X24645
transmits the byte. The read operation is terminated by
the master; by not responding with an acknowledge
and by issuing a stop condition. Refer to Figure 7 for the
sequence of address, acknowledge and data transfer.
READ OPERATIONS
Read operations are initiated in the same manner as
write operations with the exception that the R/W bit of
the slave address is set HIGH. There are three basic
read operations: current address read, random read
and sequential read.
Random Read
Random read operations allow the master to access
any memory location in a random manner. Prior to issu-
ing the slave address with the R/W bit set HIGH, the
master must first perform a “dummy” write operation.
The master issues the start condition, and the slave ad-
dress with the R/W bit set LOW, followed by the byte
address it is to read. After the word address acknowl-
edge, the master immediately reissues the start condi-
tion and the slave address with the R/W bit set HIGH.
This will be followed by an acknowledge from the
X24645 and then by the data byte. The read operation
is terminated by the master; by not responding with an
acknowledge and by issuing a stop condition. Refer to
Figure 8 for the address, acknowledge and data trans-
fer sequence.
It should be noted that the ninth clock cycle of the read
operation is not a “don’t care.” To terminate a read op-
eration, the master must either issue a stop condition
during the ninth cycle or hold SDA HIGH during the
ninth clock cycle and then issue a stop condition.
Current Address Read
Internally the X24645 contains an address counter that
maintains the address of the last byte read, increment-
ed by one or the exact address of the last byte written.
Therefore, if the last access read was to address n, the
next read operation would access data from address
n + 1. Upon receipt of the slave address with the R/W
set HIGH, the X24645 issues an acknowledge and
Figure 7. Current Address Read
S
T
A
R
T
S
T
O
P
SLAVE
ADDRESS
BUS ACTIVITY:
MASTER
SDA LINE
S
P
A
C
BUS ACTIVITY:
X24645
DATA
K
2783 ILL F11
Figure 8. Random Read
S
S
T
T
A
R
T
S
T
O
P
SLAVE
ADDRESS
BYTE
ADDRESS n
SLAVE
ADDRESS
A
R
T
BUS ACTIVITY:
MASTER
SDA LINE
S
S
P
A
C
K
A
C
K
A
C
K
BUS ACTIVITY:
X24645
DATA n
2783 ILL F12.1
7
XICOR [ XICOR INC. ]
