DS3908 MAXIM [Maxim Integrated Products], DS3908 Datasheet - Page 10

• Manufacturer Part Number: DS3908
• Description: Dual, 64-Position Nonvolatile Digital Potentiometer with Buffered Outputs
• Manufacturer: MAXIM [Maxim Integrated Products]

Dual, 64-Position Nonvolatile Digital
Potentiometer with Buffered Outputs
Byte Read: A byte read is an 8-bit information transfer
from the slave to the master plus a 1-bit ACK or NACK
from the master to the slave. The 8 bits of information
that are transferred (most significant bit first) from the
slave to the master are read by the master using the bit
read definition above, and the master transmits an ACK
using the bit write definition to receive additional data
bytes. The master must NACK the last byte read to ter-
minated communication so the slave will return control
of SDA to the master.
Slave Address Byte: Each slave on the I
responds to a slave address byte sent immediately fol-
lowing a start condition. The slave address byte con-
tains the slave address in the most significant 7 bits
and the
The DS3908’s slave address is determined by the state
of the A0, A1, and A2 address pins as shown in Figure 1.
Address pins connected to GND result in a ‘0’ in the
corresponding bit position in the slave address.
Conversely, address pins connected to V
‘1’ in the corresponding bit positions.
When the R/W bit is 0 (such as in A0h), the master is
indicating it will write data to the slave. If R/W = 1, (A1h
in this case), the master is indicating it wants to read
from the slave.
If an incorrect slave address is written, the DS3908 will
assume the master is communicating with another I
device and ignore the communication until the next
start condition is sent.
Memory Address: During an I
DS3908, the master must transmit a memory address to
identify the memory location where the slave is to store
the data. The memory address is always the second
byte transmitted during a write operation following the
slave address byte.
Writing a Single Byte to a Slave: The master must
generate a start condition, write the slave address byte
(R/W = 0), write the memory address, write the byte of
data, and generate a stop condition. The master must
read the slave’s acknowledgement during all byte write
operations.
When writing to the DS3908, the potentiometer will
adjust to the new setting once it has acknowledged the
new data that is being written, and the EEPROM (used
to make the setting nonvolatile) will be written following
the stop condition at the end of the write command. To
change the setting without changing the EEPROM, ter-
minate the write with a repeated start condition before
the next stop condition occurs. Using a repeated start
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R/W bit in the least significant bit.
I
2
2
C write operation to the
C Communication
CC
result in a
2
C bus
2
C
condition prevents the 20ms (maximum) delay required
for the EEPROM write cycle to finish.
If the master continues to write data to the DS3908,
without generating a stop condition, then the same reg-
ister will be overwritten.
Acknowledge Polling: Any time an EEPROM byte is
written, the DS3908 requires the EEPROM write time
(t
byte to EEPROM. During the EEPROM write time, the
device will not acknowledge its slave address because
it is busy. It is possible to take advantage of this phe-
nomenon by repeatedly addressing the DS3908, which
allows communication to continue as soon as the
DS3908 is ready. The alternative to acknowledge
polling is to wait for a maximum period of t
before attempting to access the device.
EEPROM Write Cycles: The DS3908’s EEPROM write
cycles are specified in the Nonvolatile Memory
Characteristics table. The specification shown is at the
worst-case temperature. It is capable of handling many
additional writes at room temperature.
Reading a Single Byte from a Slave: Unlike the write
operation that uses the specified memory address byte
to define where the data is to be written, the read oper-
ation occurs at the present value of the memory
address pointer. To read a single byte from the slave,
the master generates a start condition, writes the slave
address byte with R/W = 1, reads the data byte with a
NACK to indicate the end of the transfer, and generates
a stop condition.
Manipulating the Address Pointer for Reads: A
dummy write cycle can be used to force the address
pointer to a particular value. To do this, the master gen-
erates a start condition, writes the slave address byte
(R/W = 0), writes the memory address where it desires
to read, generates a repeated start condition, writes the
slave address byte (R/W = 1), reads data with ACK or
NACK as applicable, and generates a stop condition.
See Figure 3 for a read example using the repeated
start condition to specify the memory location.
To achieve the best results when using the DS3908,
decouple the power supply with a 0.01µF or 0.1µF
capacitor. Use a high-quality, ceramic, surface-mount
capacitor if possible. Surface-mount components mini-
mize lead inductance, which improves performance,
and ceramic capacitors tend to have adequate high-
frequency response for decoupling applications.
W
) after the stop condition to write the contents of the
Applications Information
Power-Supply Decoupling
W
to elapse