24AA65/24LC65/24C65

DS21073J-page 10

 2003 Microchip Technology Inc.

5.7

Security Options

The 24XX65 has a sophisticated mechanism for write

protecting portions of the array. This write-protect

function is programmable and allows the user to protect

0-15 contiguous 4K blocks. The user sets the security

option by sending to the device the starting block

number for the protected region and the number of

blocks to be protected. All parts will come from the

factory in the default configuration with the starting

block number set to 15 and the number of protected

blocks set to zero. THE SECURITY OPTION CAN BE

SET ONLY ONCE WITH A LENGTH GREATER THAN

ZERO.

To invoke the security option, a Write command is sent

to the device with the leading bit (bit 7) of the first

address byte set to a ‘1’ (Figure 8-1). Bits 1-4 of the first

address byte define the starting block number for the

protected region.

For example, if the starting block number is to be set to

5, the first address byte would be 1XX0101X. Bits 0, 5

and 6 of the first address byte are disregarded by the

device and can be either high or low. The device will

acknowledge after the first address byte. A byte of don’t

care bits is then sent by the master, with the device

acknowledging afterwards. The third byte sent to the

device has bit 7 (S/HE) set high and bit 6 (R) set low.

Bits 4 and 5 are don’t cares and bits 0-3 define the

number of blocks to be write-protected. For example, if

three blocks are to be protected, the third byte would be

10XX0011. After the third byte is sent to the device, it

will acknowledge and a Stop bit is then sent by the mas-

ter to complete the command.

If one of the security blocks coincides with the high

endurance block, the high endurance setting will take

precedence. Also, if the range of the security blocks

encompass the high endurance block when the secu-

rity option is set, the security block range will be set

accordingly, but the high endurance block will continue

to retain the high endurance setting. As a result, the

memory blocks preceding the high endurance block will

be set as secure sections.

During a normal write sequence, if an attempt is made

to write to a protected address, no data will be written

and the device will not report an error or abort the

command. If a Write command is attempted across a

secure boundary, unprotected addresses will be written

and protected addresses will not.

5.8

Security Configuration Read

The status of the secure portion of memory can be read

by using the same technique as programming this

option except the read bit (bit 6) of the configuration

byte is set to a one. After the configuration byte is sent,

the device will acknowledge and then send two bytes of

data to the master just as in a normal read sequence.

The master must acknowledge the first byte and not

acknowledge the second, and then send a Stop bit to

end the sequence. The upper four bits of both of these

bytes will always be read as ‘1’s. The lower four bits of

the first byte contains the starting secure block. The

lower four bits of the second byte contains the number

of secure blocks. The default starting secure block is

fifteen and the default number of secure blocks is zero

(Figure 8-1).

6.0

ACKNOWLEDGE POLLING

Since the device will not acknowledge during a write

cycle, this can be used to determine when the cycle is

complete (this feature can be used to maximize bus

throughput). Once the Stop condition for a Write

command has been issued from the master, the device

initiates the internally timed write cycle. ACK polling

can be initiated immediately. This involves the master

sending a Start condition followed by the control byte

for a Write command (R/W = 0). If the device is still

busy with the write cycle, then no ACK will be returned.

If the cycle is complete, then the device will return the

ACK and the master can then proceed with the next

Read or Write command. See Figure 6-1 for flow

diagram.

FIGURE 6-1:

ACKNOWLEDGE

POLLING FLOW

Send

Write Command

Send Stop

Condition to

Initiate Write Cycle

Send Start

Send Control Byte

with R/W = 0

Did Device

Acknowledge

(ACK = 0)?

Next

Operation

NO

YES