Flipflop aims to be a lightweight serial bootloader for PIC(L)F1919x devices. Basic functionality includes reading, writing, and erasing. UART communications is stable at baud rates ranging from 300 to 1M over both 2-wire and 1-wire UART.

Flipflop resides in a section of memory at the beginning of Program Flash Memory (PFM). This means the Reset (0x0) and Interrupt (0x4) vectors normally used by an application are taken up by the bootloader. The bootloader does not use interrupts so the hardware interrupt vector is mapped to simply jump to the application’s interrupt vector. The Reset vector is used for all bootloader functionality. This means a Reset is needed to enter the bootloader. On startup the bootloader decides whether to start the bootloader or jump to the application, based on the source of the reset.

Usage

The flipflop bootloader must be flashed to a device using an ICSP programmer. Afterwards, flipflop can be used to upload user applications to the device.

Entering the bootloader

Upon a reset event, flipflop determines if the watchdog timer caused the reset. If it was not, flipflop starts listening for the sync character (‘U’) from the UART. If the sync character is not received before the watchdog timeout, a reset is triggered. Flipflop then detects a watchdog reset, which causes a jump to the user’s application reset vector.

Uploading Applications

Applications can be uploaded with picchick or your own software. A python script is provided with examples to communicate with flipflop using pyserial.

For simple sessions you can use a serial monitor program such as screen to send and receive commands. Note that the arguments to the read/write/erase commands take a byte value, so really the only usable commands are Reset (‘U’), Start (‘X’), and Version (‘V’).

Updating the bootloader

To update the bootloader, flipflop-upload can be uploaded as an user application. This application can then be used to upload a new bootloader.

Bootloader commands

Some commands take a few of the same argument. Note that all address and word data are formatted as 2-bytes in big endian (MSB sent first).

Building

Microchip’s xc8 compiler should be used when building both flipflip and user applications. Other compilers are untested, but may work.

Building flipflop

To build flipflop, xc8 and make must be installed, along with some common *nix utils such as mkdir and rm. After installing the dependencies, running make flipflop in the project directory will generate build/flipflop.hex. This hexfile should be flashed onto the device with your favorite ICSP programmer, picchick supports this protocol with an external programmer.

An updater application can be compiled using make flipflop-update. Thiswill generate build/flipflop-update.hex which can be uploaded using flipflop.

Both binaries can be compiled with a make all.

Building user applications

User applications built with xc8 require minimal configuration to be used with flipflop.
When compiling/linking your project, you need to tell the linker to offset the starting address with -mcodeoffset by the bootloader offset 0x400:

xc8 -mcodeoffset=0x400 <sources.c> -o <output.hex>

This will produce a hex file that can be directly uploaded with flipflop.

Configuration Words

Configuration words 3-5 are unavailable for use by the user application. This includes the watchdog (CONFIG3), boot block (CONFIG4), and flash protection (CONFIG5). - The watchdog is configured to enable software control of the watchdog timer. It is disabled before starting a user application. - Boot block is disabled. - UserNVM code protection is disabled.

Copying

Copyright © 2022-2024 Rex McKinnon
This software is released under the permissive University of Illinios/NCSA Open Source License. Check the LICENSE file for more details.