PIC18F6410ESLASHPTSQTP datasheet(89/404 Pages) MICROCHIP

89 / 404 page

 2004 Microchip Technology Inc.

Preliminary

DS39635A-page 87

PIC18F6310/6410/8310/8410

EXAMPLE 6-1:

READING A FLASH PROGRAM MEMORY WORD

6.4

Writing to Program Memory Space

(PIC18F8310/8410 only)

The table write operation outputs the contents of the

TBLPTR and TABLAT registers to the external address

and data busses of the external memory interface.

Depending on the program memory mode selected, the

operation may target any byte address in the device’s

memory space. What happens to this data depends

largely on the external memory device being used.

For PIC18 devices with Enhanced Flash memory, a

single algorithm is used for writing to the on-chip

program array. In the case of external devices, however,

the algorithm is determined by the type of memory

device and its requirements. In some cases, a specific

instruction sequence must be sent before data can be

written or erased. Address and data demultiplexing,

chip select operation and write time requirements must

all be considered in creating the appropriate code.

The connection of the data and address busses to the

memory device are dictated by the interface being

used, the data bus width and the target device. When

using a 16-bit data path, the algorithm must take into

account the width of the target memory.

Another important consideration is the write time

requirement of the target device. If this is longer than

the time that a TBLWT operation makes data available

on the interface, the algorithm must be adjusted to

lengthen this time. It may be possible, for example, to

buy enough time by increasing the length of the wait

state on table operations.

In all cases, it is important to remember that instruc-

tions in the program memory space are word-aligned,

with the Least Significant bit always being written to an

even-numbered address (LSb = 0). If data is being

stored in the program memory space, word alignment

of the data is not required.

A complete overview of interface algorithms is beyond

the scope of this data sheet. The best place for timing

and instruction sequence requirements is the data

sheet of the memory device in question. For additional

information on algorithm design for the external

memory interface, refer to Microchip application note

AN869, “External Memory Interfacing Techniques for

the PIC18F8XXX” (DS00869).

6.4.1

WRITE VERIFY

Depending on the application, good programming

practice may dictate that the value written to the

memory should be verified against the original value.

This should be used in applications where excessive

writes can stress bits near the specification limit.

6.4.2

UNEXPECTED TERMINATION OF

WRITE OPERATION

If a write is terminated by an unplanned event, such as

loss of power or an unexpected Reset, the memory

location just programmed should be verified and repro-

grammed if needed. If the application writes to external

memory on a frequent basis, it may be necessary to

implement an error trapping routine to handle these

unplanned events.

6.5

Erasing External Memory

(PIC18F8310/8410 only)

Erasure is implemented in different ways on different

devices. In many cases, it is possible to erase all or part

of the memory by issuing a specific command. In some

devices, it may be necessary to write ‘0’s to the locations

to be erased. For specific information, consult the

external memory device’s data sheet for clarification.

6.6

Writing and Erasing On-Chip

Program Memory (ICSP Mode)

While the on-chip program memory is read-only in

normal operating mode, it can be written to and erased

as a function of In-Circuit Serial Programming (ICSP). In

this mode, the TBLWT operation is used in all devices to

write to blocks of 64 bytes (32 words) at one time. Write

blocks are boundary-aligned with the code protection

blocks. Special commands are used to erase one or

more code blocks of the program memory, or the entire

device.

MOVLW

CODE_ADDR_UPPER

; Load TBLPTR with the base

MOVWF

TBLPTRU

; address of the word

MOVLW

CODE_ADDR_HIGH

MOVWF

TBLPTRH

MOVLW

CODE_ADDR_LOW

MOVWF

TBLPTRL

READ_WORD

TBLRD*+

; read into TABLAT and increment

MOVF

TABLAT, W

; get data

MOVWF

WORD_EVEN

TBLRD*+

; read into TABLAT and increment

MOVFW

TABLAT, W

; get data

MOVF

WORD_ODD

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