Note that the user software can never read any code that is located inside the RWW
section during a Boot Loader software operation. The syntax “Read-While-Write sec-
tion” refers to which section that is being programmed (erased or written), not which
section that actually is being read during a Boot Loader software update.
RWW – Read-While-Write
Section
If a Boot Loader software update is programming a page inside the RWW section, it is
possible to read code from the Flash, but only code that is located in the NRWW sec-
tion. During an on-going programming, the software must ensure that the RWW section
never is being read. If the user software is trying to read code that is located inside the
RWW section (i.e., by a call/jmp/lpm or an interrupt) during programming, the software
might end up in an unknown state. To avoid this, the interrupts should either be disabled
or moved to the Boot Loader section. The Boot Loader section is always located in the
NRWW section. The RWW Section Busy bit (RWWSB) in the Store Program Memory
Control and Status Register (SPMCSR) will be read as logical one as long as the RWW
section is blocked for reading. After a programming is completed, the RWWSB must be
cleared by software before reading code located in the RWW section. See “Store Pro-
gram Memory Control and Status Register – SPMCSR” on page 260. for details on how
to clear RWWSB.
NRWW – No Read-While-Write The code located in the NRWW section can be read when the Boot Loader software is
Section
updating a page in the RWW section. When the Boot Loader code updates the NRWW
section, the CPU is halted during the entire Page Erase or Page Write operation.
Table 104. Read-While-Write Features
Which Section does the Z-
pointer Address During the
Programming?
Which Section Can
be Read During
Programming?
Is the CPU
Halted?
Read-While-Write
Supported?
RWW Section
NRWW Section
None
No
Yes
No
NRWW Section
Yes
256
ATmega48/88/168
2545D–AVR–07/04
ATMEL [ ATMEL ]
