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1016E–SMEM–12/07

AT88SC153

Programmable Chip Select (CS0–CS3): The four most significant bits (b4–b7) of every com-

mand comprise the chip select address. All AT88SC153 devices will respond to the default chip

select address of $B (1011). Each device will also respond to a second chip select address pro-

grammed into CS0–CS3 of the device configuration register. By programming each device to a

unique chip select address, it is possible to connect up to 15 devices on the same serial data

bus. The Write EEPROM and Verify Password commands can be used globally to all devices

sharing the bus by using the default chip select address $B.

Eight Trials Allowed (ETA): If enabled (ETA = “0”), the ETA extends the trials limit to eight

incorrect presentations allowed (passwords or authentication). If disabled (ETA = “1”), the PAC

and AAC will allow only four incorrect attempts.

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).

6.13 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 com-

mands. 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 command, 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 authentication. 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 automati-

cally disabled, and if the host continues to request data, the device responds as to a normal read

command, from the address $28.

Table 6-2.

Device Configuration Options

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

SME

UCR

UAT

ETA

CS3

CS2

CS1

CS0