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AN247

A CAN Bootloader for PIC18F CAN Microcontrollers

Microchip Technology 

AN247

Even with careful planning, there may be situations

where safety is actually compromised as a result of bus

contention. In these cases, the best option may be to

put all nodes in the network into a “Configuration” mode

and shut down all system functions.

Boot Mode Entry

Boot mode entry is determined by an event. This could

be a hardware event, such as pressing one or more

buttons after a device RESET. It could also be a net-

work event, such as a special set of data that tells a

device to enter Boot mode. One example is a network

boot ID that is mapped directly into the CAN ID. Then

the key, along with specific target information, could be

embedded in the data field of a CAN frame. The key

information could put one or more nodes into Boot

mode.

BOOTLOADER FIRMWARE

Basic Operation Overview

An overview of the CAN bootloader’s operation is

shown in Figure 1. A CAN Message Identifier and data

is received through the CAN module. One bit in the

identifier is used to indicate whether to PUT or GET

data. Another is used to determine if the message is to

be interpreted as data to be programmed or bootloader

control information. Writing data automatically invokes

the appropriate function to write to memory (FLASH,

Data EEPROM, or Configuration Memory). Writing to

the Control registers sets the operation of the

bootloader.

The bootloader can be configured at build time to sup-

port one of two mutually exclusive modes of operation.

In P Mode (or Put-only) mode, the microcontroller only

accepts PUT commands, and never “talks back” to the

source. In PG Mode, both PUT and GET commands

are accepted, allowing the source to both read from

and write to the target’s memory.

A more detailed explanation is provided in subsequent

sections.

Memory Organization

PROGRAM MEMORY USAGE

Currently, PIC18F devices reserve the first 512 bytes of

Program Memory as the boot block. Future devices

may expand this, depending on application require-

ments for these devices. This bootloader is designed to

occupy the current designated boot block of 512 bytes

(or 256 words) of memory using the recommended

options. Note, however, some compile time options can

grow the bootloader beyond the boot block. Figure 2

shows a memory map of the PIC18F458. The boot

area can be code protected to prevent accidental

overwriting of the boot program.

FIGURE 1:

BOOTLOADER FUNCTIONAL

BLOCK DIAGRAM

RX TX

Bootloader Firmware

CAN

Module Data

Control/Data Buffer

(Msg Identifier)

CONTROL/DATA

PUT/GET

FLASH

Program

Memory

EE Data

Memory

Configuration

Memory

D8

Bootloader

Control

Registers

D0

Memory I/O

Logic

FIGURE 2:

PROGRAM MEMORY MAP OF

THE PIC18F458

Boot Program

0000h

RESET Vector

0200h

High Priority Interrupt Vector 0208h

Low Priority Interrupt Vector 0218h

Program Memory

7FFFh

Note: Memory areas not shown to scale.

DS00247A-page 2

 2003 Microchip Technology Inc.

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