Bootloader

• Pick an address (for now)
  • In the future, I should probably store this somewhere that can be re-flashed

Memory map

Settings Page

• I2C address?
  • App
  • Bootloader
• Listen to pin(s) for bootload?
• Listen to pin(s) for I2C addr?
• Checksum

Do I want a separate command for changing settings? Or just allow it like any other subpage write?

Do sanity checks on settings?

Procedure

1. Bootloader boots
   • Disable all interrupts
2. Is button held?
   • Y: GOTO 5
   • N: Continue
3. Is bootload RAM flag set + magic word valid?
   • Y: GOTO 5
   • N: Continue
4. Boot app at 0x0000_0000
5. Bootload.
   • LED blink?
6. Listen to I2C
   • Write transactions:
     • 0x40 - Start bootload
       • 4 bytes - total checksum (later crc32)
       • 1 byte - total subpages to write
         • NOTE: must be less than 23 * 8 for now
     • 0x41 - write page command
       • Must have set 0x40 already
       • Always 1 + 1 + 256 + 4 bytes
         • 1: 0x41
         • 1: Page/Subpage
           • 0bPPPPP_SSS (32 pages, 8 sub-pages)
           • 0bPPPPP must be <= 23
         • 256: subpage contents
         • 4 byte: For now: 32-bit checksum. Later, CRC32 or Poly1305?
       • On writes to subpage zero: erase page
       • Stretch write ack until checksum + erase + write?
         • I can't use the flash while erasing (or writing?) anyway
       • On writes to 0:0: make sure the reset vector is 0x0800_C000
         • Make sure MSP is maxval?
         • 0:0 must be the last thing written
     • 0x42 - reboot to new image
     • 0x43 - abort bootload
       • No data
     • 0x44 - Offer image downstream
       • Only after finishing 0x40 transaction
   • Read transactions
     • wr-then-rd 0x10 + 16 bytes => b'sprocket boot!!!'
     • wr-then-rd 0x11 + 4 bytes => maj.min.triv.reserved
     • wr-then-rd [0x21, P:S] + 260 bytes => Read subpage
     • wr-then-rd 0x22 + 1 byte => status
     • wr-then-rd 0x23 + 4 bytes => children flashed

Status

Represent:

• Is bootload (of me): idle, active, complete, error (2-bit)
• Does my app look reasonable? yes/no (1-bit)
• Is bootload (of chain): invalid, idle, active (next), active (downstream), complete

TODO: On (critical?) error?

• Finish current transaction
• Disable I2C (auto-naks)
• reboot
• Probably not for checksum errors, unless a lot

Compile time to-do for bootloader

• Change memory range
• Disable interrupts
• Ensure Reset is at base of flash (add linker assert?)