Figure 1. X76F640 Device Operation
LOAD COMMAND BYTE
USE OF PASSWORD ACK POLLING
LOAD 2 BYTE ADDRESS
Twc OR DATA ACK POLLING
7025 FM 03
The X76F640 contains a retry counter. The retry counter
allows 8 accesses with an invalid password before any
action is taken. The counter will increment with any com-
bination of incorrect passwords. If the retry counter over-
ﬂows, all memory areas are cleared and the device is
locked by preventing any read or write array password
matches. The passwords are unaffected. If a correct
password is received prior to retry counter overﬂow, the
retry counter is reset and access is granted. In order to
reset the operation of a locked up device, a special reset
command must be used with a RESET password.
The X76F640 supports a bidirectional bus oriented pro-
tocol. The protocol deﬁnes any device that sends data
onto the bus as a transmitter and the receiving device as
a receiver. The device controlling the transfer is a master
and the device being controlled is the slave. The master
will always initiate data transfers and provide the clock for
both transmit and receive operations. Therefore, the
X76F640 will be considered a slave in all applications.
Clock and Data Conventions
Data states on the SDA line can change only during SCL
LOW. SDA changes during SCL HIGH are reserved for
indicating start and stop conditions. Refer to Figure 2 and
All commands are preceeded by the start condition,
which is a HIGH to LOW transition of SDA when SCL is
HIGH. The X76F640 continuously monitors the SDA and
SCL lines for the start condition and will not respond to
any command until this condition is met.
A start may be issued to terminate the input of a control
byte or the input data to be written. This will reset the
device and leave it ready to begin a new read or write
command. Because of the push/pull output, a start can-
not be generated while the part is outputting data. Starts
are inhibited while a write is in progress.
All communications must be terminated by a stop condi-
tion. The stop condition is a LOW to HIGH transition of
SDA when SCL is HIGH. The stop condition is also used
to reset the device during a command or data input
sequence and will leave the device in the standby power
mode. As with starts, stops are inhibited when outputting
data and while a write is in progress.
Acknowledge is a software convention used to indicate
successful data transfer. The transmitting device, either
master or slave, will release the bus after transmitting
eight bits. During the ninth clock cycle the receiver will
pull the SDA line LOW to acknowledge that it received
the eight bits of data.
The X76F640 will respond with an acknowledge after
recognition of a start condition and its slave address. If
both the device and a write condition have been
selected, the X76F640 will respond with an acknowledge
after the receipt of each subsequent eight-bit word.
Reset Device Command
The reset device command is used to clear the retry
counter and reactivate the device. When the reset device
command is used prior to the retry counter overﬂow, the
retry counter is reset and no arrays or passwords are
affected. If the retry counter has overﬂowed, all memory
areas are cleared and all commands are blocked and the
retry counter is disabled. Issuing a valid reset device
command (with reset password) to the device resets and
re-enables the retry counter and re-enables the other
commands. Again, the passwords are not affected.
Reset Password Command
A reset password command will clear both arrays and set
all passwords to all zero.