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參數(shù)資料
| 型號(hào)： | 22374A |
| 英文描述： | Migrating between Boot and Uniform Sectored Flash Devices |
| 中文描述： | 引導(dǎo)和之間的遷移統(tǒng)一扇區(qū)閃存器件 |
| 文件頁(yè)數(shù)： | 2/5頁(yè) |
| 文件大小： | 78K |
| 代理商： | 22374A |
Publication#
22374
Issue Date:
May 12, 1999
Rev:
A
Amendment/
0
Migrating Between Boot and Uniform
Sectored Flash Devices
Application Note
The purpose of this bulletin is to describe the software
algorithm changes necessary to convert from a boot
sectored flash device to a uniform sectored flash de-
vice or vise versa. Changes are required only for
applications that require sector erase capabilities in
system. If the flash device is read-only within a system,
programmed in system but not erased, or is erased
with chip erase only, there is no functional difference
between boot and uniform sectored devices and there-
fore no need to make any software changes.
The Origins of Sector Erase Flash
Memories
The first Flash devices manufactured were bulk erase
Flash devices. Bulk erase Flash devices could pro-
gram (change a “1” to a “0”) information into the device
a byte at a time, but required the entire Flash device be
erased (change “0’s” to “1’s”) every time a byte of mem-
ory needed to be erased. This wasn’t the most efficient
way of changing data because you had to erase and re-
program the whole flash device for any erase no matter
how small the change. To solve this problem, flash with
erasable sectors was developed. Now, instead of eras-
ing the entire flash device at one time, you could erase
all memory within an erase sector independent of other
sectors. This was a 16 Kbyte block of memory to begin
with, so the 1 Mb device (where erase sectors were first
implemented) had eight 16 Kbyte erase sectors which
could be erased independently of each other. Since
this device was introduced, erase sectors have further
changed into different organizations to better suit the
needs of today’s applications. Flash devices now are
organized into two different erase architectures, boot
and uniform, and erase sectors also have the ability to
be individually or group protected against accidental
programming or erase in system.
The Difference Between Boot and Uniform
Sectored Flash Architectures
The difference between a boot sectored device and a
uniform sectored device is the make up of the first or
last 64 Kbytes of memory. For a uniform device, the
64 Kbytes are a single erase block, which is similar to
the rest of the erase sectors within the device. Uniform
sectored devices are ideal for building large memory
arrays since the erase blocks are all the same size and
therefore easily managed. For boot devices, the first
or last 64 Kbytes is subdivided into several erase
blocks. Typically these smaller erase blocks are in one
of two configurations: 16 Kbyte, 8K byte, 8K byte,
32 Kbyte or eight 8 Kbyte sectors.
These smaller erase sectors are generally used to
store boot code or parametric data. The reason for this
is that boot code and parametric data usually does not
require a whole 64 Kbyte sector, but still need to be
placed in a separate sector so it can be properly pro-
tected or changed without affecting other portions of
code. Smaller boot sectors permit this without having
to waste an entire 64 Kbyte sector. Boot sectored de-
vices also have the option of placing the smaller boot
sectors within the first 64 Kbytes of the address range
(called bottom boot) or last 64 Kbytes of the address
range (called top boot). The reason for this option is
due to the way microcontrollers access the initial boot
code required to power on. Two conventions for micro-
controllers exist: top-boot convention, where the first
address fetch on re-initialization is performed at the top
of memory space, and bottom-boot, where the first
fetched address is at the bottom of memory space. Mi-
crocontrollers based of IBM, RISC, or Motorola
processors boot from bottom of memory space, and
those from Intel and DEC boot from the top of memory
space.
The following are examples of AMD’s low voltage 16
Mbit and 32 Mbit boot sectored devices and low volt-
age 64 Mbit uniform sectored device.
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