Principle analysis of bootloader

Catalog

1. The basic chapter

1.1 bootloader Realize five steps

1.1.1 Make an agreement

1.1.2 Fixed partition

1.1.3 flash Operation driven implementation

1.1.4 app Flag bit definition

1.1.5 Jump

1.2 Summary of the basic part

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1. The basic chapter

1.1 bootloader Realize five steps

1.1.1 Make an agreement

        Single frame data ： Frame head , Frame tail , Frame size , Frame check ;

        Single packet data （ Multiframe ）： Package start frame , Packet size frame , Packet data frame , Packet check frame ;

        Whole packet data （ Multi package ）： Start frame , End frame , Whole packet check frame ;

1.1.2 Fixed partition

        bootloader Partition ：boot Program store , Usually 0x8000000 Start

        Data storage access ： Firmware basic information storage area such as flag bit , Usually in boot Zoning and app In the middle of the partition ;

        app Partition ：app Program store , Leave enough space for the first two partitions , The rest can be defined as app District ;

1.1.3 flash Operation driven implementation

        flash The reading of , Write , Erase method implementation ;

        read ： Read saved in flash Some basic information in （ Such as logo information ,crc Check value , Basic information of firmware ）;

        Write ： Will receive bin File data is written to app Of flash Partition ; Write the basic information of firmware ;

        erase ： Upgrade initial stage , erase app Partition data, etc ;

1.1.4 app Flag bit definition

        One 、 Stored in flash in ： At present, the common processing logic of this method is , The flag bit exists bootloader Zoning and app A separate erasable area in the middle of the partition , When the upgrade is successful, put the flag at 1. The advantage of this method is simple logic ; The disadvantage is that the flag is not set before the upgrade process 1, Of course, you can make the flag position through single-step debugging 1.

        Two 、 Stored in ram in ： The method needs to be app Set correspondence in ram Address value ; More flexibility , Whether the burned program or the upgraded firmware can jump normally . It should be noted that bootloader and app Avoid this part ram Operation of space （ The box keil reset ram From ）.

1.1.5 Jump

        boot Jump to app, Can be said to be bootloader The core skill in , The basic functions can be realized in the following steps ：

        One 、 Close all interrupts ;

        Two 、 obtain app Entry function address （ namely app Partition start address +4）;

        3、 ... and 、 take app The entry address is assigned to the function pointer *start_app;

        Four 、 Set the main stack address （ namely app The address stored in the starting address of the partition ）;

        5、 ... and 、 call app Entry function start_app();

Be careful , These steps are in order . Think about why ？

         One , Closing all interrupts before jump is to prevent the program from running error . Because of this 5 The first step does not reset the interrupt vector table , So when you call start_app() after , Before setting the interrupt vector table , Use or bootloader The interrupt .

        Two and three , It's to get app Entry function address , Be sure to get before setting the main stack address . If you set the main stack address and then get app The starting address will be wrong . because start_app Is in boot Variables declared in , The box boot Memory requested by the stack , If you change the main stack address and then change start_app Value , It will be used app The address of the middle stack , So there will be memory usage errors .

        Four , Prepare the main stack address before jumping , Because it may appear when entering the function NMI Or other fault, You need to use the stack .

        5、 ... and , Active call into app Function of , Perform a jump to app.

1.2 Summary of the basic part

        According to the above steps, we can realize bootloader 了 , Most scenarios work .

        in addition ,app Also according to the agreed address in keil It's set up inside .

         But according to this method bootloader It's still flawed , The following questions can be pondered ：

        One 、 When upgrading to half , What happens when the upgrade is interrupted ？

        Two 、 When the upgrade fails , Can you restore the original program ？

        3、 ... and 、 When using usb As a communication interface , Whether the above steps are still applicable ？