Software Data Protection

The M28C16 offers a software controlled write

protection facility that allows the user to inhibit all

write modes to the device including the Chip Erase

instruction. This can be useful in protecting the

memory from inadvertent write cycles that may

occur due to uncontrolled bus conditions.

The M28C16 is shipped as standard in the ”unpro-

tected” state meaning that the memory contents

can be changed as required by the user. After the

Software Data Protection enable algorithm is is-

sued, the device enters the ”Protect Mode” of

operation where no further write commands have

any effect on the memory contents. The device

remains in this mode until a valid Software Data

Protection (SDP) disable sequence is received

whereby the device reverts to its ”unprotected”

state. The Software Data Protection is fully non-

volatile and is not changed by power on/off se-

quences.

To enable the Software Data Protection (SDP) the

device requires the user to write (with a Page Write)

three specific data bytes to three specific memory

locations as per Figure 5. Similarly to disable the

Software Data Protection the user has to write

specific data bytes into six different locations as per

Figure 6 (with a Page Write). This complex series

ensures that the user will never enable or disable

the Software Data Protection accidentally.

AI01509B

WRITE AAh in

Address 555h

WRITE 55h in

Address 2AAh

WRITE A0h in

Address 555h

SDP is set

WRITE AAh in

Address 555h

WRITE 55h in

Address 2AAh

WRITE A0h in

Address 555h

Write Page

(1 up to 64 bytes)

WRITE IN MEMORY

WHEN SDP IS SET

SDP ENABLE ALGORITHM

Page

Write

Instruction

(Note 1)

Page

Write

Instruction

(Note 1)

WRITE

is enabled

Figure 5. Software Data Protection Enable Algorithm and Memory Write

AI01510

WRITE AAh in

Address 555h

WRITE 55h in

Address 2AAh

WRITE 80h in

Address 555h

Unprotected State

WRITE AAh in

Address 555h

WRITE 55h in

Address 2AAh

WRITE 20h in

Address 555h

Page

Write

Instruction

Figure 6. Software Data Protection Disable

Algorithm

Note: 1. MSB Address bits (A6 to A10) differ during these specific Page Write operations.

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M28C16