maxgerhardt / minimal-esp32-sdk

Implements a no-dependencies (e.g. to ESP-IDF or Arduino-ESP32) SDK for the ESP32.

Minimal ESP32 SDK

Description

This project implements a bare-metal ESP32 firmware that is loaded as second-stage bootloader (but it isn't) from the ESP32 first-stage bootloader stored in its mask-ROM.

Startup Execution Flow

As described in ESP-IDF docs.

When the ESP32 powers on, it executes the first-stage bootloader stored in its mask-ROM (unchangable, factory-programmed).

Once it finishes basic execution and hardware init, it attempts to load the second-stage bootloader from flash offset 0x1000. It loads it into IRAM and then executes it.

The linker script provides the function call_start_cpu0() as the entry point.

After we've been loaded by the first-stage bootloader, we can do anything we want. Doesn't have to be loading another application.

Current State

The current code and PlatformIO setup flashes the application to the correct offset, so that the first-stage bootloader will load it.

The firmware then does some initialization of the BSS, clear some instruction cache stuff and then use some functions provided to us in mask-ROM to do a ets_printf().

Further, we must periodically clear the watchdogs, that is, the RTC Watchdog and the Timer group 0, timer 0 watchdog (responsible for CPU0).

Otherwise it just sits in a loop and prints Loop!.

"c:\users\max\appdata\local\programs\python\python38\python.exe" "C:\Users\Max\.platformio\packages\tool-esptoolpy\esptool.py" --chip esp32 --port "COM9" --baud 460800 --before default_reset --after hard_reset write_flash -z --flash_mode dio --flash_freq 40m --flash_size detect 0x1000 .pio\build\esp32dev\firmware.bin
[...]
--- Miniterm on COM9  115200,8,N,1 ---
--- Quit: Ctrl+C | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
ets Jun  8 2016 00:22:57

rst:0x1 (POWERON_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
ets Jun  8 2016 00:22:57

rst:0x10 (RTCWDT_RTC_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:2
load:0x3fff0000,len:28
ho 0 tail 12 room 4
load:0x40080400,len:256
entry 0x400804e0
Hello, world!
Looping!
Looping!
Looping!
Looping!

Progress and Goals

• create image that is bootable by the first-stage bootloader
• print startup message using printf()-style function
• reset watchdogs properly so that firmware runs forever
• toggle LED using GPIO registers only
• relocate interrupt vector (this is what sdk does, too, without this we can't have user-controlled ISRs )
• use timer registers and ISR
• implement drivers?
• start up second CPU?

Issues / Quirks