PIC16F87X
4.3
Reading the Data EEPROM Memory
To read a data memory location, the user must write the
address to the EEADR register, clear the EEPGD con-
trol bit (EECON1<7>) and then set control bit RD
(EECON1<0>). The data is available in the very next
instruction cycle of the EEDATA register, therefore it
can be read by the next instruction. EEDATA will hold
this value until another read operation or until it is writ-
ten to by the user (during a write operation).
EXAMPLE 4-1: DATA EEPROM READ
BSF
BCF
STATUS, RP1
;
STATUS, RP0 ;Bank 2
MOVLW DATA_EE_ADDR
MOVWF EEADR
;
;Data Memory Address to read
BSF
BCF
BSF
BCF
STATUS, RP0 ;Bank 3
EECON1, EEPGD;Point to DATA memory
EECON1, RD
STATUS, RP0 ;Bank 2
;W = EEDATA
;EEPROM Read
MOVF EEDATA, W
4.4
Writing to the Data EEPROM Memory
To write an EEPROM data location, the address must
first be written to the EEADR register and the data writ-
ten to the EEDATA register. Then the sequence in
Example 4-2 must be followed to initiate the write cycle.
EXAMPLE 4-2: DATA EEPROM WRITE
BSF
STATUS, RP1
STATUS, RP0
DATA_EE_ADDR
EEADR
;
BCF
; Bank 2
;
MOVLW
MOVWF
MOVLW
MOVWF
BSF
; Data Memory Address to write
DATA_EE_DATA
EEDATA
;
; Data Memory Value to write
; Bank 3
STATUS, RP0
BCF
EECON1, EEPGD ; Point to DATA memory
EECON1, WREN ; Enable writes
BSF
BCF
INTCON, GIE
55h
; Disable Interrupts
MOVLW
MOVWF
MOVLW
MOVWF
BSF
;
Required
Sequence
EECON2
; Write 55h
AAh
;
EECON2
; Write AAh
EECON1, WR
INTCON, GIE
; Set WR bit to begin write
; Enable Interrupts
BSF
SLEEP
BCF
; Wait for interrupt to signal write complete
EECON1, WREN ; Disable writes
The write will not initiate if the above sequence is not
exactly followed (write 55h to EECON2, write AAh to
EECON2, then set WR bit) for each byte. It is strongly
recommended that interrupts be disabled during this
code segment.
is set. The WREN bit must be set on a previous instruc-
tion. Both WR and WREN cannot be set with the same
instruction.
At the completion of the write cycle, the WR bit is
cleared in hardware and the EEPROM Write Complete
Interrupt Flag bit (EEIF) is set. EEIF must be cleared by
software.
Additionally, the WREN bit in EECON1 must be set to
enable writes. This mechanism prevents accidental
writes to data EEPROM due to unexpected code exe-
cution (i.e., runaway programs). The WREN bit should
be kept clear at all times, except when updating the
EEPROM. The WREN bit is not cleared by hardware
After a write sequence has been initiated, clearing the
WREN bit will not affect the current write cycle. The WR
bit will be inhibited from being set unless the WREN bit
1999 Microchip Technology Inc.
DS30292B-page 43
MICROCHIP [ MICROCHIP ]
