Boot Process

Max Filippov edited this page Apr 7, 2015 · 13 revisions


  • Reset vector is 0x40000080.
  • Boots into Espressif code in IROM0.
  • Loads SPI ROM data.
  • Starts executing ESP SDK-code shadowed SPI ROM (unconfirmed).

ESP Boot Modes

The Espressif code can boot in different modes, selected on power-up based on GPIO pin levels. (MTDO is equivalent to GPIO15).

MTDO GPIO0 GPIO2 Mode Description
L L H UART Download code from UART
L H H Flash Boot from SPI Flash
H x x SDIO Boot from SD-card

In the bootup message 'boot mode:(x,y)' three low bits of x are {MTDO, GPIO0, GPIO2}.



  • Set interrupt level 1
  • Set processor modes (see separate section)
  • Copy SROM data to SRAM
  • Goto _start


  • Set Ring 0
  • Clear callback vector
  • Set up stack at 3FFFFFFFh
  • Call main


  • Initialize UART0
    • 115,200 bps, 8N1
    • iomux.u0txd &= 0xE4F
    • iomux.gpio2 = (iomux.gpio2 & 0xECF) | 0x100

Processor Modes

  • Only Ring 0 used
  • TLBs (D and I) are set to
    • 00000000h-1FFFFFFFh: Illegal
    • 20000000h-5FFFFFFFh: RWX, Cache Write-Through
    • 60000000h-FFFFFFFFh: RWX, Bypass Cache