What is a bootable SD card?
- A bootable SD card is an SD card that has a bootloader installed on it.
- A bootable SD card is also known as a boot disk, which your SBC can boot-up from.
Why do we need a bootable SD card?
- You can release your custom ROM as a bootable SD card, in order to speed up the testing and development process.
- You can use a bootable SD card to do system/file recovery, in the event that your SBC is unable to boot from the eMMC.
The process for VIM1, VIM2, VIM3, VIM3L and VIM4 is similar, so we will use VIM1 as an example.
Download U-Boot for (VIM1/VIM2/VIM3/VIM4), or get the correct bootloader blob for your SD card and manually build U-Boot. Regardless of which method you choose, you need to remember that there are different bootloader blobs for different boot disks/media:
u-boot.bin.sd.binis for SD-Cards
u-boot.binis for eMMC storage
u-boot.bin.sd.bin.signedis for SD cards
u-boot.bin.signedis for eMMC storage
u-boot.bin.spi.bin.signedis for SPI flash
Insert the SD card into your PC, and unmount it:
$ sudo umount /dev/sdX1
Format the SD card as FAT32:
$ sudo mkfs.vfat /dev/sdX1
dd to write the U-Boot blob into the first sector of SD card:
$ sudo dd if=u-boot.bin.sd.bin of=/dev/sdX conv=fsync,notrunc bs=1 count=444
Eject the SD card from your PC:
$ sudo eject /dev/sdX
sdXwith the correct device node on your PC.
Connect your PC to your SBC using a Serial to USB tool.
Open Terminal and type
sudo minicom to open a serial connection to your SBC.
Insert the Bootable SD card into your SBC and power on.
If your SBC has successfully booted from the SD card, you should get this Terminal print-out:
In rare cases, you may need to erase the eMMC in order to boot from an SD card.