PIC16F87XA
3.6
Writing to FLASH Program
Memory
To transfer data from the buffer registers to the program
memory, the EEADR and EEADRH must point to the
last location in the four-word block (EEADR<1:0> =
‘11’). Then the following sequence of events must be
executed:
1.
2.
3.
Set the EEPGD control bit (EECON1<7>)
Write 55h, then AAh, to EECON2 (FLASH pro-
gramming sequence)
Set control bit WR (EECON1<1>) to begin the
write operation
FLASH program memory may only be written to if the
destination address is in a segment of memory that is
not write protected, as defined in bits WRT1:WRT0 of
the device configuration word (Register 14-1). FLASH
program memory must be written in four-word blocks.
A block consists of four words with sequential
addresses, with a lower boundary defined by an
address, where EEADR<1:0> = ‘00’. At the same time,
all block writes to program memory are done as erase-
and-write operations. The write operation is edge-
aligned, and cannot occur across boundaries.
To write program data, it must first be loaded into the
buffer registers (see Figure 3-1). This is accomplished
by first writing the destination address to EEADR and
EEADRH, and then writing the data to EEDATA and
EEDATH. After the address and data have been set up,
then the following sequence of events must be exe-
cuted:
1.
2.
3.
Set the EEPGD control bit (EECON1<7>)
Write 55h, then AAh, to EECON2 (FLASH pro-
gramming sequence)
Set the WR control bit (EECON1<1>)
The user must follow the same specific sequence to ini-
tiate the write for each word in the program block, writ-
ing each program word in sequence (00,01,10,11).
When the write is performed on the last word
(EEADR<1:0> = ‘11’), the block of four words are
automatically erased, and the contents of the buffer
registers are written into the program memory.
After the “BSF
EECON1,WR“
instruction, the processor
requires two cycles to set up the erase/write operation.
The user must place two
NOP
instructions after the WR
bit is set. Since data is being written to buffer registers,
the writing of the first three words of the block appears
to occur immediately. The processor will halt internal
operations for the typical 4 ms, only during the cycle in
which the erase takes place (i.e., the last word of the
four-word block). This is not SLEEP mode, as the
clocks and peripherals will continue to run. After the
write cycle, the processor will resume operation with
the third instruction after the EECON1 write instruction.
If the sequence is performed to any other location, the
action is ignored.
All four buffer register locations
MUST
be written to with
correct data. If only one, two, or three words are being
written to in the block of four words, then a read from
the program memory location(s) not being written to
must be performed. This takes the data from the pro-
gram location(s) not being written and loads it into the
EEDATA and EEDATH registers. Then the sequence of
events to transfer data to the buffer registers must be
executed.
FIGURE 3-1:
BLOCK WRITES TO FLASH PROGRAM MEMORY
7
5
EEDATH
0 7
EEDATA
0
Four words of FLASH
are erased, then
all buffers are
transferred
to FLASH
automatically
after this word
is written
6
First word of block
to be written
8
14
EEADR<1:0>
= ‘
00
’
Buffer Register
EEADR<1:0>
= ‘
01
’
14
EEADR<1:0>
= ‘
10
’
14
EEADR<1:0>
= ‘
11
’
14
Buffer Register
Buffer Register
Buffer Register
Program Memory
2001 Microchip Technology Inc.
Advance Information
DS39582A-page 35
MICROCHIP [ MICROCHIP TECHNOLOGY ]
