ATmega325PA Atmel Corporation, ATmega325PA Datasheet - Page 267

• Manufacturer Part Number: ATmega325PA
• Manufacturer: Atmel Corporation

26.4
26.4.1
26.4.2
Table 26-1.
8285D–AVR–06/11
Address During the Programming?
Which Section does the Z-pointer
Read-While-Write and No Read-While-Write Flash Sections
RWW – Read-While-Write Section
NRWW – No Read-While-Write Section
NRWW Section
RWW Section
Read-While-Write Features
ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P
Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-
ware update is dependent on which address that is being programmed. In addition to the two
sections that are configurable by the BOOTSZ Fuses as described above, the Flash is also
divided into two fixed sections, the Read-While-Write (RWW) section and the No Read-While-
Write (NRWW) section. The limit between the RWW- and NRWW sections is given in
7 on page 279
• When erasing or writing a page located inside the RWW section, the NRWW section can be
• When erasing or writing a page located inside the NRWW section, the CPU is halted during the
Note that the user software can never read any code that is located inside the RWW section dur-
ing a Boot Loader software operation. The syntax “Read-While-Write section” 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.
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 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 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 and Status Register (SPMCSR) will be read
as logical one as long as the RWW section is blocked for reading. After a programming is com-
pleted, the RWWSB must be cleared by software before reading code located in the RWW
section.
details on how to clear RWWSB.
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.
read during the operation.
entire operation.
See ”SPMCSR – Store Program Memory Control and Status Register” on page 281.
and
Figure 26-2 on page
Read During Programming?
Which Section Can be
NRWW Section
None
269. The main difference between the two sections is:
CPU Halted?
Is the
Yes
No
Read-While-Write
Supported?
Yes
No
Table 26-
267
for