X24128
Random Read
ation is that the new address is loaded into the
address counter, but no data is output by the device.
Random read operation allows the master to access
any memory location in the array. Prior to issuing
the Slave Address Byte with the R/W bit set to one,
the master must first perform a “Dummy” write oper-
ation. The master issues the start condition and the
Slave Address Byte with the R/W bit low, receives
an acknowledge, then issues the Word Address
Byte 1, receives another acknowledge, then issues
the Word Address Byte 0. After the device acknowl-
edges receipt of the Word Address Byte 0, the
master issues another start condition and the Slave
Address Byte with the R/W bit set to one. This is
followed by an acknowledge and then eight bits of
data from the device. The master terminates the
read operation by not responding with an acknowl-
edge and then issuing a stop condition. Refer to
figure 9 for the address, acknowledge, and data
transfer sequence.
The next Current Address Read operation will read
from the newly loaded address.
Sequential Read
Sequential reads can be initiated as either a current
address read or random read. The first Data Byte is
transmitted as with the other modes; however, the
master now responds with an acknowledge, indicating
it requires additional data. The device continues to
output data for each acknowledge received. The
master terminates the read operation by not
responding with an acknowledge and then issuing a
stop condition.
The data output is sequential, with the data from address
n followed by the data from address n + 1. The address
counter for read operations increments through all byte
addresses, allowing the entire memory contents to be
read during one operation. At the end of the address
space the counter “rolls over” to address 0000h and the
device continues to output data for each acknowledge
received. Refer to figure 10 for the acknowledge and
data transfer sequence.
The device will perform a similar operation called “Set
Current Address” if a stop is issued instead of the
second start shown in figure 9. The device will go into
standby mode after the stop and all bus activity will be
ignored until a start is detected. The effect of this oper-
Figure 9. Random Read Sequence
S
S
SIGNALS
FROMTHE
MASTER
T
A
R
T
A
R
WORDADDRESS WORD ADDRESS
SLAVE
ADDRESS
S
T
O
P
SLAVE
ADDRESS
BYTE 1
BYTE 0
T
T
1
SDA BUS
S 1 0 1 0
0
S
P
A
C
K
A
C
K
A
C
K
A
C
K
SIGNALS
FROMTHE
SLAVE
DATA
7027 FM 11
Figure 10. Sequential Read Sequence
SIGNALS
FROM THE
MASTER
A
C
K
A
C
K
A
C
K
S
T
O
P
SLAVE
ADDRESS
SDA BUS
1
P
S
A
C
K
SIGNALS
FROM THE
SLAVE
DATA
(1)
DATA
(2)
DATA
(n–1)
DATA
(n)
(n is any integer greater than 1)
7027 FM 12
8
XICOR [ XICOR INC. ]
