modified on 9 February 2012 at 21:15 ••• 124,922 views

Boot from SD/MMC

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Boot from SD/MMC

This procedure explains how to boot a kernel image and root filesystem from a microSD card. Booting from a microSD card gives applications a wide range of options including portability, multiple distributions, larger storage space, etc. The microSD card has lower disk I/O performance than NAND, but the difference is not significant for embedded applications that perform most operations in RAM. A NAND is not required when booting from a microSD card if the kernel is included with the root filesystem.

Note: The EXT2 filesystem does not implement journaling. Proper shutdown procedures should be followed.

Creating a Bootable Filesystem on microSD card using a Linux PC

1) Partition the microSD card and create a filesystem on the first partition. Generally this is done on a Linux PC. Insert the microSD card and enter the following commands:

$ fdisk /dev/mmcblk0

Command (m for help): o                          # create a new partition table
Building a new DOS disklabel with disk identifier 0xc57e78f0.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.

Command (m for help): n                          # create a new partition
Command action
   e   extended
   p   primary partition (1-4)
p                                                # select primary partition
Partition number (1-4): 1                        # select first partition (default)
First cylinder (1-1203, default 1):              # select first cylinder (default)
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-1203, default 1203): 
                                                 # select last cylinder (default)
Using default value 1203

Command (m for help): a                          # set boot blag
Partition number (1-4): 1                        # for first partition

Command (m for help): w                          # write changes
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
$ mkfs -t ext2  /dev/mmcblk0p1
$ mkdir /mnt/sdcard
$ mount /dev/mmcblk0p1 /mnt/sdcard

2) Copy the desired root filesystem to the microSD card. The filesystem should contain a kernel (e.g. uImage- in the top directory and the corresponding modules directory in /lib/modules. The command below assumes the root filesystem is in the /home/electrum/rootfs/ directory of your Linux PC. On most Linux distributions you need to be running as root to copy device (/dev) nodes. This copy may take several minutes depending on the filesystem contents and the speed (class) of your card.

# cp -pr /home/electrum/rootfs/* /mnt/sdcard

3) The microSD card should now contain your root filesystem. Confirm the disk usage with the df command prior to unmounting.

$ df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mmcblk0p1         7707056    516216   6799340   8% /mnt/sdcard
$ umount /mnt/sdcard

Your microSD card now has a bootable system. The next step is to change the bootloader configuration to boot from it instead of NAND.

Creating a Bootable Filesystem on microSD card using a Windows PC

1) Download the file.

2) Download the latest Linux and file system image file.

3) Unzip the files into a single directory.

4) Insert the microSD card into a USB to SD card adapter. Note: win32diskimager does not function well with card readers built into PCs.

5) Open the Win32DiskImager application.

6) If Windows asks allow the Win32DiskImager application to run.

7) Browse for the desired image file in the directory where it was unzipped.

8) Make sure the correct drive is selected.

9) Click on the "Write" button.

Booting from the microSD Card

1) Enter the U-boot console via the serial console port (COM1) to change the bootloader configuration you need to . If the board is up, reboot using

# shutdown -r now

When the U-Boot messages appear on the console, stop the automatic boot by pressing SPACE twice.

2) Insert the microSD card and confirm it is accessible using the rescan command.

Electrum> mmc rescan 0

3) Change the bootargs and bootcmd parameters. List the current values in case you need to restore them. Use the kernel version loaded in your card (uImage- in the example).

Electrum> printenv bootargs
bootargs=console=ttyS0,115200 root=/dev/mtdblock6 mtdparts=atmel_nand:256k(bootstrap)ro,256k(uboot)ro,256k(env1),
256k(env2),1024k(user),2M(linux),-(root) rw rootfstype=jffs2

Electrum> setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p1 rw ip=

Electrum> printenv bootcmd
bootcmd=nand read 0x22000000 0x200000 0x200000; bootm

Electrum> setenv bootcmd mmcinfo\; ext2load mmc 0:1 0x22000000 uImage-\; bootm

4) Save the bootloader environment and boot the board.

Electrum>> saveenv
Electrum>> boot