PIC18F23K20 Datasheet, PDF (89/420 Pages) Microchip Technology – 28/40/44-Pin Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F2XK20/4XK20
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 one byte at
a time. A write to program memory is executed on
blocks of 64, 32 or 8 bytes at a time depending on the
specific device (See Table 6-1). Program memory is
erased in blocks of 64 bytes at a time. The difference
between the write and erase block sizes requires from
1 to 8 block writes to restore the contents of a single
block erase. A bulk erase operation can not be issued
from user code.
TABLE 6-1: WRITE/ERASE BLOCK SIZES
Device
Write Block Erase Block
Size (bytes) Size (bytes)
PIC18F43K20,
8
64
PIC18F23K20
PIC18F24K20,
32
64
PIC18F25K20,
PIC18F44K20,
PIC18F45K20
PIC18F26K20,
64
64
PIC18F46K20
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.
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.
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).
The table read operation retrieves one byte of data
directly from program memory and places it into the
TABLAT register. Figure 6-1 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 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. Tables contain-
ing 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,
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 register points to a byte in program memory.
© 2007 Microchip Technology Inc.
Advance Information
DS41303B-page 87