ATmega64A datasheet(282/392 Pages) ATMEL

282 / 392 page

282

8160C–AVR–07/09

ATmega64A

26.4

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

Whether the CPU supports Read-While-Write or if the CPU is halted during a Boot Loader soft-

ware update is dependent on which address that is being programmed. 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

“ATmega64A Boot Loader Parameters” on page 292 and Figure 26-2 on page 284. 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.

26.4.1

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 ongo-

ing 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 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 Register” on page 293. for details on how to clear

RWWSB.

26.4.2

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 26-1.

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

ATmega64A bảng dữ liệu(PDF) 282 Page - ATMEL Corporation

ATmega64A bảng dữ liệu(HTML) 282 Page - ATMEL Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

bảng dữ liệu tải về

Link URL