Boot Process

The boot process is a bit of madness, but justifiable madness.

Overview

The LPC4320 bootloader initializes the Cortex-M4F core to boot from the start of external SPI flash. The M0 core stays in reset. The bootstrap code runs from SPI flash, on the Cortex-M4F. The bootstrap initializes the Cortex-M0 to execute the application code from SPI flash, then sleeps. The application code copies the baseband code into RAM, configures the Cortex-M4F to run from RAM, then resets the Cortex-M4F to begin baseband execution.

(TODO: A diagram would be helpful, showing the M4F and M0 activities vs. time.)

Bootstrap

In the PortaPack image, the Cortex-M4F "bootstrap" image is located at the start of SPI flash. It's executed by the LPC4320 built-in bootloader. The M4 clock is already set to 96MHz. The bootstrap code configures SPIFI to run at (approximately) maximum speed. Then, it initializes the Cortex-M0's memory map to point at the "application" image in SPI flash, and releases the Cortex-M0 from reset. The bootstrap then sleeps the Cortex-M4 until the M0 application needs to run baseband firmware on it.

Application

On the Cortex-M0 core, boot time looks like this:

ResetHandler:
    Initialize process stack pointer
    Initialize stack RAM regions (fill with pattern)
    __early_init()
        Enable extra processor exceptions for debugging
    Init data segment (copy SPI flash -> data region in RAM)
    Initialize BSS (fill RAM region with 0)
    __late_init()
        reset()
            Reset most peripherals -- not SCU, SPIFI, or M0APP
        halInit()
            hal_lld_init()
                Init timer 3 as cycle counter (no DWT on M0)
                Init RIT as SysTick (no SysTick on M0)
            palInit()
            gptInit()
            i2cInit()
            sdcInit()
            spiInit()
            rtcInit()
            boardInit()
        chSysInit()
            port_init()
            _scheduler_init()
            _vt_init()
            _core_init()
            _heap_init()
            chSysEnable()
            chThdCreateStatic(_idle_thread_wa, ...)
    Constructors
    main()
    Destructors
    _default_exit()
        while(1);

Baseband

On the Cortex-M4F core, these are the stages a baseband image moves through:

ResetHandler:
    Initialize process stack pointer
    Initialize FPU
    Initialize stack RAM regions (fill with pattern)
    __early_init()
        Enable extra processor exceptions for debugging
    Init data segment (copy SPI flash -> data region in RAM)
    Initialize BSS (fill RAM region with 0)
    __late_init()
        halInit()
            hal_lld_init()
                Init SysTick
                Init DWT as cycle counter
            # Baseband controls no hardware, so no hardware init here.
            boardInit()
        chSysInit()
            port_init()
            _scheduler_init()
            _vt_init()
            _core_init()
            _heap_init()
            chSysEnable()
            chThdCreateStatic(_idle_thread_wa, ...)
    Constructors
    main()
    Destructors
    _default_exit()
        while(1);

Original Wiki by sharebrained at Boot Process