PIC16 datasheet(96/418 Pages) MICROCHIP

96 / 418 page

PIC16(L)F1508/9

DS40001609C-page 96

 2011-2013 Microchip Technology Inc.

10.2.2

FLASH MEMORY UNLOCK

SEQUENCE

The unlock sequence is a mechanism that protects the

Flash program memory from unintended self-write pro-

gramming or erasing. The sequence must be executed

and completed without interruption to successfully

complete any of the following operations:

•Row Erase

• Load program memory write latches

• Write of program memory write latches to

program memory

• Write of program memory write latches to User

IDs

The unlock sequence consists of the following steps:

1. Write 55h to PMCON2

2. Write AAh to PMCON2

3. Set the WR bit in PMCON1

4. NOP instruction

5. NOP instruction

Once the WR bit is set, the processor will always force

two NOP instructions. When an Erase Row or Program

Row operation is being performed, the processor will stall

internal operations (typical 2 ms), until the operation is

complete and then resume with the next instruction.

When the operation is loading the program memory write

latches, the processor will always force the two NOP

instructions and continue uninterrupted with the next

instruction.

Since the unlock sequence must not be interrupted,

global interrupts should be disabled prior to the unlock

sequence and re-enabled after the unlock sequence is

completed.

FIGURE 10-3:

FLASH PROGRAM

MEMORY UNLOCK

SEQUENCE FLOWCHART

Start

Unlock Sequence

End

Unlock Sequence

Write 0x55 to

PMCON2

Write 0xAA to

PMCON2

Initiate

Write or Erase operation

(WR =

Instruction fetched ignored

NOP execution forced

Instruction fetched ignored

NOP execution forced

Rev. 10-000047A

7/30/2013

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