AN247 datasheet(2/32 Pages) MICROCHIP

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AN247

DS00247A-page 2

 2003 Microchip Technology Inc.

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

FIGURE 2:

PROGRAM MEMORY MAP OF

THE PIC18F458

CAN

PUT/GET

Memory I/O

FLASH

Program

Memory

Configuration

Data

Memory

Bootloader Firmware

Module

Logic

Control/Data Buffer

CONTROL/DATA

Bootloader

Control

Registers

Data

(Msg Identifier)

RESET Vector

Low Priority Interrupt Vector

0200h

0218h

Program Memory

High Priority Interrupt Vector 0208h

7FFFh

Boot Program

Note:

Memory areas not shown to scale.

0000h

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