PIC18F2450/4450
DS39760A-page 76
Advance Information
2006 Microchip Technology Inc.
6.2.2
TABLE LATCH REGISTER (TABLAT)
The Table Latch (TABLAT) is an 8-bit register mapped
into the SFR space. The Table Latch register is used to
hold 8-bit data during data transfers between program
memory and data RAM.
6.2.3
TABLE POINTER REGISTER
(TBLPTR)
The Table Pointer (TBLPTR) register addresses a byte
within the program memory. The TBLPTR is comprised
of three SFR registers: Table Pointer Upper Byte, Table
Pointer High Byte and Table Pointer Low Byte
(TBLPTRU:TBLPTRH:TBLPTRL). These three registers
join to form a 22-bit wide pointer. The low-order 21 bits
allow the device to address up to 2 Mbytes of program
memory space. The 22nd bit allows access to the device
ID, the user ID and the Configuration bits.
The Table Pointer, TBLPTR, is used by the TBLRD and
TBLWT
instructions. These instructions can update the
TBLPTR in one of four ways based on the table opera-
tion. These operations are shown in
Table 6-1. These
operations on the TBLPTR only affect the low-order
21 bits.
6.2.4
TABLE POINTER BOUNDARIES
TBLPTR is used in reads, writes and erases of the
Flash program memory.
When a TBLRD is executed, all 22 bits of the TBLPTR
determine which byte is read from program memory
into TABLAT.
When a TBLWT is executed, the four LSbs of the Table
Pointer register (TBLPTR<3:0>) determine which of the
16 program memory holding registers is written to.
When the timed write to program memory begins (via
the
WR
bit),
the
16
MSbs
of
the
TBLPTR
(TBLPTR<21:4>) determine which program memory
block of 16 bytes is written to. For more detail, see
When an erase of program memory is executed, the
16 MSbs of the Table Pointer register (TBLPTR<21:6>)
point to the 64-byte block that will be erased. The Least
Significant bits (TBLPTR<5:0>) are ignored.
Figure 6-3 describes the relevant boundaries of the
TBLPTR based on Flash program memory operations.
TABLE 6-1:
TABLE POINTER OPERATIONS WITH TBLRD AND TBLWT INSTRUCTIONS
FIGURE 6-3:
TABLE POINTER BOUNDARIES BASED ON OPERATION
Example
Operation on Table Pointer
TBLRD*
TBLWT*
TBLPTR is not modified
TBLRD*+
TBLWT*+
TBLPTR is incremented after the read/write
TBLRD*-
TBLWT*-
TBLPTR is decremented after the read/write
TBLRD+*
TBLWT+*
TBLPTR is incremented before the read/write
21
16
15
87
0
TABLE ERASE
TABLE READ – TBLPTR<21:0>
TBLPTRL
TBLPTRH
TBLPTRU
TBLPTR<21:6>
TABLE WRITE – TBLPTR<21:4>