PSD813F1 datasheet(19/110 Pages) STMICROELECTRONICS

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PSD813F1

Memory Operation

The main Flash and EEPROM memory are ad-

dressed through the microcontroller interface on

the PSD device. The microcontroller can access

these memories in one of two ways:

–

The microcontroller can execute a typical bus

WRITE or READ operation just as it would if

accessing a RAM or ROM device using

standard bus cycles.

–

The microcontroller can execute a specific

instruction that consists of several WRITE and

READ operations. This involves writing

specific data patterns to special addresses

within the Flash or EEPROM to invoke an

embedded algorithm. These instructions are

summarized in Table 8., page 20.

Typically, Flash memory can be read by the micro-

controller using READ operations, just as it would

read a ROM device. However, Flash memory can

only be erased and programmed with specific in-

structions. For example, the microcontroller can-

not write a single byte directly to Flash memory as

one would write a byte to RAM. To program a byte

into Flash memory, the microcontroller must exe-

cute a program instruction sequence, then test the

status of the programming event. This status test

is achieved by a READ operation or polling the

Ready/Busy pin (PC3).

The Flash memory can also be read by using spe-

cial instructions to retrieve particular Flash device

information (sector protect status and ID).

The EEPROM is a bit different. Data can be written

to EEPROM memory using write operations, like

writing to a RAM device, but the status of each

WRITE event must be checked by the microcon-

troller. A WRITE event can be one to 64 contigu-

ous bytes. The status test is very similar to that

used for Flash memory (READ operation or

Ready/Busy). Optionally, the EEPROM memory

may be put into a Software Data Protect (SDP)

mode where it requires instructions, rather than

operations, to alter its contents. SDP mode makes

writing to EEPROM much like writing to Flash

memory.

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