PIC18F242 datasheet(67/332 Pages) MICROCHIP

67 / 332 page

DS39564C-page 65

PIC18FXX2

6.0

DATA EEPROM MEMORY

The Data EEPROM is readable and writable during

normal operation over the entire VDD range. The data

memory is not directly mapped in the register file

space. Instead, it is indirectly addressed through the

Special Function Registers (SFR).

There are four SFRs used to read and write the

program and data EEPROM memory. These registers

are:

• EECON1

• EECON2

• EEDATA

• EEADR

The EEPROM data memory allows byte read and write.

When interfacing to the data memory block, EEDATA

holds the 8-bit data for read/write and EEADR holds the

address of the EEPROM location being accessed.

These devices have 256 bytes of data EEPROM with

an address range from 0h to FFh.

The EEPROM data memory is rated for high erase/

write cycles. A byte write automatically erases the loca-

tion and writes the new data (erase-before-write). The

write time is controlled by an on-chip timer. The write

time will vary with voltage and temperature, as well as

from chip to chip. Please refer to parameter D122

(Electrical Characteristics, Section 22.0) for exact

limits.

6.1

EEADR

The address register can address up to a maximum of

256 bytes of data EEPROM.

6.2

EECON1 and EECON2 Registers

EECON1 is the control register for EEPROM memory

accesses.

EECON2 is not a physical register. Reading EECON2

will read all '0's. The EECON2 register is used

exclusively in the EEPROM write sequence.

Control bits RD and WR initiate read and write opera-

tions, respectively. These bits cannot be cleared, only

set, in software. They are cleared in hardware at the

completion of the read or write operation. The inability

to clear the WR bit in software prevents the accidental

or premature termination of a write operation.

The WREN bit, when set, will allow a write operation.

On power-up, the WREN bit is clear. The WRERR bit is

set when a write operation is interrupted by a MCLR

Reset, or a WDT Time-out Reset during normal opera-

tion. In these situations, the user can check the

WRERR bit and rewrite the location. It is necessary to

reload the data and address registers (EEDATA and

EEADR), due to the RESET condition forcing the

contents of the registers to zero.

Note:

Interrupt flag bit, EEIF in the PIR2 register,

is set when write is complete. It must be

cleared in software.

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