The eMMC v5.1 controller can serve as boot interface for one of the autonomous boot modes. The controller supports the FAT 16/32 file systems and select raw partition combinations for reading the boot images. Raw partitions, not formatted and without a file system, are supported for primary boot mode, the boot partition 1, boot partition 2, or the user area of the eMMC. Using eMMC FAT 16/32 file system for primary boot mode and eMMC raw as secondary boot mode is not supported. The image search limit for eMMC boot mode is listed in Boot Search Limit section.
In the eMMC boot mode, the RCU BootROM runs at an eMMC device clock frequency between 8.7 MHz and 19.3 MHz dependent on the REF_CLK setting. The eMMC boot mode supports 1.8V and 1-bit, 4-bit, and 8-bit data interfaces. The BootROM uses auto-width detection to determine the data bus width for initial boot. The auto-bus width detection starts by checking the 8-bit data bus width, followed by 4-bit data bus width, and then 1-bit data bus width.