PIC16F877-20L Datasheet, PDF (43/218 Pages) Microchip Technology – 28/40-Pin 8-Bit CMOS FLASH Microcontrollers

4.2 Reading the EEPROM Data
Memory
Reading EEPROM data memory only requires that the
desired address to access be written to the EEADR
register and clear the EEPGD bit. After the RD bit is set,
data will be available in the EEDATA register on the
very next instruction cycle. EEDATA will hold this value
until another read operation is initiated or until it is writ-
ten by firmware.
The steps to reading the EEPROM data memory are:
1. Write the address to EEDATA. Make sure that
the address is not larger than the memory size
of the PIC16F87X device.
2. Clear the EEPGD bit to point to EEPROM data
memory.
3. Set the RD bit to start the read operation.
4. Read the data from the EEDATA register.
EXAMPLE 4-1: EEPROM DATA READ
BSF
BCF
MOVF
MOVWF
BSF
BCF
BSF
BCF
STATUS, RP1
STATUS, RP0
ADDR, W
EEADR
STATUS, RP0
EECON1, EEPGD
EECON1, RD
STATUS, RP0
;
;Bank 2
;Write address
;to read from
;Bank 3
;Point to Data memory
;Start read operation
;Bank 2
MOVF EEDATA, W
;W = EEDATA
4.3 Writing to the EEPROM Data
Memory
There are many steps in writing to the EEPROM data
memory. Both address and data values must be written
to the SFRs. The EEPGD bit must be cleared, 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 EEPROM data. 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 com-
pletes will not terminate the write in progress.
Writes to EEPROM data memory must also be pref-
aced with a special sequence of instructions, that pre-
vent inadvertent write operations. This is a sequence of
five instructions that must be executed without interrup-
tions. The firmware should verify that a write is not in
progress, before starting another cycle.
PIC16F87X
The steps to write to EEPROM data memory are:
1. If step 10 is not implemented, check the WR bit
to see if a write is in progress.
2. Write the address to EEADR. Make sure that the
address is not larger than the memory size of
the PIC16F87X device.
3. Write the 8-bit data value to be programmed in
the EEDATA register.
4. Clear the EEPGD bit to point to EEPROM data
memory.
5. Set the WREN bit to enable program operations.
6. Disable interrupts (if enabled).
7. 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
8. Enable interrupts (if using interrupts).
9. Clear the WREN bit to disable program opera-
tions.
10. At the completion of the write cycle, the WR bit
is cleared and the EEIF interrupt flag bit is set.
(EEIF must be cleared by firmware.) If step 1 is
not implemented, then firmware should check
for EEIF to be set, or WR to clear, to indicate the
end of the program cycle.
EXAMPLE 4-2: EEPROM DATA WRITE
BSF
BSF
BTFSC
GOTO
BCF
MOVF
MOVWF
MOVF
MOVWF
BSF
BCF
BSF
BCF
MOVLW
MOVWF
MOVLW
MOVWF
BSF
BSF
BCF
STATUS, RP1 ;
STATUS, RP0 ;Bank 3
EECON1, WR ;Wait for
$-1
;write to finish
STATUS, RP0 ;Bank 2
ADDR, W
;Address to
EEADR
;write to
VALUE, W
;Data to
EEDATA
;write
STATUS, RP0 ;Bank 3
EECON1, EEPGD ;Point to Data memory
EECON1, WREN ;Enable writes
;Only disable interrupts
INTCON, GIE ;if already enabled,
;otherwise discard
0x55
;Write 55h to
EECON2
;EECON2
0xAA
;Write AAh to
EECON2
;EECON2
EECON1, WR ;Start write operation
;Only enable interrupts
INTCON, GIE ;if using interrupts,
;otherwise discard
EECON1, WREN ;Disable writes
 1998-2013 Microchip Technology Inc.
DS30292D-page 43