PIC16F753 Datasheet, PDF (32/238 Pages) Microchip Technology – 14/16-Pin, Flash-Based 8-Bit CMOS Microcontrollers

PIC16F753/HV753
3.5 Writing the Flash Program
Memory
A word of the Flash program memory may only be
written to if the word is in an unprotected segment of
memory.
Flash program memory must be written in four-word
blocks. See Figure 3-2 and Figure 3-3 for more details.
A block consists of four words with sequential
addresses, with a lower boundary defined by an
address, where PMADRL<1:0> = 00. All block writes to
program memory are done as 16-word erase by four-
word 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 3-2). This is accomplished
by first writing the destination address to PMADRL and
PMADRH and then writing the data to PMDATL and
PMDATH. After the address and data have been set
up, then the following sequence of events must be
executed:
1. Write 55h, then AAh, to PMCON2 (Flash
programming sequence).
2. Set the WR control bit of the PMCON1 register.
All four buffer register locations should be written to
with correct data. If less than four 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
program location(s) not being written and loads it into
the PMDATL and PMDATH registers. Then the
sequence of events to transfer data to the buffer
registers must be executed.
To transfer data from the buffer registers to the program
memory, the PMADRL and PMADRH must point to the
last location in the four-word block (PMADRL<1:0> =
11). Then the following sequence of events must be
executed:
1. Write 55h, then AAh, to PMCON2 (Flash
programming sequence).
2. Set control bit WR of the PMCON1 register to
begin the write operation.
The user must follow the same specific sequence to
initiate the write for each word in the program block,
writing each program word in sequence (000, 001,
010, 011). When the write is performed on the last
word (PMADRL<1:0> = 11), a block of sixteen words is
automatically erased and the content of the four-word
buffer registers are written into the program memory.
After the “BSF PMCON1,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
sixteen-word block erase). This is not Sleep mode as
the clocks and peripherals will continue to run. After
the four-word write cycle, the processor will resume
operation with the third instruction after the PMCON1
write instruction. The above sequence must be
repeated for the higher 12 words.
3.6 Protection Against Spurious Write
There are conditions when the device should not write
to the program memory. To protect against spurious
writes, various mechanisms have been built in. On
power-up, WREN is cleared. Also, the Power-up Timer
(64 ms duration) prevents program memory writes.
The write initiate sequence and the WREN bit help
prevent an accidental write during brown-out, power
glitch or software malfunction.
3.7 Operation During Code-Protect
When the device is code-protected, the CPU is able to
read and write unscrambled data to the program
memory.
3.8 Operation During Write Protect
When the program memory is write-protected, the CPU
can read and execute from the program memory. The
portions of program memory that are write-protected
can be modified by the CPU using the PMCON
registers, but the protected program memory cannot be
modified using ICSP mode.
DS40001709A-page 32
Preliminary
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