window manager. You may want to check and modify the last few lines of this file to suit your needs.