AT90SCR100 datasheet(364/433 Pages) ATMEL

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AT90SCR100

28.3

Read-While-Write and No Read-While-Write Flash Sections

Regarding the address targeted by a programming sequence, the CPU supports Read-While-

Write or is halted during a Boot Loader software update.

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 Table 28-1 and Figure 28-1 on page 365. The main differences between the two sec-

tions are:

• The NRWW section can be read when erasing or writing a page located inside the RWW

section.

• The CPU is stopped when erasing or writing a page located inside the NRWW section.

28.3.1

RWW – Read-While-Write Section

If a Boot Loader software update is programming a page inside the RWW section you can only

read code from the NRWW section.

During an on-going programming, the software must ensure that the RWW section won’t be

read. If the user software is trying to read code located inside the RWW section (i.e., by load pro-

gram memory, call, or jump instructions or an interrupt) during programming sequence, the

software may 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 read operation. After the end of a pro-

gramming sequence, the RWWSB must be cleared by software before reading code located in

the RWW section. See “SPMCSR – Store Program Memory Control and Status Register” on

page 367. for details on how to clear RWWSB.

28.3.2

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.

The user software can never read any code located inside the RWW section dur-

ing a Boot Loader software operation. The syntax “Read-While-Write section”

refers to the section that is being programmed (erased or written), not the section

executed during a Boot Loader software update.

Note

Table 28-1.

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

Yes

NRWW Section

None

Yes

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