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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

bit),

the

MSbs

the

TBLPTR

(TBLPTR<21:4>) determine which program memory

block of 16 bytes is written to. For more detail, see

Section 6.5 “Writing to Flash Program Memory”.

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

TABLE ERASE

TABLE READ – TBLPTR<21:0>

TBLPTRL

TBLPTRH

TBLPTRU

TBLPTR<21:6>

TABLE WRITE – TBLPTR<21:4>

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