AN1302

DS01302A-page 2

 2009 Microchip Technology Inc.

BASIC OPERATION

The basic functionality of a bootloader is to receive,

interpret and execute a set of commands known both

by the host software and the target firmware. The

command set can be more or less complex than shown

in the application note, but it must generally support

commands to read, erase and write the Flash memory.

Additionally, it must support a command to jump from

the bootloader code to the application code. This

command is typically issued at the end of a

program/verify cycle. The application code might

require a command to exit the application code and

enter the bootloader code.

ERASING PROGRAM MEMORY

The program memory can be erased under software

control only in rows of instructions. A row consists of 32

instruction words of program memory. Please note that

the address must be aligned to a 32-word boundary

(EEADRL <4:0> = 0 0 0 0).

Because the erase sequence can modify existing data,

the special unlock sequence is needed. The unlock

sequence consists in consecutive write values 0x55,

0xAA to the EECON2 register.

UPDATING PROGRAM MEMORY

Before writing to the program memory, an erase

operation should be performed. The program memory

is written in blocks. A block consists of 8 words of

program memory data with sequential address, aligned

to an 8-word boundary (EEADRL <2:0> = 0 0 0). If

smaller blocks of memory need to be modified, then a

read-modify-erase program approach should be used,

as the program memory can only be written in 8-word

blocks.

An erase operation sets the value of 0x3FFF to all

erased locations. This corresponds to an all-ones in

binary.

BOOTLOADER COMMANDS

The bootloader firmware used in this design supports a

set of commands used to read, erase and write to the

Flash memory of the target device. The commands are

shown in Table 1.

TABLE 1:

BOOTLOADER COMMANDS

Command

Name

Read/Write

Description

0x01

Set/Get

Address

Pointer

R/W

Sets/Reads the Address Pointer. Data is written/read in two bytes format,

MSB first.

0x02

Download

data to

internal buffer

-/W

8 words of data are downloaded from the host into an internal buffer.

Data is downloaded MSB first starting from the lower address. This

command is usually followed by a Write command (0x05).

0x03

Read Flash

command

R/-

Reads 8 words from the Flash memory starting with the address

specified by the Address Pointer. The Address Pointer is automatically

incremented by 8 words upon a read.

0x04

Erase Flash

command

R/-

Erases a row of 32 words starting with the address specified by the

Address Pointer. The Address Pointer must be aligned to a 32-word

boundary (0x0000, 0x0020, 0x0040, 0x0060……). The Address Pointer

is automatically advanced by 32 words. This command uses clock

stretching and returns a dummy 0x00 byte after command completion.

0x05

Write Flash

command

R/-

The data that is stored into the buffer is written into the Flash memory at

the address pointed to by the Address Pointer. The Address Pointer is

automatically incremented by 8 words upon a successful write. This

command uses clock stretching and returns a dummy 0x00 byte after

command completion.

0x06

Jump to

application

code

R/-

Usually used after a successful firmware download. The application

resets itself and jumps to the application segment. This command uses

clock stretching and returns a dummy 0x00 byte after command

completion.