ATmega640/1280/1281/2560/2561
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 Table 29-
1 and Figure 29-1 on page 320. The main difference between the two sections is:
•
When erasing or writing a page located inside the RWW section, the NRWW section can be
read during the operation.
•
When erasing or writing a page located inside the NRWW section, the CPU is halted during
the entire operation.
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.
29.3.1
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 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 load program
memory, call, or jump instructions 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 “SPMCSR – Store Program Memory Control and Status Reg-
ister” on page 334. for details on how to clear RWWSB.
29.3.2
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.
Table 29-1. Read-While-Write Features
Which Section does the Z-pointer
Address during the Programming?
Which Section can be
Read-While-Write
Supported?
Read during Programming?
NRWW Section
None
CPU Halted?
RWW Section
No
Yes
NRWW Section
Yes
No
319
2549L–AVR–08/07
ATMEL [ ATMEL ]
