 2009 Microchip Technology Inc.

DS01302A-page 3

AN1302

BOOTLOADER FIRMWARE

As mentioned before, the bootloader firmware resides

in the lower part of the Flash memory. The firmware

mands (the complete list of commands is described in

firmware must access the Flash memory. For some

commands that involve operations with the internal

Flash memory that takes time to complete the opera-

command. They will return a dummy 0x00 byte after

the operation has completed. During this time, the

line low to signal the master device that the operation

has not completed. The download of data to the inter-

nal buffer does not need any response from the target

system, thus the command is implemented as a normal

following. These are implemented as the target being

Table 2):

1.

Send a "Start" condition to the slave device

2.

Send the device address with the R/W bit low

(even address). If the sent address matches the

slave device address, then the slave device

sends an ACK signal.

3.

Send the internal register number you want to

write to.

4.

Send data. Depending on the command used,

the master can send multiple data bytes. Please

note that the master device can send a multiple

number of bytes.

5.

Send a "Stop" condition.

TABLE 2:

Table 3):

1.

Send a “Start” condition.

2.

Send the device address with the R/W bit low

(even address). Notice that the slave sends an

Acknowledge signal if the address matches.

3.

Send the bootloader command number you

want to read from.

4.

Send a “Restart” condition.

5.

Send the device address again. This time with

the R/W bit high (odd address).

6.

Read a number of bytes from the slave device.

If data is not yet available, the slave will use

clock stretching (pull SCL line low).

7.

Send a “Stop” condition

TABLE 3:

The bootloader code must exit after completing the

download of the new firmware. This is typically imple-

mented using a Reset instruction. The decision to go to

bootloader code or application code is based on a boot-

loader flag placed in the Flash memory. The location of

this flag is at the highest address of Flash memory for

the target device. In the current implementation using

the PIC16F1937, this location is 0x1FFF. Other devices

might have more or less available Flash program mem-

ory and the last available Flash memory location will

vary accordingly. Following an Erase command, the

data at address 0x1FFF is 0x3FFF. After a successful

code download and verify, the data contained at this

address will be changed to a special value (0x55), indi-

cating that an application is loaded. When the applica-

tion code needs to jump to the bootloader code it will

write an all-zeros value to the bootloader flag address.

When writing an all-zeros value to a Flash memory

location there is no need for a previous erase opera-

tion. An erase operation sets the Flash memory loca-

tion to an all-ones value. The special value can be

either contained in the application hex file or written by

the bootloader upon a successful code update.

Alternatively, any means of code-present flags can be

used, such as flags in EEPROM memory or any other

address in the Flash memory.

The full source code of the bootloader is available with

this application note. The firmware runs on the F1 Eval-

uation Platform containing a PIC16F1937. Three demo

applications are provided along with this application

note. These are relatively simple and are provided with

the purpose of demonstrating firmware upload. The

first two applications are just "Hello world" kind of firm-

ware. The third application showcases the use of inter-

rupt forwarding.

Start

Address

Register

Data (nbytes)

Stop

Start Address Register Restart Address

Data

(nbytes)

Stop