13/30

DEVICE FIRMWARE UPGRADE (DFU) IMPLEMENTATION IN ST7 USB DEVICES

3.4.3 OVERLAP AREA

The Overlap area is not used by the Library of the FS project. So no precautions are neces-

sary for this project.

But the Library of the LS project is strongly impacted. A lot of Library functions have parame-

ters. When more than 2 bytes are used, the parameter passing is done through the Overlap

area. An overlapping problem can occur when a firmware is upgraded. See the example

below:

Initial state: the linker has placed a variable used by the application at address 50h and a var-

iable used by a Library function at address 51h. The Library and Application firmware are

written in Sectors 0, 1, 2 using an EPB programming board:

Figure 3. Initial State of Overlap area prior to DFU

The problem appears when a new application firmware is written in the Flash and the overlap

area is modified. Suppose now that the application function uses a second variable. The linker

will locate this second variable at address 51h and the Library function variable at address

52h. But in DFU programming, the Library in Sector 0 is not changed and still uses the ad-

dress 51h:

Library

(sector0)

Application 1

(sectors 1 & 2)

FLASH memory

RAM memory (Overlap area)

50h

51h