RK3399 can boot from SPI/SD card/eMMC, the SPI buses for booting is SPI1. On ROCK Pi 4 V1.3 and earlier, the design has included the SPI flash but it was not soldered. There are several reasons:
- The SPI is also exported on the 40PIN GPIO header, if we use it as boot rom, then we will not able to use the SPI bus on the GPIO header
- If the bootloader in the SPI flash corrupted, it will brick the ROCK Pi 4, which is hardware for normal user to recover.
- The bootloader doesn't support booting from USB3 or NVMe before and there are tools missing to write to SPI flash easily.
Some advanced users may want to solder the SPI flash by themself. The footprint for the SPI flash is SOP8, the voltage level is 3.3V. The size should be larger than 4MB(32Mbit) to store the bootloader. The verified working models are: