Datasheet

278
2490R–AVR–02/2013
ATmega64(L)
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 section. During an ongo-
ing 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 (that is, 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 sec-
tion. The Boot Loader section is always located in the NRWW section. The RWW section Busy
Bit (RWWSB) in the Store Program Memory Control 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
“SPMCSR – Store Program Memory Control Register” on page 281. for details on how to clear
RWWSB.
NRWW – No Read-
While-Write Section
The code located in the NRWW section can be read when the Boot Loader software is 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.
Figure 135. Read-While-Write vs. No Read-While-Write
Table 107. 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 No Yes
NRWW section None Yes No
Read-While-Write
(RWW) Section
No Read-While-Write
(NRWW) Section
Z-pointer
Addresses RWW
Section
Z-pointer
Addresses NRWW
Section
CPU is Halted
During the Operation
Code Located in
NRWW Section
Can be Read During
the Operation