Datasheet

PIC18F1220/1320
DS39605F-page 58 © 2007 Microchip Technology Inc.
FIGURE 6-2: TABLE WRITE OPERATION
6.2 Control Registers
Several control registers are used in conjunction with
the TBLRD and TBLWT instructions. These include the:
• EECON1 register
• EECON2 register
• TABLAT register
• TBLPTR registers
6.2.1 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 and erase sequences.
Control bit, EEPGD, determines if the access will be to
program or data EEPROM memory. When clear,
operations will access the data EEPROM memory.
When set, program memory is accessed.
Control bit, CFGS, determines if the access will be to
the configuration registers, or to program memory/data
EEPROM memory. When set, subsequent operations
access configuration registers. When CFGS is clear,
the EEPGD bit selects either program Flash or data
EEPROM memory.
The FREE bit controls program memory erase opera-
tions. When the FREE bit is set, the erase operation is
initiated on the next WR command. When FREE is
clear, only writes are enabled.
The WREN bit enables and disables erase and write
operations. When set, erase and write operations are
allowed. When clear, erase and write operations are
disabled – the WR bit cannot be set while the WREN bit
is clear. This process helps to prevent accidental writes
to memory due to errant (unexpected) code execution.
Firmware should keep the WREN bit clear at all times,
except when starting erase or write operations. Once
firmware has set the WR bit, the WREN bit may be
cleared. Clearing the WREN bit will not affect the
operation in progress.
The WRERR bit is set when a write operation is
interrupted by a Reset. In these situations, the user can
check the WRERR bit and rewrite the location. It will be
necessary to reload the data and address registers
(EEDATA and EEADR) as these registers have cleared
as a result of the Reset.
Control bits, RD and WR, start read and erase/write
operations, respectively. These bits are set by firmware
and cleared by hardware at the completion of the
operation.
The RD bit cannot be set when accessing program
memory (EEPGD = 1). Program memory is read using
table read instructions. See Section 6.3 “Reading the
Flash Program Memory” regarding table reads.
Table Pointer
(1)
Table Latch (8-bit)
TBLPTRH TBLPTRL
TABLAT
Program Memory
(TBLPTR)
TBLPTRU
Instruction: TBLWT*
Note 1: Table Pointer actually points to one of eight holding registers, the address of which is determined by
TBLPTRL<2:0>. The process for physically writing data to the program memory array is discussed
in Section 6.5 “Writing to Flash Program Memory”.
Holding Registers
Program Memory
Note: Interrupt flag bit, EEIF in the PIR2 register,
is set when the write is complete. It must
be cleared in software.