© 2008 Microchip Technology Inc.
Writing to Flash Program Memory
The minimum programming block is 32 words or
64 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 64 holding registers used by the table writes for
Since the Table Latch (TABLAT) is only a single byte,
the TBLWT instruction may need to be executed
64 times for each programming operation. All of the
table write operations will essentially be short writes
because only the holding registers are written. At the end
of updating the 64 holding registers, the EECON1
register must be written to in order to start the
programming operation with a long write.
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.
The EEPROM on-chip timer controls the write time.
The write/erase voltages are generated by an on-chip
charge pump, rated to operate over the voltage range
of the device.
TABLE WRITES TO FLASH PROGRAM MEMORY
FLASH PROGRAM MEMORY
The sequence of events for programming an internal
program memory location should be:
Read 64 bytes into RAM.
Update data values in RAM as necessary.
Load Table Pointer register with address being
Execute the row erase procedure.
Load Table Pointer register with address of first
byte being written.
Write the 64 bytes into the holding registers with
Set the EECON1 register for the write operation:
• set EEPGD bit to point to program memory;
• clear the CFGS bit to access program memory;
• set WREN to enable byte writes.
Write 55h to EECON2.
10. Write 0AAh to EECON2.
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).
13. Re-enable interrupts.
14. Verify the memory (table read).
This procedure will require about 6 ms to update one
row of 64 bytes of memory. An example of the required
code is given in Example 7-3.
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 64 holding registers
before executing a write operation.
TBLPTR = xxxx3F
TBLPTR = xxxxx1
TBLPTR = xxxxx0
TBLPTR = xxxxx2
Before setting the WR bit, the Table
Pointer address needs to be within the
intended address range of the 64 bytes in
the holding register.