AN247 datasheet(6/32 Pages) MICROCHIP

6 / 32 page

AN247

DS00247A-page 6

 2003 Microchip Technology Inc.

It is also important to note some configuration bits are

single direction bits in Normal mode; they can only be

changed to one state, and cannot be changed back.

The code protection bits in Configuration registers 5L

and 5H are a good example. If any type of code protec-

tion is enabled for a block, it cannot be disabled without

a device programmer. Essentially, the bootloader

cannot reverse code protection.

The Device ID (addresses 3FFFFEh and 3FFFFFh) is

also considered Program Memory. While they can be

accessed, however, they are read only and cannot be

altered.

Write Latency

When writing data, there is a specific time that the pro-

gramming source must wait for to complete the pro-

gramming operation. Fortunately, the CAN module

actually buffers received data; therefore, receiving can

actually overlap memory write operations (Figure 6). In

general, it takes about 2 ms for Program Memory write

operations, while EEDATA takes about 4 ms. Not all

PIC18F devices have the same time specifications, so

it is important to verify the write times for the specific

device to be used.

FIGURE 6:

CAN RECEIVE VS.

MEMORY WRITE

WRITING CODE

The bootloader operates as a separate entity, which

means that an application can be developed with very

little concern about what the bootloader is doing. This

is as it should be; the bootloader should be dormant

code until an event initiates a boot operation. Under

ideal circumstances, bootloader code should never be

running during an application’s intended normal

operation.

When developing an application with a resident

bootloader, some basic principles must be kept in mind.

Writing in Assembly

When writing in assembly, the boot block and new vec-

tors must be considered. For modular code, this is gen-

erally just a matter of changing the linker script file for

the project. An example is given in Appendix C. If an

absolute address is assigned to a code section, the

address must point somewhere above the boot block.

For those who write absolute assembly, all that is nec-

essary to remember is that the new RESET vector is at

200h, and the interrupt vectors are at 208h and 218h.

No code except the bootloader should reside in the

boot block.

Writing in C

When using the MPLAB® C18 C compiler to develop

PIC18F firmware for an application, the standard

start-up object (c018.o or c018i.o) must be rebuilt

with the new RESET vector. Like modular assembly,

the linker file must be changed to incorporate the pro-

tected boot block and new vectors. Appendix C shows

an example linker file.

Users of other compilers should check with the com-

piler’s software user guide to determine how to change

the start-up code and vectors.

Bootloader Re-Entry

If the need exists to re-enter Boot mode from the appli-

cation (and it usually does), the last location of the data

EEPROM must be set to FFh. The code in Example 1

demonstrates how this might be done in an application.

Since the bootloader assumes RESET conditions, a

RESET

instruction should be initiated after setting the

last location.

EXAMPLE 1:

SETTING THE LAST

LOCATION OF THE

DATA MEMORY

CAN

Msg

CAN

Message

Receive

Memory

Write

SETF

EEADR

; Point to the last byte

SETF

EEADRH

SETF

EEDATA

; Bootmode control byte

MOVLW

b'00000100 ; Setup for EEData

MOVWF

EECON1

MOVLW

0x55

; Unlock

MOVWF

EECON2

MOVLW

0xAA

MOVWF

EECON2

BSF

EECON1, WR ; Start the write

NOP

BTFSC

EECON1, WR ; Wait

BRA

$ - 2

RESET

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