The U-Boot environment is a block of memory in persistent storage. It is used to store variables in the form name=value.

Where is the U-Boot environment?

The ConnectCore 8M Nano reads the U-Boot environment (including the MAC addresses) from the boot media, which means:

  • If the device boots from the eMMC, it reads the environment from the eMMC.

  • If the device boots from microSD card, it reads the environment from the microSD card.

  • If it does not find a valid environment on the boot media, it uses a hard-coded default environment.

When the ConnectCore 8M Nano boots from the eMMC, it reads U-Boot from eMMC boot partition 1 (device /dev/mmcblk0boot0 in Linux) and reads the U-Boot environment (including the MAC addresses) from eMMC boot partition 2 (device /dev/mmcblk0boot1 in Linux).

  • The environment is located at the offset determined by constant CONFIG_ENV_OFFSET in U-Boot source code

  • The size of the environment is determined by constant CONFIG_ENV_SIZE in U-Boot source code

  • There is a back-up copy of the environment in the same partition (where the original copy is) at the offset determined by constant CONFIG_ENV_OFFSET_REDUND in U-Boot source code

Find these constants at the configuration file configs/ccimx8mn_dvk_defconfig.

Access U-Boot environment from Linux

You can access the U-Boot environment from a Linux application. In the U-Boot source tree, you can find the environment tools in the directory tools/env. Digi Embedded Yocto compiles this tool by default and installs it on your target’s root file system as two executable files:

  • fw_printenv: to print the value of variables.

  • fw_setenv: to set the value of variables.

The tools work with the configuration file /etc/fw_env.config, which must contain two entries in the form: Device name, offset, size. The first entry must point to the U-Boot environment location. The second must point to its redundant copy. For example:

/etc/fw_env.config
# Configuration file for fw_(printenv/setenv) utility.
# Up to two entries are valid, in this case the redundant
# environment sector is assumed present.
# Device name        Offset      Size
/dev/mmcblk0boot1    0x1C0000    0x4000
/dev/mmcblk0boot1    0x1E0000    0x4000

Append boot arguments to the kernel command line

You can use the extra_bootargs variables to append commands to the default kernel command line.

Important U-Boot environment variables on the ConnectCore 8M Nano

The following U-Boot environment variables are worth mentioning:

MAC addresses

Variable Description Flags

$ethaddr

MAC address of the first wired Ethernet interface

change-default

$eth1addr

MAC address of the second wired Ethernet interface if there is one

change-default

$wlanaddr

MAC address of the Wi-Fi interface

change-default

$btaddr

MAC address of the Bluetooth interface

change-default

About flags

  • Variables with the write-once flag are protected and will not be overwritten by setenv or env default commands (unless manually forced with -f option).

  • Variables with the change-default flag can only be written once using setenv command (unless manually forced with -f option).

Digi programs the MAC addresses of the ConnectCore 8M Nano during manufacturing and saves them in the U-Boot environment on the eMMC.

If the device boots from an external media different from the eMMC (for example microSD card), the system reads the U-Boot environment from it, so MAC addresses will not match the ones assigned by Digi during manufacturing. You must program the MAC addresses of the available interfaces and save the environment on your boot storage media.

You can find the Digi-assigned MAC address on the ConnectCore 8M Nano module label. See Determine Digi MAC addresses for more information.

Wireless virtual MAC addresses

The wireless interface on the ConnectCore 8M Nano module allows you to define up to three optional virtual interfaces so that the platform can run concurrently as station, p2p, and SoftAP. Digi neither reserves nor programs unique MAC addresses for these virtual interfaces. A user who wishes to assign unique MAC addresses to such virtual interfaces can use the following U-Boot environment variables:

Variable Description Flags

$wlan1addr

MAC address of virtual wireless interface 1

change-default

$wlan2addr

MAC address of virtual wireless interface 2

change-default

$wlan3addr

MAC address of virtual wireless interface 3

change-default

MMC boot device

Variable Description Flags

$mmcbootdev

MMC device index where U-Boot booted from

write-once

During start-up, U-Boot initially sets this variable to the MMC device index where the device booted from:

  • 0 if booting from internal eMMC (USDHC3)

  • 1 if booting from microSD card (USDHC2)

You can use this variable to determine which MMC device the ConnectCore 8M Nano booted from.

MMC device (update target)

Variable Description Flags

$mmcdev

MMC device index to update firmware to

-

The update command uses this variable to determine the MMC device index to write the firmware to:

  • 0 for the internal eMMC (USDHC3)

  • 1 for the microSD card (USDHC2)

You can modify this variable as needed.

Module variant

Variable Description Flags

$module_variant

Variant ID code for the System-On-Chip

write-once

During start-up, U-Boot automatically sets this variable to the ConnectCore 8M Nano System-On-Chip variant ID number (a hexadecimal code programmed in the SOM one-time programmable bits).

Carrier board version and ID

Variable Description Flags

$board_version

Version number of the carrier board

write-once

$board_id

ID number of the carrier board

write-once

These optional write-once variables can be set to the carrier board’s version and ID numbers during manufacturing. See Carrier board version and ID.

Determine Digi MAC addresses

Sequential MAC address scheme

Digi assigns MAC addresses according to a sequential scheme. The order of assignment depends on the available interfaces, but the scheme always respects the order Ethernet 1, Ethernet 2, Wi-Fi, Bluetooth and skips non-available interfaces.

Determine MAC addresses

You can determine the MAC addresses for your device either by reading the Ethernet MAC off the label and using the scheme to calculate subsequent addresses, or by reading them from your device. To read the stock environment variables from the device using the printenv command executed at the U-Boot prompt:

To get first Ethernet MAC address:

=> printenv ethaddr

If your SOM has a second Ethernet MAC address:

=> printenv eth1addr

To get Wi-Fi MAC address:

=> printenv wlanaddr

To get Bluetooth MAC address:

=> printenv btaddr