PIC18FXX20
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 5-1 shows the operation of a Table Read with
program memory and data RAM.
Table Write operations store data from the data mem-
ory space into holding registers in program memory.
The procedure to write the contents of the holding reg-
isters into program memory is detailed in Section 5.5,
'”Writing to FLASH Program Memory”. Figure 5-2
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.
5.0
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 64 bytes at a time. A bulk erase operation
may not be issued from user code.
Writing or erasing program memory will cease instruc-
tion fetches until the operation is complete. The pro-
gram memory cannot be accessed during the write or
erase, therefore, code cannot execute. An internal pro-
gramming 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 loca-
tion that forms an invalid instruction results in a NOP.
5.1
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)
FIGURE 5-1:
TABLE READ OPERATION
Instruction: TBLRD*
Program Memory
(1)
Table Pointer
Table Latch (8-bit)
TABLAT
TBLPTRU TBLPTRH TBLPTRL
Program Memory
(TBLPTR)
Note 1: Table Pointer points to a byte in program memory.
2003 Microchip Technology Inc.
Advance Information
DS39609A-page 61
MICROCHIP [ MICROCHIP ]
