Here is the video where it is explained step by step how to prevent U-boot console from interrupting autoboot and sending debug messages on UART. I know links only answers are frowned upon, so here' s a quick breakdown of a solution:
Install the dependencies
sudo apt install git make gcc gcc-aarch64-linux-gnu
Git clone the official u-boot repository. Alternatively you can git clone my fork of repository, where I already have the necessary changes for silent autoboot - but if you need the latest version, then you need to clone the official repository and make changes yourself.
git clone --depth 1 git://git.denx.de/u-boot.git
cd u-boot
Find the raspberry pi config files - they depend on the model, rpi_3_defconfig for Raspberry Pi 3, rpi_4_defconfig for Raspberry Pi 4 and so on.
Add the following lines to the end of the file
CONFIG_BOOTDELAY=-2
CONFIG_SILENT_CONSOLE=y
CONFIG_SYS_DEVICE_NULLDEV=y
CONFIG_SILENT_CONSOLE_UPDATE_ON_SET=y
CONFIG_SILENT_U_BOOT_ONLY=y
The first line removes the boot delay, so autoboot will not be interrupted by messages sent on UART interface. Next four lines enable silent boot, so U-boot will not send any messages on UART itself, because the messages might in turn confuse your device. One more little thing left, set silent boot environmental variable.Change include/configs/rpi.h file
#define CONFIG_EXTRA_ENV_SETTINGS \
"dhcpuboot=usb start; dhcp u-boot.uimg; bootm\0" \
"silent=1\0" \
ENV_DEVICE_SETTINGS \
ENV_DFU_SETTINGS \
ENV_MEM_LAYOUT_SETTINGS \
BOOTENV
Now configure with
make rpi_3_defconfig
from repository main folder
And build with
make CROSS_COMPILE=aarch64-linux-gnu-
When build process finishes you will have a u-boot.bin file, which you need to rename and copy to Raspberry Pi SD card. Now you Raspberry Pi will not be disturbed by any messages on UART during boot. The UART functionality after boot will not be affected.
Relevant docs:
https://gitlab.denx.de/u-boot/u-boot/blob/HEAD/doc/README.autoboot
https://gitlab.denx.de/u-boot/u-boot/blob/HEAD/doc/README.silent
https://wiki.ubuntu.com/ARM/RaspberryPi
sensors connected send garbage
.... the "garbage" could still contain the predefined key press