.How do I make U-boot run independently of the operating system?

U-boot seems to keep rebooting,The following is displayed:
DDR Version V1.26
REGFB: 0x00000032, 0x00000032
Col=10 Bank=8 Row=15 Size=512MB
Returning to boot ROM…
INFO: Preloader serial: 0
NOTICE: BL31: v1.3(release):3a579d3
NOTICE: BL31: Built : 09:09:17, Jan 29 2019
INFO: Lastlog: last=0x100000, realtime=0x102000, size=0x2000
INFO: ARM GICv2 driver initialized
INFO: Using opteed sec cpu_context!
INFO: boot cpu mask: 1
INFO: plat_rockchip_pmu_init: pd status 0xe
INFO: BL31: Initializing runtime services
WARNING: No OPTEE provided by BL2 boot loader, Booting device without OPTEE initialization. SMC`s destined for OPTEE will return SMC_UNK
ERROR: Error initializing runtime service opteed_fast
INFO: BL31: Preparing for EL3 exit to normal world
INFO: Entry point address = 0x600000
INFO: SPSR = 0x3c9
No serial driver found

DDR Version V1.26
REGFB: 0x00000033, 0x00000032
Col=10 Bank=8 Row=15 Size=512MB
Returning to boot ROM…
Is u-boot on the automatic boot system?I did not use the dd command to copy the Kernel image to the SD card.
I want the U-boot to run separately and then run the built-in command to load the kernel image into memory.
Is there any way to get it to cancel the boot system?
Build Debianhttps://wiki.radxa.com/RockpiS/dev/Debian

Press Ctrl-C to get to the u-boot shell. But there’s a super small delay for it by default (like 1 second or so)
(You’ll see the prompt for Ctrl-C in boot log when turn on the device)

Ctrl-c doesn’t work.I even changed the u-boot configuration file,setting “CONFIG_BOOTDELAY” to -1 won’t work (disable auto boot).
Maybe I made a mistake. It might not be the output of uboot.img.
This might be the output of idbloader.img.
The format of this other Uboot output is quite different.
“No serial driver found” is in the uboot source file。
./drivers/serial/serial-uclass.c:142: panic_str(“No serial driver found”);

What system image do you use? As I know, it is okay to enter U-Boot shell when using Radxa Official images.

I haven’t installed the operating system in the sd card, just want to let u-boot run independently first.
Then use the built-in nfs or tftp commands of uboot to load into the memory and run through the network.
I only put idbloader.img, uboot.img,trust.img files on the SD card。

You could follow the steps in this repo to get your uboot working :

Alternatively, you can build the system and run the SDcard, I can confirm that ctl-c works with the version of uboot written to sdcard in this buildroot os.

If for example, you comment out (with “#”) the boot and rootfs partitions, then the kernel and rootfs will not be written to the sdcard, only the bootloader, uboot and trust.img.