PIC16F872
The value written to program memory does not need to
be a valid instruction. Therefore, up to 14-bit numbers
can be stored in memory for use as calibration param-
eters, serial numbers, packed 7-bit ASCII, etc. Execut-
ing a program memory location containing data that
forms an invalid instruction results in a NOP.
4.0
DATA EEPROM AND FLASH
PROGRAM MEMORY
The Data EEPROM and FLASH Program Memory are
readable and writable during normal operation over the
entire VDD range. A bulk erase operation may not be
issued from user code (which includes removing code
protection). The data memory is not directly mapped in
4.1
EEADR
the register file space.
addressed through the Special Function Registers
(SFR).
Instead, it is indirectly
The address registers can address up to a maximum of
256 bytes of data EEPROM or up to a maximum of 8K
words of program FLASH. However, the PIC16F872
has 64 bytes of data EEPROM and 2K words of pro-
gram FLASH.
There are six SFRs used to read and write the program
and data EEPROM memory. These registers are:
• EECON1
• EECON2
• EEDATA
• EEDATH
• EEADR
When selecting a program address value, the MSByte
of the address is written to the EEADRH register and
the LSByte is written to the EEADR register. When
selecting a data address value, only the LSByte of the
address is written to the EEADR register.
On the PIC16F872 device, the upper two bits of the
EEADR must always be cleared to prevent inadvertent
access to the wrong location in data EEPROM. This
also applies to the program memory. The upper five
MSbits of EEADRH must always be clear during pro-
gram FLASH access.
• EEADRH
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. The
registers EEDATH and EEADRH are not used for data
EEPROM access. The PIC16F872 device has 64 bytes
of data EEPROM with an address range from 0h to
3Fh.
4.2
EECON1 and EECON2 Registers
EECON1 is the control register for memory accesses.
EECON2 is not a physical register. Reading EECON2
will read all '0's. The EECON2 register is used
exclusively in the memory write sequence.
The EEPROM data memory is rated for high erase/
write cycles. The write time is controlled by an on-chip
timer. The write time will vary with voltage and temper-
ature, as well as from chip-to-chip. Please refer to the
specifications for exact limits.
Control bit EEPGD determines if the access will be a
program or a data memory access. When clear, any
subsequent operations will operate on the data mem-
ory. When set, any subsequent operations will operate
on the program memory.
The program memory allows word reads and writes.
Program memory access allows for checksum calcula-
tion and calibration table storage. A byte or word write
automatically erases the location and writes the new
data (erase before write). Writing to program memory
will cease operation until the write is complete. The pro-
gram memory cannot be accessed during the write,
therefore code cannot execute. During the write opera-
tion, the oscillator continues to clock the peripherals,
and therefore, they continue to operate. Interrupt
events will be detected and essentially “queued” until
the write is completed. When the write completes, the
next instruction in the pipeline is executed and the
branch to the interrupt vector address will occur.
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 operation.
In these situations, following reset, the user can check
the WRERR bit and rewrite the location. The value of
the data and address registers and the EEPGD bit
remains unchanged.
When interfacing to the program memory block, the
EEDATH:EEDATA registers form a two byte word,
which holds the 14-bit data for read/write. The
EEADRH:EEADR registers form a two byte word,
which holds the 13-bit address of the FLASH location
being accessed. The PIC16F872 device has 2K words
of program FLASH with an address range from 0h to
7FFh. The unused upper bits in both the EEDATH and
EEDATA registers all read as “0’s”.
Interrupt flag bit EEIF, in the PIR2 register, is set when
write is complete. It must be cleared in software.
1999 Microchip Technology Inc.
Preliminary
DS30221A-page 29
MICROCHIP [ MICROCHIP ]
