Boot Process

Overview

• 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}.

Flow

ResetHandler

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

_start

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

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

References

• Chinese seller with boot modes
• Translated boot modes
• ESP8266_Module Application Design Guide Chinese.pdf
• OTA bootloader, stage 1