PIC18LC658-I/CL View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC18LC658-I/CL

High-Performance Microcontrollers with CAN Module

Microchip Technology 

PIC18CXX8

5.2 Program Memory Read/Writes

5.2.1 TABLE READ OVERVIEW (TBLRD)

The TBLRD instructions are used to read data from pro-

gram memory to data memory.

TBLPTR points to a byte address in program space.

Executing TBLRD places the byte pointed to into

TABLAT. In addition, TBLPTR can be modified auto-

matically for the next Table Read operation.

Table Reads from program memory are performed one

byte at a time. The instruction will load TABLAT with the

one byte from program memory pointed to by TBLPTR.

5.2.2 PROGRAM MEMORY WRITE BLOCK SIZE

The program memory of PIC18CXX8 devices is written

in blocks. For PIC18CXX8 devices, the write block size

is 2 bytes. Consequently, Table Write operations to

program memory are performed in pairs, one byte at a

time.

When a Table Write occurs to an even program mem-

ory address (TBLPTR<0> = 0), the contents of TABLAT

are transferred to an internal holding register. This is

performed as a short write and the program memory

block is not actually programmed at this time. The

holding register is not accessible by the user.

When a Table Write occurs to an odd program memory

address (TBLPTR<0> = 1), a long write is started. Dur-

ing the long write, the contents of TABLAT are written

to the high byte of the program memory block and the

contents of the holding register are transferred to the

low byte of the program memory block.

Figure 5-3 shows the holding register and the program

memory write blocks.

If a single byte is to be programmed, the low (even)

byte of the destination program word should be read

using TBLRD*, modified or changed, if required, and

written back to the same address using TBLWT*+. The

high (odd) byte should be read using TBLRD*, modified

or changed if required, and written back to the same

address using TBLWT. The write to an odd address will

cause a long write to begin. This process ensures that

existing data in either byte will not be changed unless

desired.

FIGURE 5-3: HOLDING REGISTER AND THE WRITE

Program Memory

n-1

n

n+1

n+2

DataLow

DataHigh

Holding Register

MSB

LSB

DataLow

MSB

DataHigh

LSB

DataLow

Instruction Execution

; TABLPTR points to address n

MOVLW DataLow ; Load low data

MOVWF TABLAT ; byte to TABLAT

TBLWT*+

; Write it to LSB

; of Holding register

MOVLW DataHigh ; Load high data

MOVWF TABLAT ; byte to TABLAT

TBLWT*

; Write it to MSB

; of Holding

; register and

; begin long

; write

EXAMPLE 5-1: TABLE READ CODE EXAMPLE

; Read a byte from location 0x0020

CLRF TBLPTRU

; Load upper 5 bits of

; 0x0020

CLRF TBLPTRH

; Load higher 8 bits of

; 0x0020

MOVLW 0x20

; Load 0x20 into

MOVWF TBLPTRL

; TBLPTRL

MOVWF TBLRD*

; Data is in TABLAT

DS30475A-page 68

Advanced Information

 2000 Microchip Technology Inc.

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