8

AT88SC153

1016D–SMEM–04/04

Unlimited Authentication Trials (UAT): If enabled (UAT = “0”), the AAC is disabled,

allowing an unlimited number of authentication attempts. The PACs are not affected by

the UAT bit.

Unlimited Checksum Reads (UCR): If enabled (UCR = “0”), the device will allow an

unlimited number of checksums without requiring a new authentication.

Supervisor Mode Enable (SME): If enabled (SME = “0”), verification of the Write 1

password will allow the user to write and read the entire passwords zone (including the

PACs).

Checksum

Authentication Register

After a valid authentication has been completed, the internal pseudo-random generator

(PRG) will compute a secure checksum after one write command or several consecutive

write commands. This checksum certifies that the data sent by the host during the write

commands were received and therefore written in the memory. For every write com-

mand, the device clocks the data bytes into the PRG and its output is the Checksum

Authentication Register (CAR), which is a function of Ci, Gc, Q, and the data bytes

written.

After a valid authentication, any write command will enable the checksum mode and

cause AAC to become the virtual location of the 8-byte CAR. When all data have been

transmitted, the host may perform a Read CAR command by sending a read command

with the AAC address ($20). The first 8 bytes transmitted by the device form the secure

checksum.

The checksum mode allows only a single Read CAR operation for each valid authenti-

cation. The checksum mode is disabled at the end of the Read CAR command,

whatever the number of bytes transmitted, or by a read command with any other

address. The checksum mode can only be enabled once for a given authentication.

Note: During the Read CAR command, the internal address counter is incremented just

as in a normal read command. Once 8 bytes have been transmitted, the checksum

mode is automatically disabled, and if the host continues to request data, the device

responds as to a normal read command, from the address $28.

User Zones

Three zones are dedicated to the user data. The access rights of each zone are pro-

grammable separately via the access registers. If several zones share the same

password set, this set will be entered only once (after the part is powered up), so several

zones might be combined in one larger zone.

Security Operations

Write Lock

If a user zone is configured in the write lock mode (access register bit 2), the lowest

address byte of a page constitutes a write access byte for the bytes of that page.

Table 8. Write Lock

Example: The write lock byte (WLB) at $00 controls the bytes from $00 to $07.

The WLB can also lock itself by writing its least significant (right most) bit to “0”. The

WLB can only be programmed, i.e., bits written to “0” cannot return to “1”.

$0 - WLB

$1$2$3

$4$5$6

$7

@

11011001

x x

Lock

x x

Lock

x x

x x

x x

Lock

x x

x x

$00