DS1991

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DS1991 COMMAND STRUCTURE Figure 4

2

3

Command

1

B7

B6

B5

B4

B3

B2

B1 B0

write

scratchpad

96H

read

scratchpad

69H

1

1

any value

00H to 3FH

copy

scratchpad

3CH

0

0

0

0

0

0

read

SubKey

66H

write

SubKey

99H

any value

10H to 3FH

write

password

5AH

Sub-Key

Nr.:

0

0

or

0

1

or

1

0

0

0

0

0

0

0

ones complement

of 2

SCRATCHPAD COMMANDS

The 64-byte read/write scratchpad of the DS1991 is not password-protected. Its normal use is to build up

a data structure to be verified and then copied to a secure subkey.

Write Scratchpad [96H]

The Write Scratchpad command is used to enter data into the scratchpad. The starting address for the

write sequence is specified in the command. Data can be continuously written until the end of the

scratchpad is reached or until the DS1991 is reset. The command sequence is shown in Figure 5, first

page, left column.

Read Scratchpad [69H]

The Read Scratchpad command is used to retrieve data from the scratchpad. The starting address is

specified in the command word. Data can be continuously read until the end of the scratchpad is reached

or until the DS1991 is reset. The command sequence is shown in Figure 5, first page, center column.

Copy Scratchpad [3CH]

The Copy Scratchpad command is used to transfer specified data blocks from the scratchpad to a selected

subkey. This command should be used when data verification is required before storage in a secure

subkey. Data can be transferred in single 8-byte blocks or in one large 64-byte block. There are nine valid

block selector codes that are used to specify which block is to be transferred (Figure 6). As a further

precaution against accidental erasure of secure data, the 8-byte password of the destination subkey must

be entered. If the password does not match, the operation is terminated. After the block of data is

transferred to the secure subkey, the original data in the corresponding block of the scratchpad is erased.

The command sequence is shown in Figure 5, first page, right column.

SUBKEY COMMANDS

Each of the subkeys within the DS1991 is accessed individually. Transactions to read and write data to a

secured subkey start at the address defined in the command and proceed until the device is reset or the

end of the subkey is reached.