18F258 datasheet(97/484 Pages) MICROCHIP

97 / 484 page

 2004 Microchip Technology Inc.

Preliminary

DS39637A-page 95

PIC18F2480/2580/4480/4580

6.0

FLASH PROGRAM MEMORY

The Flash program memory is readable, writable and

erasable, during normal operation over the entire VDD

range.

A read from program memory is executed on one byte

at a time. A write to program memory is executed on

blocks of 8 bytes at a time. Program memory is erased

in blocks of 32 bytes at a time. A bulk erase operation

may not be issued from user code.

Writing or erasing program memory will cease

instruction fetches until the operation is complete. The

program memory cannot be accessed during the write

or erase, therefore, code cannot execute. An internal

programming timer terminates program memory writes

and erases.

6.1

Table Reads and Table Writes

In order to read and write program memory, there are

two operations that allow the processor to move bytes

between the program memory space and the data RAM:

• Table Read (TBLRD)

• Table Write (TBLWT)

The program memory space is 16 bits wide, while the

data RAM space is 8 bits wide. Table reads and table

writes move data between these two memory spaces

through an 8-bit register (TABLAT).

Table read operations retrieve data from program

memory and place it into the data RAM space.

Figure 6-1 shows the operation of a table read with

program memory and data RAM.

Table write operations store data from the data memory

space into holding registers in program memory. The

procedure to write the contents of the holding registers

into program memory is detailed in Section 6.5 “Writing

to Flash Program Memory”. Figure 6-2 shows the

operation of a table write with program memory and data

RAM.

Table operations work with byte entities. A table block

containing data, rather than program instructions, is not

required to be word aligned. Therefore, a table block can

start and end at any byte address. If a table write is being

used to write executable code into program memory,

program instructions will need to be word aligned.

FIGURE 6-1:

TABLE READ OPERATION

Table Pointer(1)

Table Latch (8-bit)

Program Memory

TBLPTRH

TBLPTRL

TABLAT

TBLPTRU

Instruction: TBLRD*

Note

1: Table Pointer register points to a byte in program memory.

Program Memory

(TBLPTR)

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