Purpose

To provide a relatively easy-to-use bootloader that is compatible with most PIC24 and DSPIC33 series processors. This bootloader does NOT use interrupts, so your default compilation steps should work with only minor changes to the linker script.

The communications protocol is described in the comm-protocol.rst document.

Use the command-line utility booty to upload your hex file!

Features

• uses no interrupts - you only need to change a couple of lines in the linker file
• simple
  • the application is located at the same location in memory across devices
  • easy to write your own loader
• small - the bootloader is located between 0x400 and 0x1000 on most devices, leaving lots of room for the application above 0x1000
• protects itself - the bootloader will not allow a self-write
• configurable - see boot_config.h file, pull a pin low to keep bootloader activated or simply keep communicating with the board
• linker scripts protect application area - if you make a change to the code which results in a bootloader overrunning its allotted space, then the linker will throw an error

Supported Devices

This list will be updated as I test more devices (which will be slowly). Each supported device will have a linker script for the bootloader (this project) and the application located in the gld directory. You must add the proper linker file to your linker files in your MPLAB project in order to get the bootloader to operate correctly. Modification of the stock linker files to be compatible with bootypic is described in the gld directory readme.

• dsPIC33EP32MC204
• dsPIC33EP64MC504
• PIC24FJ256GB106

Contributions in this area are welcome!

This bootloader utilizes 1 UART, 2 timers, and 1 GPIO. There is no reason that the application cannot assume control of these peripherals. No interrupts are utilized so the application has full control of the device once control is passed to the application.

Environment

The anticipated environment is MPLAB X with an XC16 compiler. This will probably work in other environments, but I am only testing with Microchip-provided tools at this time.

In order to be most compatible with devices, the bootloader must fit within a small and well-defined memory footprint. As a result, optimizations must be turned up to -O1 or the application will have to be moved to a higher memory location on some devices with larger flash erase pages.

Bootloader

Your MPLAB X project for the bootloader should contain:

• devices/<my_device>_boot.gld - bootloader linker script (device specific)
• devices/<my_device>/config.h - configuration defines
• devices/<my_device>/boot_config.h - some handy defines for your device
• devices/<my_device>/bootloaderasm.s - implementations in assembly (device specific)
• bootloader.h - header for bootloader
• bootloader.c - implementations in C

The intent is for the config.h and boot_config.h should be customized for your device and application while all else simply works. I still haven't worked out how I should set up the oscillator on all platforms for consistency, but I will get to that.

Application

Your MPLAB X project for the application should be structured as you please. The only difference between the default application and the bootloader is that the linker script must be modified to move the application to a higher place in memory. The <my_device>_app.gld scripts located in this repository provide a pretty decent place to start.

Compiling/Loading

Supported Devices

1. Copy bootloader.c and bootloader.h into a directory.
2. Create your MPLAB X project, add bootloader.h, bootloaderasm.s, and <my device>_boot.gld.
3. Create an appropriate config.h file, add to your project (examples provided).
4. Modify the boot_config.h as required for your application (pin settings, etc) and add to your project.
5. Compile, load using MPLAB ICD3 or similar to get into your device
6. Use booty to load your application hex file

Unsupported Devices

There are many devices that will work well, but may require additional defines to allocate the correct code. This should be a relatively simple task for anyone who has done any PIC24 or dsPIC programming.

Troubleshooting

You can step through this code just like any other project. There are a couple of timers, one UART, and a few IO that need to be set up. In addition, different devices support somewhat different interfaces to flash memory. Most devices will support some flavor of what is already here. If you are having trouble, try to answer these questions:

• is the oscillator correctly set up, including configuration bytes?
• are the rx/tx and boot pins correctly set up?
• are the timers correctly set up on my device?

Please post any issues within the issues, and if you add device support, please do a pull request!

Performance

The current default transmission unit is 128 instructions and may be adjusted in bootloader.h under the MAX_PROG_SIZE define. The 128 value was chosen since it is a value that should perform well enough on all platforms. This value results in a loading time of 17.1s for a 32kB device at 115200 baud, using booty. This could likely be significantly improved if the MAX_PROG_SIZE were increased.

Linker Scripts

The linker scripts herein are slight modifications of those that can be found as part of the default installation of MPLAB XC16 compilers. The program memory has been offset so that it makes room for the booloader at or near the beginning of flash memory. On some devices, the bootloader will reside at 0x400 while on others, it will reside at 0x800 (depending on page erase size). On all of these devices, the application should reside at 0x1000.

By locating the application memory further back than the default 0x200, the application will have fewer instructions in program memory in which to reside. For instance, a dsPIC33EP32MC204 has 32226 bytes of program memory available (10742 instructions). The application will reside at 0x1000 instead of 0x200, so it will lose access to 0xe00 addresses (3584 addresses, or 5376 bytes) due to allocated space for the bootloader.

Sizes

In the first few pages of most device datasheets, there is a table for the family that lists the 'Page Erase Size' in instructions. This is the same table in which the peripherals, packages, and memory are listed out. Use that to determine where the bootloader and application should be located.

erase page size	bootloader address	application address
512	0x400	0x1000
1024	0x800	0x1000

Creating a New Linker Script (Bootloader)

1. Copy the linker script from the <XC16 installation dir>/support/<device>/gld
2. Rename to <device>_boot.gld (optional)
3. Find the MEMORY region, modify the program (xr) line
   1. ORIGIN should be 0x400 or 0x800 depending on the page erase memory
   2. LENGTH should be the current LENGTH - 0xe00 for bootloaders at 0x400 or LENGTH - 0xa00 for bootloaders located at 0x800 (you can do this in the google search engine, simply type 0x55ec - 0xe00)
4. Scroll down a bit, find __CODE_BASE, make it equal to ORIGIN
5. Find __CODE_LENGTH, make it equal to your computed LENGTH

Creating a New Linker Script (Application)

1. Copy the linker script from the <XC16 installation dir>/support/<device>/gld
2. Rename to <device>_app.gld (optional)
3. Find the MEMORY region, modify the program (xr) line
   1. ORIGIN should be 0x1000
   2. LENGTH should be the current LENGTH - 0xe00 (you can do this in the google search engine, simply type 0x55ec - 0xe00)
4. Scroll down a bit, find __CODE_BASE, make it equal to ORIGIN
5. Find __CODE_LENGTH, make it equal to your computed LENGTH