X24325
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.
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 X24325 contains an address counter that
maintains the address of the last word read, increment-
ed by one or the exact address of the last word 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 X24325 issues an acknowledge and
transmits the eight-bit word. The read operation is ter-
minated by the master; by not responding with an ac-
knowledge and by issuing a stop condition. Refer to
Figure 7 for the sequence of address, acknowledge
and data transfer.
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 word
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
X24325 and then by the eight-bit word. The read oper-
ation 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 transfer sequence.
Figure 7. Current Address Read
S
T
A
R
T
S
A
C
K
DATA
6552 ILL F11
BUS ACTIVITY:
MASTER
SDA LINE
BUS ACTIVITY:
X24325
SLAVE
ADDRESS
S
T
O
P
P
Figure 8. Random Read
S
T
A
R
T
S
A
C
K
A
C
K
S
T
A
R
T
S
A
C
K
DATA n
6552 ILL F12.1
BUS ACTIVITY:
MASTER
SDA LINE
BUS ACTIVITY:
X24325
SLAVE
ADDRESS
WORD
ADDRESS n
SLAVE
ADDRESS
S
T
O
P
P
7
XICOR [ XICOR INC. ]
