PIC16F877-20L Datasheet, PDF (44/218 Pages) Microchip Technology – 28/40-Pin 8-Bit CMOS FLASH Microcontrollers

PIC16F87X
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:
1. Write the address to EEADRH:EEADR. Make
sure that the address is not larger than the mem-
ory size of the PIC16F87X device.
2. Set the EEPGD bit to point to FLASH program
memory.
3. Set the RD bit to start the read operation.
4. Execute two NOP instructions to allow the micro-
controller to read out of program memory.
5. Read the data from the EEDATH:EEDATA
registers.
EXAMPLE 4-3: FLASH PROGRAM READ
BSF
BCF
MOVF
MOVWF
MOVF
MOVWF
BSF
BSF
BSF
NOP
NOP
BCF
MOVF
MOVWF
MOVF
MOVWF
STATUS, RP1 ;
STATUS, RP0 ;Bank 2
ADDRL, W
;Write the
EEADR
;address bytes
ADDRH,W
;for the desired
EEADRH
;address to read
STATUS, RP0 ;Bank 3
EECON1, EEPGD ;Point to Program memory
EECON1, RD ;Start read operation
;Required two NOPs
;
STATUS, RP0 ;Bank 2
EEDATA, W
;DATAL = EEDATA
DATAL
;
EEDATH,W
;DATAH = EEDATH
DATAH
;
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:
1. Write the address to EEADRH:EEADR. Make
sure that the address is not larger than the mem-
ory size of the PIC16F87X device.
2. Write the 14-bit data value to be programmed in
the EEDATH:EEDATA registers.
3. Set the EEPGD bit to point to FLASH program
memory.
4. Set the WREN bit to enable program operations.
5. Disable interrupts (if enabled).
6. 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
7. Execute two NOP instructions to allow the micro-
controller to setup for write operation.
8. Enable interrupts (if using interrupts).
9. Clear the WREN bit to disable program
operations.
DS30292D-page 44
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