PIC18F242 datasheet(60/332 Pages) MICROCHIP

60 / 332 page

PIC18FXX2

DS39564C-page 58

5.2.2

TABLAT - TABLE LATCH REGISTER

The Table Latch (TABLAT) is an 8-bit register mapped

into the SFR space. The Table Latch is used to hold

8-bit data during data transfers between program

memory and data RAM.

5.2.3

TBLPTR - TABLE POINTER

The Table Pointer (TBLPTR) 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 regis-

ters join to form a 22-bit wide pointer. The low order 21

bits allow the device to address up to 2 Mbytes of pro-

gram 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 5-1. These

operations on the TBLPTR only affect the low order

21 bits.

5.2.4

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 Table

Pointer determine which byte is read from program

memory into TABLAT.

When a TBLWT is executed, the three LSbs of the Table

Pointer (TBLPTR<2:0>) determine which of the eight

program memory holding registers is written to. When

the timed write to program memory (long write) begins,

the

MSbs

the

Table

Pointer,

TBLPTR

(TBLPTR<21:3>), will determine which program mem-

ory block of 8 bytes is written to. For more detail, see

Section 5.5 (“Writing to FLASH Program Memory”).

When an erase of program memory is executed, the 16

MSbs of the Table Pointer (TBLPTR<21:6>) point to the

64-byte block that will be erased. The Least Significant

bits (TBLPTR<5:0>) are ignored.

Figure 5-3 describes the relevant boundaries of

TBLPTR

based

FLASH

program

memory

operations.

TABLE 5-1:

TABLE POINTER OPERATIONS WITH TBLRD AND TBLWT INSTRUCTIONS

FIGURE 5-3:

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

ERASE - TBLPTR<21:6>

WRITE - TBLPTR<21:3>

READ - TBLPTR<21:0>

TBLPTRL

TBLPTRH

TBLPTRU

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