ATmega128 datasheet(274/386 Pages) ATMEL

274 / 386 page

274

2467S–AVR–07/09

ATmega128

section that is being programmed (erased or written), not which section that actually is being

read during a Boot Loader software update.

Read-While-Write

Section – RWW

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. See “Store Program Memory Control and Status Register – SPMCSR” on page 277. for

details on how to clear RWWSB.

No Read-While-Write

Section – NRWW

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.

Figure 132. Read-While-Write vs. No Read-While-Write

Table 107. 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

Read-While-Write

(RWW) Section

No Read-While-Write

(NRWW) Section

Z-pointer

Addresses RWW

Section

Z-pointer

Addresses NRWW

Section

CPU is Halted

During the Operation

Code Located in

NRWW Section

Can be Read During

the Operation

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