REFERENCE.md

reference for stm8lxx-bootloader

further explanation and description for the customized bootloader

TWO MODES: REMAP & OVERLAP

the key point for a customized IAP is about the handling for interrupt vectors

• REMAP

principle for remap mode interrupt vectors

1. for detail information about the standard customized bootloader, go to read AN2659
2. the first 128 bytes (0x8000-0x8080) contains 32 fix interrupt vectors
3. the first interrupt vector (0x8000-0x8003) is for the hardware reset interrupt and it should be kept unchanged
4. the last 31 interrupt vectors (0x8004-0x8080) should be redirected to 0x8284-0x82ff, the value for 0x8004-0x8080 is calculated by 0x8200 << 16 & (0x8280 + n*4), n=1,2,...,31
5. in file lnkstm8l103k3.icf, change define region NearFunCode = [from 0x8280 to 0x9FFF], compile and get the app bin
6. memcyp the app bin including the interrupt vector table and the code to the flash from the address 0x8280, the code actually starts from 0x8300
7. when bootloader finished, the program will jump to the app JP $8280

• OVERLAP

principle for overlap mode interrupt vectors

1. the first 128 bytes (0x8000-0x8080) contains 32 fix interrupt vectors
2. the first interrupt vector (0x8000-0x8003) is for the hardware reset interrupt and it should be kept unchanged
3. the last 31 interrupt vectors (0x8004-0x8080) should be changed according to the app's 1-31th interrupt vectors
4. store the reset interrupt vector for app at the end of the user-bootloader 0x827c-0x827f
5. in file lnkstm8l103k3.icf, change define region NearFunCode = [from 0x8200 to 0x9FFF], compile and get the app bin
6. just memcyp the app bin code except the interrupt vector table to the flash from the address 0x8280
7. when bootloader finished, the program will jump to the app JP $827c

• comparison between REMAP & OVERLAP
  1. REMAP mode is the standard mode provide by the official manual reference AN2659, so it is easy to understand and realize
  2. in REMAP mode, the content for the bootloader including both the interrupt vectors and user-bootloader code is constant, so you can define this area as UBC and design protection mechanism to prevent write operation during IAP; while in OVERLAP mode, as the last 31 interrupt vectors vary as the app code, it can not easily be protected
  3. compared to OVERLAP mode, you will waste 124 bytes storing the redirecting addresses, which is fatal when the flash resource is very limited
  4. compared to OVERLAP mode, you will waste at least a CPU cycle when handling the interrupt service. When an interrupt happens, the PC first jump to the fix interrupt vector, then jump to the redirected address, and then jump to the entry of the ISR, which is fatal when the interrupt service is very time limited.

TIPs

1. when flashing the program, you can use byte programming, fast byte programming, word programming, fast word programming, block programming and fast block programming.
   the time consumption for byte programming and block programming scales greatly. For a binary file whose size is about 6K bytes, you may need minutes to finish programming using byte programming, at the meantime, 2-3 seconds using block programming
   When using block programming, you should map the code into the RAM, for example:

   IN_RAM(void FLASH_ProgBlock(uint16_t StartAddress, uint8_t *Buffer));

   for further information about the programming guide, refer to AN2659, PM0047, UM0560 and SWIM

2. you should pay attention to the size of the bootloader.bin, in this example, we reserve 640bytes for bootloader. If you use more complicated algorithms such as transfer verification, flash verification or CRC verification, the size of the bootloader.bin will increase.
   When the space revered for the bootloader is not enough, fatal error will happen.

Q & A

1. In the OVERLAP mode, the space address storing the app reset interrupt can be changed?

   Absolutely yes!
   Actually, when the bootloader ends, it will jump to the space address storing the app reset interrupt vector which stores the entry for main function. You can even just fetch the entry for main function, and put it into the jump space address, this will save a jump cycle.
   In this example, we just choose the jump space address at the end of the user-bootloader 0x807c. You can choose wherever you want just to avoid the occupation.

2. Can the interrupt enabled during the bootloader?

   Generally not!
   But if you change the bootloader a little, it can!
   The reason why we should not enable interrupt during the bootloader is that we have damaged the interrupt vector, both in REMAP or OVERLAP mode after we rewrite the interrupt vector table.
   In REMAP mode, if you enable specified interrupt during bootloader, you can make a branch like this:

   if(booting){
       "excute the ISR for booting"
   }else{
       "jump to the specified redirected address for app"
   }

   booting is a global boolen variable indicating that whether the process now is in boot or app