DATA EEPROM AND FLASH
The data EEPROM and Flash program memory is read-
able and writable during normal operation (over the full
range). This memory is not directly mapped in the
register file space. Instead, it is indirectly addressed
through the Special Function Registers. There are six
SFRs used to read and write this memory:
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 128 or 256 bytes of data EEPROM
(depending on the device), with an address range from
00h to FFh. On devices with 128 bytes, addresses from
80h to FFh are unimplemented and will wraparound to
the beginning of data EEPROM memory. When writing
to unimplemented locations, the on-chip charge pump
will be turned off.
When interfacing the program memory block, the
EEDATA and EEDATH registers form a two-byte word
that holds the 14-bit data for read/write and the EEADR
and EEADRH registers form a two-byte word that holds
the 13-bit address of the program memory location
being accessed. These devices have 4 or 8K words of
program Flash, with an address range from 0000h to
0FFFh for the PIC16F873A/874A and 0000h to 1FFFh
for the PIC16F876A/877A. Addresses above the range
of the respective device will wraparound to the
beginning of program memory.
The EEPROM data memory allows single-byte read and
write. The Flash program memory allows single-word
reads and four-word block writes. Program memory
write operations automatically perform an erase-before-
write on blocks of four words. A byte write in data
EEPROM memory automatically erases the location
and writes the new data (erase-before-write).
The write time is controlled by an on-chip timer. The
write/erase voltages are generated by an on-chip
charge pump, rated to operate over the voltage range
of the device for byte or word operations.
When the device is code-protected, the CPU may
continue to read and write the data EEPROM memory.
Depending on the settings of the write-protect bits, the
device may or may not be able to write certain blocks
of the program memory; however, reads of the program
memory are allowed. When code-protected, the device
programmer can no longer access data or program
memory; this does NOT inhibit internal reads or writes.
2001-2013 Microchip Technology Inc.
EEADR and EEADRH
The EEADRH:EEADR register pair can address up to
a maximum of 256 bytes of data EEPROM or up to a
maximum of 8K words of program EEPROM. When
selecting a data address value, only the LSByte of the
address is written to the EEADR register. When select-
ing a program address value, the MSByte of the
address is written to the EEADRH register and the
LSByte is written to the EEADR register.
If the device contains less memory than the full address
reach of the address register pair, the Most Significant
bits of the registers are not implemented. For example,
if the device has 128 bytes of data EEPROM, the Most
Significant bit of EEADR is not implemented on access
to data EEPROM.
EECON1 and EECON2 Registers
EECON1 is the control register for memory accesses.
Control bit, EEPGD, determines if the access will be a
program or data memory access. When clear, as it is
when reset, any subsequent operations will operate on
the data memory. When set, any subsequent
operations will operate on the program memory.
Control bits, RD and WR, initiate read and write or
erase, respectively. These bits cannot be cleared, only
set, in software. They are cleared in hardware at com-
pletion of the read or write operation. The inability to
clear the WR bit in software prevents the accidental,
premature termination of a write operation.
The WREN bit, when set, will allow a write or erase
operation. On power-up, the WREN bit is clear. The
WRERR bit is set when a write (or erase) operation is
interrupted by a MCLR or a WDT Time-out Reset dur-
ing normal operation. In these situations, following
Reset, the user can check the WRERR bit and rewrite
the location. The data and address will be unchanged
in the EEDATA and EEADR registers.
Interrupt flag bit, EEIF in the PIR2 register, is set when
the write is complete. It must be cleared in software.
EECON2 is not a physical register. Reading EECON2
will read all ‘0’s. The EECON2 register is used
exclusively in the EEPROM write sequence.
The self-programming mechanism for Flash
program memory has been changed. On
previous PIC16F87X devices, Flash pro-
gramming was done in single-word erase/
write cycles. The newer PIC18F87XA
devices use a four-word erase/write
cycle. See Section 3.6 “Writing to Flash
Program Memory” for more information.