13
May 25, 2006
Stops and Write Modes
Stop conditions that terminate write operations must
be sent by the master after sending at least 1 full data
byte plus the subsequent ACK signal. If a stop is
issued in the middle of a data byte, or before 1 full data
byte plus its associated ACK is sent, then the device
will reset itself without performing the write. The con-
tents of the array will not be effected.
Acknowledge Polling
The disabling of the inputs during high voltage cycles
can be used to take advantage of the typical 5ms write
cycle time. Once the stop condition is issued to indi-
cate the end of the master’s byte load operation, the
device initiates the internal high voltage cycle.
Acknowledge polling can be initiated immediately. To
do this, the master issues a start condition followed by
the Slave Address Byte for a write or read operation. If
the device is still busy with the high voltage cycle then
no ACK will be returned. If the device has completed
the write operation, an ACK will be returned and the
host can then proceed with the read or write operation.
See Figure 13.
Serial 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 Byte is set to one. There are three
basic read operations: Current Address Reads, Ran-
dom Reads, and Sequential Reads.
Current Address Read
Internally the device contains an address counter that
maintains the address of the last word read incre-
mented by one. Therefore, if the last read was to
address n, the next read operation would access data
from address n+1. On power-up, the address of the
address counter is undefined, requiring a read or write
operation for initialization.
Upon receipt of the Slave Address Byte with the R/W
bit set to one, the device issues an acknowledge and
then transmits the eight bits of the Data Byte. The
master terminates the read operation when it does not
respond with an acknowledge during the ninth clock
and then issues a stop condition. See Figure 14 for the
address, acknowledge, and data transfer sequence.
Figure 13. Acknowledge Polling Sequence
It should be noted that the ninth clock cycle of the read
operation is not a “don’t care.” To terminate a read
operation, the master must either issue a stop condi-
tion during the ninth cycle or hold SDA HIGH during
the ninth clock cycle and then issue a stop condition.
Random Read
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 operation. The master
issues the start condition and the Slave Address Byte,
receives an acknowledge, then issues the Word Address
Bytes. After acknowledging receipts of the Word Address
Bytes, the master immediately issues another start con-
dition and the Slave Address Byte with the R/W
bit set to
one. This is followed by an acknowledge from the device
and then by the eight bit word. The master terminates the
read operation by not responding with an acknowledge
and then issuing a stop condition. See Figure 15 for the
address, acknowledge, and data transfer sequence.
ACK
Returned?
Issue Slave Address
Byte (Read or Write)
Byte Load Completed
by Issuing STOP.
Enter ACK Polling
Issue STOP
Issue START
NO
YES
High Voltage Cycle
Complete. Continue
Command Sequence?
Issue STOP
NO
Continue Normal
Read or Write
Command Sequence
PROCEED
YES
X40420, X40421
