PIC16F87X datasheet(44/218 Pages) MICROCHIP

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PIC16F87X

DS30292D-page 44

 1998-2013 Microchip Technology Inc.

4.4

Reading the FLASH Program

Memory

Reading FLASH program memory is much like that of

EEPROM data memory, only two NOP instructions must

be inserted after the RD bit is set. These two instruction

cycles that the NOP instructions execute, will be used

by the microcontroller to read the data out of program

memory

and

insert

the

value

into

the

EEDATH:EEDATA registers. Data will be available fol-

lowing the second NOP instruction. EEDATH and

EEDATA will hold their value until another read opera-

tion is initiated, or until they are written by firmware.

The steps to reading the FLASH program memory are:

Write the address to EEADRH:EEADR. Make

sure that the address is not larger than the mem-

ory size of the PIC16F87X device.

Set the EEPGD bit to point to FLASH program

memory.

Set the RD bit to start the read operation.

Execute two NOP instructions to allow the micro-

controller to read out of program memory.

Read the data from the EEDATH:EEDATA

registers.

EXAMPLE 4-3:

FLASH PROGRAM READ

4.5

Writing to the FLASH Program

Memory

Writing to FLASH program memory is unique, in that

the microcontroller does not execute instructions while

programming is taking place. The oscillator continues

to run and all peripherals continue to operate and

queue interrupts, if enabled. Once the write operation

completes (specification D133), the processor begins

executing code from where it left off. The other impor-

tant difference when writing to FLASH program mem-

ory, is that the WRT configuration bit, when clear,

prevents any writes to program memory (see Table 4-1).

Just like EEPROM data memory, there are many steps

in writing to the FLASH program memory. Both address

and data values must be written to the SFRs. The

EEPGD bit must be set, and the WREN bit must be set

to enable writes. The WREN bit should be kept clear at

all times, except when writing to the FLASH Program

memory. The WR bit can only be set if the WREN bit

was set in a previous operation, i.e., they both cannot

be set in the same operation. The WREN bit should

then be cleared by firmware after the write. Clearing the

WREN bit before the write actually completes will not

terminate the write in progress.

Writes to program memory must also be prefaced with

a special sequence of instructions that prevent inad-

vertent write operations. This is a sequence of five

instructions that must be executed without interruption

for each byte written. These instructions must then be

followed by two NOP instructions to allow the microcon-

troller to setup for the write operation. Once the write is

complete, the execution of instructions starts with the

instruction after the second NOP.

The steps to write to program memory are:

Write the address to EEADRH:EEADR. Make

sure that the address is not larger than the mem-

ory size of the PIC16F87X device.

Write the 14-bit data value to be programmed in

the EEDATH:EEDATA registers.

Set the EEPGD bit to point to FLASH program

memory.

Set the WREN bit to enable program operations.

Disable interrupts (if enabled).

Execute the special five instruction sequence:

• Write 55h to EECON2 in two steps (first to W,

then to EECON2)

• Write AAh to EECON2 in two steps (first to W,

then to EECON2)

• Set the WR bit

Execute two NOP instructions to allow the micro-

controller to setup for write operation.

Enable interrupts (if using interrupts).

Clear the WREN bit to disable program

operations.

BSF

STATUS, RP1

;

BCF

STATUS, RP0

;Bank 2

MOVF

ADDRL, W

;Write the

MOVWF

EEADR

;address bytes

MOVF

ADDRH,W

;for the desired

MOVWF

EEADRH

;address to read

BSF

STATUS, RP0

;Bank 3

BSF

EECON1, EEPGD ;Point to Program memory

BSF

EECON1, RD

;Start read operation

NOP

;Required two NOPs

NOP

;

BCF

STATUS, RP0

;Bank 2

MOVF

EEDATA, W

;DATAL = EEDATA

MOVWF

DATAL

;

MOVF

EEDATH,W

;DATAH = EEDATH

MOVWF

DATAH

;

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