16F87x BOOT LOADER/DOWNLOADER Copyright (C)2000 HI-TECH Software.
Freely distributable.
Included in this package is an implementation of a boot loader for
the 16F87x, a downloading program which it communicates with and
all of the associated source code.
Introduction
The Microchip 16F87x PIC processors are able to write to their own program
memory while executing from it. By taking advantage of this fact, a
bootloader program can be designed to accept new code via the serial
port (or other method) and re-program itself.
On startup the bootloader code is the first to execute. By default, it
checks the RB0 push-button to see if it is on. If it is, it goes into
download mode waiting for commands via the serial port. If it is not
on, it checks to see if there has been a user program installed and if
so, executes it.
How it Works
So as to maximize the amount of available program memory, the bootloader
has been designed to occupy less than 256 words. Another reason for doing
this is because this is how much space Microchip's ICD debug code requires
and by positioning the bootloader code at the same location, the -ICD
compiler switch can be used when compiling downloadable programs.
Using this switch ensures that the program being downloaded won't overlap
the program locations that the bootloader may have used.
Since the bootloader code must execute before any user program, it uses
the first 4 program locations to jump to its code. Programs that have
been compiled for download will also try to use these locations. To avoid
a conflict, the bootloader code will relocate the first 4 user program
words to a different location. It is assumed that the downloaded user program
has been compiled with HI-TECH PIC C and therefore these first four
instructions simply implement a long jump. Below is a typical memory map with
the bootloader installed.
Microchip PIC 16F877 memory map with the bootloader installed
<![CDATA[
Address
0 ----------------------------
| Jump to bootloader | <--- Bootloader uses 1st 4 words (0-3H)
4 |--------------------------|
| |
| |
| |
| |
| |
| |
| Program space |
| available for | <--- Available space for user programs
| downloaded programs |
| |
| |
| |
| |
| |
| |
| |
1F00 |--------------------------|
| Jump to user program | <--- 1st 4 user program words (1F00-1F03H)
1F04 |--------------------------|
| Program ID | <--- Identifying word (1F04H)
1F05 |--------------------------|
| |
| Bootloader code | <--- The bootloader code (1F05-1FFFH)
| |
1FFF ----------------------------
]]>
Also used is a program ID. This is needed so that the bootloader can check to
see if a user program has been downloaded yet. If it has been set to a specific
value, then it knows that it is safe to run the user program. If it is not set,
it assumes that no user program has successfully been downloaded and therefore
will not attempt to run the program on startup.
Any program locatations of the downloaded program that correspond to locations
in the bootloader, will be ignored. This ensures that program data being
received cannot overwrite any bootloader code.
Compiling Programs for Download
Except for the fact that there are 256 words less space available, nothing
special has to be done in the design of downloadable programs. The only thing
that must be done is to make sure the program doesn't use any program space
where the bootloader resides. By default, the bootloader is positioned at
0x1F00 - 0x1FFF. To avoid these locations use the compiler switch -ICD
(available from version 7.85). Included with the source code are HPDPIC
projects pre-configured with the appropriate linker options for other
16F87x processors. If you need to position the bootloader
elsewhere, then you will have to modify the CODE and CONST ranges
in the linker options (see the tutorial section of the user manual for
information about changing linker options). The "intentry" psect should be set to
to the address where the bootloader code starts. The "LoaderStart" define in the
bootloader code should also be changed to the start address of where you would
like the bootloader to go.
Downloading Programs
Included are two versions of the command line downloading program. These
are: NTLOAD.EXE and DOWNLDR.EXE. NTLOAD is designed to work with Windows NT,
and DOWNLDR is a generic version for other operating systems with DOS command
lines. The command to the downloader takes the following form:
<![CDATA[DOWNLDR options hex_file]]>
Or,
<![CDATA[NTLOAD options hex_file]]>
Where 'options' is zero or more of the following options and 'hex_file' is
the name file the HEX file you wish to download.
<![CDATA[Options:
-Sbaud_rate Specify the baud rate for communications. Eg: -S9600
-PCOMn Specify the com port to use. Eg: -PCOM1
-DEBUG_MODE=n Show debug information. The value for n can be 1 or 2. If set
to 1 it will show information about the hex file being
downloaded: addresses,etc... Set to 2 it will show all
comm information being sent to/from the bootloader.
]]>
The following is a typical command line used to download a program:
<![CDATA[DOWNLDR -S9600 -PCOM1 file.hex]]>
Converted to HTML by Philippe Corbes (Email:philippe.corbes@laposte.net) 01/12/2002.