PIC18F4685-ISLASHSP datasheet(103/484 Pages) MICROCHIP

103 / 484 page

Preliminary

DS39761B-page 101

PIC18F2682/2685/4682/4685

6.5

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

programming.

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

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.

FIGURE 6-5:

TABLE WRITES TO FLASH PROGRAM MEMORY

6.5.1

FLASH PROGRAM MEMORY WRITE

SEQUENCE

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

erased.

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

auto-increment.

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.

Disable interrupts.

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 18 ms to update one

row of 64 bytes of memory. An example of the required

code is given in Example 6-3.

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 64 holding registers

before executing a write operation.

TBLPTR = xxxx3F

TBLPTR = xxxxx1

TBLPTR = xxxxx0

TBLPTR = xxxxx2

Program Memory

Holding Register

TABLAT

Write Register

Note:

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.

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