PIC18F2221T-I/SS【PDF 92/110 页】IC PIC MCU FLASH 2KX16 28SSOP

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PIC18F2221/2321/4221/4321 FAMILY

DS39689F-page 82

2009 Microchip Technology Inc.

7.2.2

TABLAT – TABLE LATCH REGISTER

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.

7.2.3

TBLPTR – TABLE POINTER

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 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

program memory space. The 22nd bit allows access to

the device ID, the user ID and the Configuration bits.

The Table Pointer register, TBLPTR, is used by the

TBLRD and TBLWT instructions. These instructions can

update the TBLPTR in one of four ways based on the

table operation. These operations are shown in

Table 7-1. These operations on the TBLPTR only affect

the low-order 21 bits.

7.2.4

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 the timed write to program memory begins (via

the WR bit), the 19 MSbs of the TBLPTR

(TBLPTR<21:3>) determine which program memory

block of 8 bytes is written to. The Table Pointer regis-

ter’s three LSBs (TBLPTR<2:0>) are ignored. For more

detail, see Section 7.5 “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 7-3 describes the relevant boundaries of

TBLPTR based on Flash program memory operations.

TABLE 7-1:

TABLE POINTER OPERATIONS WITH TBLRD AND TBLWT INSTRUCTIONS

FIGURE 7-3:

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 WRITE

TABLE READ – TBLPTR<21:0>

TBLPTRL

TBLPTRH

TBLPTRU

TBLPTR<21:3>

TBLPTR<21:6>

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