AD5241/AD5242
4.
Unlike the write mode, the data byte follows immediately
after the acknowledgment of the slave address byte in
Frame 2 read mode. Data is transmitted over the serial bus
in sequences of nine clock pulses (slightly different from
the write mode, there are eight data bits followed by a no
acknowledge Logic 1 bit in read mode). Similarly, the
transitions on the SDA line must occur during the low
period of SCL and remain stable during the high period of
SCL (see Figure 5).
When all data bits have been read or written, a stop condition
is established by the master. A stop condition is defined as
a low-to-high transition on the SDA line while SCL is high.
In write mode, the master pulls the SDA line high during
the tenth clock pulse to establish a stop condition (see
for the ninth clock pulse (that is, the SDA line remains high).
The master then brings the SDA line low before the tenth
clock pulse, which goes high to establish a stop condition
(see Figure 5).
MULTIPLE DEVICES ON ONE BUS
Each has a different slave address because the state of their AD0
and AD1 pins are different. This allows each RDAC within each
device to be written to or read from independently. The master
device output bus line drivers are open-drain pull-downs in a
fully I
2
C-compatible interface. Note, a device is addressed properly
only if the bit information of AD0 and AD1 in the slave address
byte matches with the logic inputs at the AD0 and AD1 pins of
that particular device.
5.
LEVEL-SHIFT FOR BIDIRECTIONAL INTERFACE
While most old systems can operate at one voltage, a new
component may be optimized at another. When they operate
the same signal at two different voltages, a proper method of
level-shifting is needed. For instance, a 3.3 V E
2
PROM can be
used to interface with a 5 V digital potentiometer. A level-shift
scheme is needed to enable a bidirectional communication so that
the setting of the digital potentiometer can be stored to and
retrieved from the E
2
PROM. Figure 32 shows one of the techniques.
M1 and M2 can be N-channel FETs (2N7002) or low threshold
FDV301N if V
DD
falls below 2.5 V.
V
DD
= 3.3V
V
DD
= 5V
A repeated write function gives the user flexibility to update the
RDAC output a number of times after addressing and instructing
the part only once. During the write cycle, each data byte updates
the RDAC output. For example, after the RDAC has acknowledged
its slave address and instruction bytes, the RDAC output is
updated. If another byte is written to the RDAC while it is still
addressed to a specific slave device with the same instruction,
this byte updates the output of the selected slave device. If
different instructions are needed, the write mode has to start a
completely new sequence with a new slave address, instruction,
and data bytes transferred again. Similarly, a repeated read
function of the RDAC is also allowed.
R
P
SDA1
R
P
S
G
D
M1
S
M2
G
D
R
P
R
P
SDA2
SCL1
3.3V
E
2
PROM
SCL2
AD5242
5V
Figure 32. Level-Shift for Different Voltage Devices Operation
READBACK RDAC VALUE
Specific to the AD5242 dual-channel device, the channel of
interest is the one that was previously selected in the write mode.
In addition, to read both RDAC values consecutively, users have to
perform two write-read cycles. For example, users may first specify
the RDAC1 subaddress in write mode (it is not necessary to issue
the data byte and stop condition), and then change to read mode
to read the RDAC1 value. To continue reading the RDAC2 value,
users have to switch back to write mode, specify the subaddress,
and then switch once again to read mode to read the RDAC2
value. It is not necessary to issue the write mode data byte or
the first stop condition for this operation. Users should refer to
5V
R
P
R
P
MASTER
SDA SCL
AD1
AD0
V
DD
SDA
SCL
SDA SCL
AD1
AD0
V
DD
V
DD
SDA SCL
AD1
AD0
SDA SCL
AD1
AD5242
AD5242
AD5242
AD5242
Figure 33. Multiple AD5242 Devices on One Bus
Rev. C | Page 14 of 20
00926-023
AD0
00926-024
AD [ ANALOG DEVICES ]
