Linker Script Generator for PIC24 ================================== About ------ This is a linker script generator for PIC24. This project exists because the linker scripts that ship with Microchip's XC16 are non-free in the free/libre software sense of the word and can't be redistributed as open source as part of open source projects, according to the XC16 license. In addition to being free, this linker script generator will generate two types of linker script: standard and bootloader. Standard linker scripts behave similarly to the default linker scripts which come with the XC16 compiler. Bootloader linker scripts implement the needed functionality which make them able to be used with the Signal 11 PIC24 Bootloader, part of the Signal 11 USB Device Stack. With the generated bootloader linker scripts, the same script can be used for both bootloader firmware and for application firmware which is to be loaded by the bootloader. To specify which mode to use (bootloader or application), either define the BOOTLOADER or BOOTLOADER_APP preprocessor symbol from MPLABX's Linker Symbols field (under Symbols & Macros for the Linker (xc16-ld) in the Project Properties in MPLAB X). Bootloader Linker Script Theory of Operation --------------------------------------------- To implement a bootloader, a custom linker script (different than the default linker script that comes with the compiler) must be used to instruct the linker where to put the bootloader firmware in flash memory and which area should be left empty for the application (which will be loaded later by the bootloader). The application then must also be built with a custom linker script which instructs the linker to put the application in a different section of memory than that which the bootloader occupies. Two other factors come into play. First, the device needs to be unbrickable (or as close as it can possibly be made to be). Since the bootloader is to be run at the beginning of execution, the GOTO instruction at the bottom of the flash memory must be under bootloader control (and not written by the application). Since this is in the same flash erase block as the interrupt vector table, it makes sense to put the bootloader at the bottom of memory. The other important factor is interrupt remapping. Since the application will be unable to write the interrupt vectors (since that flash is part of the bootloader's flash area), the interrupt vectors must be remapped. To implement this, a map is made which is in the application's flash space (ivt_map in the generated bootloader files). This map contains a GOTO instruction for each implemented interrupt which jumps to the application's interrupt handler. The bootloader then, in each entry in the actual interrupt vector table (ivt in the generated bootloader files), references the appropriate entry in the interrupt map (in application space). This adds a small overhead to interrupt service routines (the single GOTO instruction). See the comments in the generated linker scripts for more information. Supported Microcontrollers --------------------------- See gen_linker_scripts.sh for a list of the supported MCUs. Generating ----------- Run ./gen_linker_scripts.sh . All the generated files will go in output/ . Adding Support for New Microcontrollers --------------------------------------- To add an MCU in an already supported family (currently PIC24F), all you should need to generate is a *_mem.txt and a *_regs.txt for your micro. For the _mem.txt file, start with one that exists already and modify it. For the *_regs.txt file, you'll need to make one using the datasheet for your MCU. Use the exact names and addresses of the registers in the datasheet in the *_regs.txt file. Use the full name of the MCU as the prefix to the filenames. Add another line to gen_linker_scripts.sh with your microcontroller in it. Use pic24fj64gb002_*.txt as a template if you want. If you add a microcontroller, please send me the files so I can put them in the main repo. License -------- These files are owned by Signal 11 Software, and may be used by anyone for any purpose. Alan Ott Signal 11 Software 407-222-6975