Datasheet

337
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
The limit between the RWW- and NRWW sections is given in Table 27-1 on page 337 and Figure 27-1 on page
338. 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 during 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.
27.3.1 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.
Refer to “Store Program Memory Control and Status Register – SPMCSR” on page 341 for details on how to
clear RWWSB.
27.3.2 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 27-1. 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