34VL02 datasheet(12/32 Pages) MICROCHIP

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34VL02

DS22079A-page 12

7.0

WRITE PROTECTION

The 34VL02 has two software write-protect features

(SWP and PSWP) that allows the lower half of the array

(addresses 00h-7Fh) to be write-protected, as well as

a WP pin that can be used to protect the entire array.

The permanent software write-protect feature is

enabled by sending the device a special command.

Once this feature has been enabled, it cannot be

reversed. The resettable software write-protect feature

is also enabled by sending the device a special

command but can be reset by issuing another special

command. In addition to the software protect features,

there is a WP pin that can be used to write-protect the

entire array, regardless of whether the software write-

protect register has been written or not.

Table 7-2 and Table 7-3 describe how the 34VL02 will

acknowledge specific commands under various

circumstances.

7.1

Hardware Write Protection

The WP pin allows the user to write-protect the entire

array (00-FF) when the pin is tied to VCC. If the pin is

tied to VSS the write protection is disabled.

7.2

Software Write Protection (SWP)

and Clear Software Write

Protection (CSWP)

In addition to hardware write-protect the 34VL02 has

an additional software write-protect feature that, when

set, protects the first 128 bytes (00-7Fh) of the array

from being written.

Setting the software write protection is done by sending

the SWP instruction. SWP can also then be cleared by

issuing a CSWP instruction (see Figure 7-1).

These two instructions follow the same format as the

BYTE

WRITE instruction with the exception of the

Device Type Identifier, (typically ‘1010’, instead

changes to ‘0110’). Once this identifier is recognized

by the device, the rest of the Byte Write command,

address and data, are “don’t cares”. In addition to the

identifier, high voltage must be applied to the A0 pin of

the device and specific levels must be present on A1

and A2. See Table 7-1 for the available commands.

7.3

Permanent Software Write-Protect

(PSWP)

The Permanent software write protection, or PSWP is

another instruction that may be used to permanently

protect the first 128 byte of the array. Once this

command is issued, the user will no longer have the

ability to clear this feature regardless of instruction,

power cycling, or state of the WP pin. Also, once this

instruction has been executed, the device will no

longer acknowledge the device identifier ‘0110’.

FIGURE 7-1:

SOFTWARE WRITE PROTECTION FOR SWP, CSWP, PSWP, OR CPSWP

Bus Activity

Master

SDA Line

Bus Activity

Control

Byte

Address

Byte

Data

S 011 0

“Don’t Care”

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