TS-4100 U-boot Sections

This platform includes U-Boot as the bootloader to load and boot the full operating system. The i.MX6UL processor loads U-Boot from the eMMC flash at power-on. U-Boot allows booting images from the microSD, eMMC, NFS, or USB. U-Boot is a general purpose bootloader that is capable of booting into common Linux distributions, Android, QNX, or others.

On a normal boot, output from U-Boot will be similar to the following:

U-Boot 2016.03-00367-gb3ce405827 (Jul 02 2019 - 16:40:10 -0700)

CPU:   Freescale i.MX6UL rev1.1 at 396 MHz
Reset cause: POR
Board: Technologic Systems TS-4100
FPGA:  Rev 11
Silab: Rev 2
I2C:   ready
DRAM:  1 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
Baseboard ID: 0x8
Baseboard Rev: 0
Net:   FEC0 [PRIME]
Press Ctrl+C to abort autoboot in 1 second(s)
starting USB...
USB0:   USB EHCI 1.00
scanning bus 0 for devices... 1 USB Device(s) found
USB1:   USB EHCI 1.00
scanning bus 1 for devices... 1 USB Device(s) found
       scanning usb for storage devices... 0 Storage Device(s) found
No storage devices, perhaps not 'usb start'ed..?
Booting from the SD card ...
** File not found /boot/boot.scr **
** File not found /boot/ts4100-fpga.vme **
29205 bytes read in 146 ms (195.3 KiB/s)
8 detected
5244088 bytes read in 390 ms (12.8 MiB/s)
Kernel image @ 0x80800000 [ 0x000000 - 0x5004b8 ]
## Flattened Device Tree blob at 83000000
   Booting using the fdt blob at 0x83000000
   Using Device Tree in place at 83000000, end 8300a214

Starting kernel ...

When running standalone, the TS-4100 will default to booting from eMMC. This behavior can be changed by modifying the U-Boot environment variable "force_jpsdboot". Normally this variable does not exist in the U-Boot environment, if it does exist, setting it to a value of 1 will force U-Boot to attempt to boot from microSD. If it is set to a value of 0, then U-Boot will attempt to boot from eMMC. Additionally, running U-Boot commands are used direct booting to another source such as NFS, USB, or microSD.

Many of our compatible baseboards offer an "SD Boot" jumper to control the behavior between SD and eMMC. See the specific baseboard manual for the combination being used for more detailed information. If designing a custom baseboard, we recommend reviewing our custom baseboard design recommendations for better controlling boot flow with U-Boot.

Entering U-Boot Shell

The U-Boot shell is a powerful tool. It allows modification of the environment, as well as the ability to run commands directly.

When the TS-4100 is booted standalone, a prompt is provided by U-Boot to press "ctrl+c" followed by a 1 second window. If "ctrl+c" is found incoming on the serial terminal within this time window then U-Boot will drop to its shell. This behavior can be modified by setting the U-Boot environment variable "force_bootdelay". Normally this variable does not exist in U-Boot environment, if it does, it will force the timeout for pressing "ctrl+c" to be however many seconds the variable is set to. If it is set to 0, then the "ctrl+c" prompt is skipped.

Note:

Use caution when setting "force_bootdelay", a value of 0 means it is no longer possible to enter the U-Boot shell without booting the device on a compatible baseboard that has a "U-Boot" jumper!

On our compatible baseboards or custom designed baseboards there are two ways to break in to the U-Boot shell. Note that some compatible baseboards may still offer a prompt as outlined above. See the specific baseboard manual for the combination being used for more detailed information.

The baseboard has a "U-Boot" jumper. When set, this will cause U-Boot to read a script file from an attached USB mass storage device before dropping to the U-Boot shell. When the jumper is not set, the unit will boot as normal. See our custom baseboard design recommendations for information on how the U-Boot jumper should be set up in order to be supported by our default U-Boot configuration.

The baseboard offers a push switch which is broken out from pin 9 on CN1 of the TS-4100. This button can be held down before applying power, and at least 5 seconds after power is applied.

This allows for out of the box functionality and customized production via a USB drive. In order to provide increased security, the push switch entry to U-Boot can be disabled via the U-Boot environment variable rstuboot. The following U-Boot commands can be used to disable the push switch check, thereby preventing it from being able to enter the U-Boot shell.

env set rstuboot 0
env save

Removing the rstuboot environment variable, or setting it to a 1, will re-enable the ability for the push switch to interrupt U-Boot and drop to its shell. See our custom baseboard design recommendations for information on how the push switch should be set up in order to be supported by our default U-Boot configuration.

On baseboards that support both a jumper and the push switch, disabling the push switch will still allow the jumper method of entering the U-Boot shell.