I am learning some development kits, and find some confusing about the boot mode config. Hope some experts can share some ideas. I found some suppliers development kits boot setting as below:
For eMMC boot：
My confusing is why they both select 4 bit mode, instead of 8 bit, although the schematic design is 8bit. connect both SD0-SD7. I also see some board set 8bit also can work fine. Does the eMMC can be work under both 4bit and 8bit? what's the difference? If the development kits use 4bits mode, any impact if I change to the 8 bits then? Will the eMMC still working? any impact to the SW load?
For SD boot:
Same confusing, as the table, they use 1bit mode, but the schematic connect 4bits SD0-SD3 to the TF card. what will happen if I change to 4 bit boot?
For the USB download:
According the user manual, should set the boot_mode[1:0]=0x01, but the board has fixed the boot_mode=0x10, then with above config setting, still can go into USB mode. Does this use the feature "once some error happened with internal boot, it will directly jump to USB mode"? So this config lead the MCU check the unconnected interface, like SD3, MCU find nothing at SD3, then directly go into USB mode. Is this the idea?