The Kontron SMARC-sAL28 board is a TSN-enabled dual-core ARM A72 processor module with an on-chip 6-port TSN switch and a 3D GPU.


Compile U-Boot

Configure and compile the binary:

$ make kontron_sl28_defconfig
$ CROSS_COMPILE=aarch64-linux-gnu make

Copy u-boot.rom to a TFTP server.

Install the bootloader on the board

Please note, this bootloader doesn’t support the builtin watchdog (yet), therefore you have to disable it, see below. Otherwise you’ll end up in the failsafe bootloader on every reset:

> tftp path/to/u-boot.rom
> sf probe 0
> sf update $fileaddr 0x210000 $filesize

The board is fully failsafe, you can’t break anything. But because you’ve disabled the builtin watchdog you might have to manually enter failsafe mode by asserting the FORCE_RECOV# line during board reset.

Disable the builtin watchdog

  • boot into the failsafe bootloader, either by asserting the FORCE_RECOV# line or if you still have the original bootloader installed you can use the command:

    > wdt dev cpld_watchdog@4a; wdt expire 1
  • in the failsafe bootloader use the “sl28 nvm” command to disable the automatic start of the builtin watchdog:

    > sl28 nvm 0008
  • power-cycle the board

Useful I2C tricks

The board has a board management controller which is not supported in u-boot (yet). But you can use the i2c command to access it.

  • reset into failsafe bootloader:

    > i2c mw 4a 5.1 0; i2c mw 4a 6.1 6b; i2c mw 4a 4.1 42
  • read board management controller version:

    > i2c md 4a 3.1 1

Non-volatile Board Configuration Bits

The board has 16 configuration bits which are stored in the CPLD and are non-volatile. These can be changed by the sl28 nvm command.

Bit Description
0 Power-on inhibit
1 Enable eMMC boot
2 Enable watchdog by default
3 Disable failsafe watchdog by default
4 Clock generator selection bit 0
5 Clock generator selection bit 1
6 Disable CPU SerDes clock #2 and PCIe-A clock output
7 Disable PCIe-B and PCIe-C clock output
8 Keep onboard PHYs in reset
9 Keep USB hub in reset
10 Keep eDP-to-LVDS converter in reset
11 Enable I2C stuck recovery on I2C PM and I2C GP busses
12 Enable automatic onboard PHY H/W reset
13 reserved
14 Used by the RCW to determine boot source
15 Used by the RCW to determine boot source

Please note, that if the board is in failsafe mode, the bits will have the factory defaults, ie. all bits are off.

Power-On Inhibit

If this is set, the board doesn’t automatically turn on when power is applied. Instead, the user has to either toggle the PWR_BTN# line or use any other wake-up source such as RTC alarm or Wake-on-LAN.

eMMC Boot

If this is set, the RCW will be fetched from the on-board eMMC at offset 1MiB. For further details, have a look at the Reset Configuration Word Documentation.


By default, the CPLD watchdog is enabled in failsafe mode. Using bits 2 and 3, the user can change its mode or disable it altogether.

Bit 2 Bit 3 Description
0 0 Watchdog enabled, failsafe mode
0 1 Watchdog disabled
1 0 Watchdog enabled, failsafe mode
1 1 Watchdog enabled, normal mode

Clock Generator Select

The board is prepared to supply different SerDes clock speeds. But for now, only setting 0 is supported, otherwise the CPU will hang because the PLL will not lock.

Clock Output Disable And Keep Devices In Reset

To safe power, the user might disable different devices and clock output of the board. It is not supported to disable the “CPU SerDes clock #2” for now, otherwise the CPU will hang because the PLL will not lock.

Automatic reset of the onboard PHYs

By default, there is no hardware reset of the onboard PHY. This is because for Wake-on-LAN, some registers have to retain their values. If you don’t use the WOL feature and a soft reset of the PHY is not enough you can enable the hardware reset. The onboard PHY hardware reset follows the power-on reset.