PIC16LF870SLASHSP datasheet(44/156 Pages) MICROCHIP

44 / 156 page

PIC16F870/871

DS30569A-page 44

Preliminary

4.7

Write Verify

Depending on the application, good programming prac-

tice 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.

Generally a write failure will be a bit which was written

as a ’1’, but reads back as a ’0’ (due to leakage off the

bit).

4.8

Protection Against Spurious Write

4.8.1

EEPROM DATA MEMORY

There are conditions when the device may not want to

write to the data EEPROM memory. To protect against

spurious EEPROM writes, various mechanisms have

been built-in. On power-up, the WREN bit is cleared.

Also, the Power-up Timer (72 ms duration) prevents

EEPROM write.

The write initiate sequence and the WREN bit together

help prevent an accidental write during brown-out,

power glitch, or software malfunction.

4.8.2

PROGRAM FLASH MEMORY

To protect against spurious writes to FLASH program

memory, the WRT bit in the configuration word may be

programmed to ‘0’ to prevent writes. The write initiate

sequence must also be followed. WRT and the configu-

ration word cannot be programmed by user code, only

through the use of an external programmer.

4.9

Operation during Code Protect

Each reprogrammable memory block has its own code

protect mechanism. External Read and Write opera-

tions are disabled if either of these mechanisms are

enabled.

4.9.1

DATA EEPROM MEMORY

The microcontroller itself can both read and write to the

internal Data EEPROM, regardless of the state of the

code protect configuration bit.

When data memory is code protected (CONFIG<8>=0)

any further external programming access of program

memory is disabled. To reenable programming access

to program memory, both bulk erase and removal of

code protection must be performed on program and

data memory.

4.9.2

PROGRAM FLASH MEMORY

The microcontroller can read and execute instructions

out of the internal FLASH program memory, regardless

of the state of the code protect configuration bits. How-

ever, the WRT configuration bit and the code protect

bits have different effects on writing to program mem-

ory. Table 4-1 shows the various configurations and

status of reads and writes. To erase the WRT or code

protection bits in the configuration word requires that

the device be fully erased.

TABLE 4-1:

READ/WRITE STATE OF INTERNAL FLASH PROGRAM MEMORY

Configuration Bits

Memory Location

Internal

Read

Internal

Write

ICSP Read ICSP Write

CP1

CP0

WRT

All program memory

Yes

All program memory

Yes

All program memory

Yes

All program memory

Yes

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