PIC18F2450/4450
The long write is necessary for programming the
internal Flash. Instruction execution is halted while in a
long write cycle. The long write will be terminated by
the internal programming timer.
6.5
Writing to Flash Program Memory
The minimum programming block is 8 words or
16 bytes. Word or byte programming is not supported.
Table writes are used internally to load the holding
registers needed to program the Flash memory. There
are 16 holding registers used by the table writes for
programming.
The write/erase voltages are generated by an on-chip
charge pump, rated to operate over the voltage range
of the device.
Note:
The default value of the holding registers on
device Resets and after write operations is
FFh. A write of FFh to a holding register
does not modify that byte. This means that
individual bytes of program memory may be
modified, provided that the change does not
attempt to change any bit from a ‘0’ to a ‘1’.
When modifying individual bytes, it is not
necessary to load all 16 holding registers
before executing a write operation.
Since the Table Latch (TABLAT) is only a single byte, the
TBLWTinstruction may need to be executed 16 times for
each programming operation. All of the table write oper-
ations will essentially be short writes because only the
holding registers are written. At the end of updating the
16 holding registers, the EECON1 register must be
written to in order to start the programming operation
with a long write.
FIGURE 6-5:
TABLE WRITES TO FLASH PROGRAM MEMORY
TABLAT
Write Register
8
8
8
8
TBLPTR = xxxxx0
TBLPTR = xxxxx1
TBLPTR = xxxxx2
TBLPTR = xxxxxF
Holding Register
Holding Register
Holding Register
Holding Register
Program Memory
10. Write 0AAh to EECON2.
6.5.1
FLASH PROGRAM MEMORY WRITE
SEQUENCE
11. Set the WR bit. This will begin the write cycle.
12. The CPU will stall for duration of the write (about
2 ms using internal timer).
The sequence of events for programming an internal
program memory location should be:
13. Re-enable interrupts.
1. Read 64 bytes into RAM.
14. Repeat steps 6 through 14 once more to write
64 bytes.
2. Update data values in RAM as necessary.
3. Load Table Pointer register with address being
erased.
15. Verify the memory (table read).
This procedure will require about 8 ms to update one
row of 64 bytes of memory. An example of the required
code is given in Example 6-3.
4. Execute the Row Erase procedure.
5. Load Table Pointer register with address of first
byte being written.
Note:
Before setting the WR bit, the Table
Pointer address needs to be within the
intended address range of the 16 bytes in
the holding register.
6. Write 16 bytes into the holding registers with
auto-increment.
7. Set the EECON1 register for the write operation:
• clear the CFGS bit to access program memory;
• set WREN to enable byte writes.
8. Disable interrupts.
9. Write 55h to EECON2.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 79
MICROCHIP [ MICROCHIP ]
