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 MP15 reads the U-Boot environment (including the MAC addresses) from a UBI volume called uboot_config or from its redundant copy in UBI volume uboot_config_r in the NAND flash. If it does not find a valid environment on the NAND, U-Boot uses a hard-coded default environment:

  • The size of the environment is determined by constant CONFIG_ENV_SIZE.

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       Erase-size    No.Blocks
/dev/ubi0:uboot_config     0x0      0x20000    0x20000
/dev/ubi0:uboot_config_r   0x0      0x20000    0x20000

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 MP15

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 MP15 during manufacturing and stores them in the Wi-Fi chip OTP bits, keeping a copy in the U-Boot environment on the NAND.

You can find the Digi-assigned MAC address on the ConnectCore MP15 module label. See Determine Digi MAC addresses for more information.

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 MP15 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

The MAC address and serial number of the SOM are encoded in the data matrix on the SOM label.

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