AT90USB646 datasheet(353/434 Pages) ATMEL

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7593A–AVR–02/06

AT90USB64/128

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 354. 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.

28.3.1

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 “Store Program Memory Control and Status Register –

SPMCSR” on page 358. 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.

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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