PIC18F2220/2320/4220/4320
6.0
FLASH PROGRAM MEMORY
The Flash program memory is readable, writable and
erasable during normal operation over the entire V
DD
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.
While writing or erasing program memory, instruction
fetches cease 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.
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 TABLAT in 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 TABLAT in 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
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 (TBLPTRL<0> =
0).
The EEPROM on-chip timer controls the write and
erase times. The write and erase voltages are gener-
ated by an on-chip charge pump rated to operate over
the voltage range of the device for byte or word
operations.
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)
FIGURE 6-1:
TABLE READ OPERATION
Instruction:
TBLRD*
Table Pointer
(1)
TBLPTRU
TBLPTRH
TBLPTRL
Program Memory
Table Latch (8-bit)
TABLAT
Program Memory
(TBLPTR)
Note 1:
Table Pointer points to a byte in program memory.
2003 Microchip Technology Inc.
DS39599C-page 71
MICROCHIP [ MICROCHIP TECHNOLOGY ]
