PIC16F87x(A) BOOT LOADER v3.01A Copyright (C)2005 HI-TECH Software.
Freely distributable.

Included in this package is an implementation of a bootloader for
the PIC16 flash devices.

Introduction
############

The Microchip PIC16F87x(A) 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. The bootloader will
transmit a count-down on the serial port. During this time the bootloader is
awaiting to recieve a response on the serial port. Sending a character to the 
serial port during the count-down will indicate to the device that the user
wishes to engage the bootloader facility and download a new program. If so
a prompt will be displayed and the new program may now be transferred
to the device via the serial connection.

If the user does not respond to the processor before the count-down expires,
the bootloader facility will not be engaged and the processor will begin
to execute a previously downloaded program.

If there has been no response from the user, and the device was previously
unprogrammed, the bootloader countdown will simply restart.

How it Works
############

So as to maximize the amount of available program memory, the bootloader 
has been designed to occupy less than 100H words.

The bootloader code executes before any user program. If it is given a program to
download during its countdown, it will write the program code to the device's flash
memory. It is assumed that the program code being downloaded is an INTEL hex file
which avoids the address ranges occupied by the bootloader (the last 100H
words of program memory).

The hex file is read by the bootloader program and stores the data in the according
memory region: either EEPROM data or flash memory. Configuration and ID
location words are ignored.


Microchip PIC16F87x(A) memory map with the bootloader installed

     Address
        0       |--------------------------|
               	| Reset jumps to bootloader|	
        0004    |--------------------------| 
               	|                          |
               	|                          |
        00FF    |                          |
               	|                          |
               	|                          |
               	|                          |
               	|                          |
               	|      Program space       |
               	|      available for       |	<--- Available space for user programs
               	|   downloaded programs    |
               	|                          |
               	|                          |
               	|                          |
               	|                          |
               	|--------------------------|
               	|       Bootloader         |
        Flash   |                          |
        TOP     |--------------------------|	<--- Top of flash memory is device specific


        0       |--------------------------|
               	|                          |	
               	|       Data EEPROM        |	<--- Data EEPROM size is device specific
        EEPROM  |                          |
        TOP     |--------------------------|



Any program locatations of the downloaded program that would overwrite the
bootloader will be ignored. This is to ensure the integrity of the
bootloader during its execution.

Compiling the Bootloader
#######################

The source code for the bootloader must be compiled prior to download into the
target device. Typical command lines used to compile the bootloader are:

picc --chip=16F877a bootldr.c --ROM=1F00-1FFF
or
picc --chip=16F877 bootldr.c -DVERBOSE --ROM=1E80-1FFF
to put the bootloader in verbose mode - prints a dot for each hex record read.
or
picc --chip=16F874A bootldr.c -DVERBOSE=2 --ROM=E80-FFF
to put the bootloader in a debug mode - all data is echoed back to the terminal

The bootloader can be put into modes that are more informative if the symbol
VERBOSE is defined when compiling the bootloader source code. However, using the
bootloader in these modes will consume more program memory.

The --ROM option causes the bootloader to be linked at the address specified.
Below is a list of addresses to use depending on which processor is being
used and which mode is required:

=====================================================================
| Processor | Normal | Verbose | Normal with ICD | Verbose with ICD |
=====================================================================
| 16F877    | 1F00   | 1E80    | 1E00            | 1D80             |
---------------------------------------------------------------------
| 16F877A   | 1F00   | 1E80    | 1E00            | 1D80             |
---------------------------------------------------------------------
| 16F876    | 1F00   | 1E80    | 1E00            | 1D80             |
---------------------------------------------------------------------
| 16F876A   | 1F00   | 1E80    | 1E00            | 1D80             |
---------------------------------------------------------------------
| 16F874    | F00    | E80     | E00             | D80              |
---------------------------------------------------------------------
| 16F874A   | F00    | E80     | E00             | D80              |
---------------------------------------------------------------------
| 16F873    | F00    | E80     | E00             | D80              |
---------------------------------------------------------------------
| 16F873A   | F00    | E80     | E00             | D80              |
---------------------------------------------------------------------
| 16F871    | 700    | 680     | 600             | 580              |
---------------------------------------------------------------------
| 16F870    | 700    | 680     | 600             | 580              |
=====================================================================

The above information can also be obtained from the BOOT_START define
located in the bootloader's header file.

Compiling Programs for Download
############################### 

When compiling a program to be downloaded via this bootloader, the following
command line options should be added to the link stage:
 --runtime=+download
 --ROM=default,-XXX-1FFF
Where XXX is the start of the bootloader code (from table above).

The --runtime option will condition the data in the hexfile so that it is
optimal for bootloader to process.
The --ROM option is to ensure that the new application does not try to reside
in the same memory location as the bootloader.

Downloading Programs
####################

Once the bootloader itself has been downloaded into the processor, there is no
special program required to operate this bootloader. Any basic serial
communications program (such as HyperTerminal) will be sufficient.

By default the data trasfer rate is 2400 Bits per second. This again can be
changed, by modifying the BAUD definition in bootldr.h.

Note: The bootloader uses XON/XOFF software flow control for receiving the
downloaded program. Be sure you have this turned on in your terminal program.

Basic flowchart to describe bootloader operation
################################################

[START]
     |
     v
[init comms and device]
     |
     v
<Wait for user response>----> [EXIT: execute existing code!]
     |responded         no response
     |
     v
[Await incoming hex record] <-------------------------+-------------+
     |                                                |             |
     v                                                |             |
<hex record: EOF?>----> [EXIT:execute new program!]   |             |
     |             yes                                |             |
     | no                                             |             |
     |                                                |             |
     v                                                |             |
[data record]                                         |             |
     |                                                |yes          |
     v                                                |             |
<EEPROM/flash?>----> [select flash]--> <bootloader threatened?>--> [Write!]
     |         flash                                           no   |
     |EE                                                            |
     |                                                              |
     v                                                              |
  [select EE]-------------------------------------------------------+