The basic idea of this design is to connect to an external PC through a DFU. The PC stores three bootloaders. The first bootloader is a Keywriter, which is used to burn the key. The second bootloader is based on the SBL_Uniflash provided by the SDK. SBL_Uniflash is pre-signed and encrypted with the key from the customer. After booting, SBL_Uniflash burns the normal application files and bootloader3 (pre-signed and encrypted with the customer's key) to related offsets in the flash memory. Upon power up, the Boot ROM reads the application bootloader from the primary boot mode, which is OSPI and boots the SOC.

Requirements

Boot mode => Primary: OSPI, Backup: UART/DFU (DFU preferable).

Flash is completely empty. More importantly, the flash is erased at 0x0 and 0x400000 offsets so that ROM fallbacks to backup boot.

Procedure

1. On First POR: Device state: HS-FS. ROM boots OTP Keywriter from the selected backup boot media. Program the Keys according to the Keywriter guide. Convert HS-FS to HS-SE.
2. On Second POR: Device state: HS-SE.ROM boots flash writer from the selected backup boot media. Flash the SDK images (SBL/Applications)
3. On Third POR: Device state: HS-SE. ROM boots from the primary boot media OSPI.