S4242/S42WD42/S4261/S42WD61
Acknowledge Polling
READ OPERATIONS
WhentheS42xxxisperforminganinternalWRITEopera-
tion, it will ignore any new START conditions. Since the Read operations are initiated with the R/W bit of the
device will only return an acknowledge after it accepts the identification field set to “1.” There are four different read
START, the part can be continuously queried until an options:
acknowledgeisissued, indicatingthattheinternalWRITE
1. Current Address Byte Read
cycle is complete.
2. Random Address Byte Read
To poll the device, give it a START condition, followed by
a slave address for a WRITE operation (See Figure 9).
3. Current Address Sequential Read
4. Random Address Sequential Read
Current Address Byte Read
Internal WRITE Cycle
In Progress;
Begin ACK Polling
The S42xxx contains an internal address counter which
maintains the address of the last word accessed,
incremented by one. If the last address accessed (either
a read or write) was to address location n, the next read
operation would access data from address location n+1
and increment the current address pointer. When the
S42xxx receives the slave address field with the R/W bit
set to “1,” it issues an acknowledge and transmits the 8-
bit word stored at address location n+1.
Issue Start
Issue Slave
Address and
R/W = 0
Issue Stop
ACK
Returned?
No
The current address byte read operation only accesses a
singlebyteofdata. Themasterdoesnotacknowledgethe
transfer, but does generate a stop condition. At this point,
the S42xxx discontinues data transmission. See
Figure 12 for the address acknowledge and data transfer
sequence.
Yes (Internal WRITE Cycle is completed)
Next
operation a
WRITE?
No
Yes
Issue Byte
Address
Issue Stop
Await Next
Command
Proceed with
WRITE
2025 ILL11.0
FIGURE 11. ACKNOWLEDGE POLLING
A
C
K
A
10
A
9
A
8
R
W
Data Byte
1
SDA Bus Activity
D
7
D
6
D
5
D
4
D
3
D
2
D
1
D
0
1
1 0 1 0
1
S
T
A
R
T
S
T
O
P
Device
Type
Address
A10,A9,A8
Read/Write
1= Read
Lack of ACK (low)
from Master
determines last
data byte to be read
Slave Address
Slave sends
Data to Master
Master sends Read
request to Slave
Slave Transmitter
to
Shading Denotes
Master Transmitter
to
42xxx
Master Receiver
SDA Output Active
Slave Receiver
2025 ILL12.1
FIGURE 12. CURRENT ADDRESS BYTE READ MODE
2025 6.0 4/17/00
10
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
