Microchip Technology MA160014 Data Sheet
2010-2012 Microchip Technology Inc.
DS41412F-page 95
PIC18(L)F2X/4XK22
6.0
FLASH PROGRAM MEMORY
The Flash program memory is readable, writable and
erasable during normal operation over the entire V
erasable during normal operation over the entire V
DD
range.
A read from program memory is executed one byte at
a time. A write to program memory is executed on
blocks of 64 bytes at a time. Program memory is
erased in blocks of 64 bytes at a time. A bulk erase
operation cannot be issued from user code.
a time. A write to program memory is executed on
blocks of 64 bytes at a time. Program memory is
erased in blocks of 64 bytes at a time. A bulk erase
operation cannot 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.
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.
A value written to program memory does not need to be
a valid instruction. Executing a program memory
location that forms an invalid instruction results in a
NOP
a valid instruction. Executing a program memory
location that forms an invalid instruction results in a
NOP
.
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:
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).
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).
The table read operation retrieves one byte of data
directly from program memory and places it into the
TABLAT register.
directly from program memory and places it into the
TABLAT register.
shows the operation of a
table read.
The table write operation stores one byte of data from the
TABLAT register into a write block holding register. The
procedure to write the contents of the holding registers
into program memory is detailed in
TABLAT register into a write block holding register. The
procedure to write the contents of the holding registers
into program memory is detailed in
shows the
operation of a table write with program memory and data
RAM.
RAM.
Table operations work with byte entities. Tables
containing data, rather than program instructions, are
not required to be word aligned. Therefore, a table 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.
containing data, rather than program instructions, are
not required to be word aligned. Therefore, a table 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)
(TBLPTR)