AD5243(RevC) 데이터 시트보기 (PDF) - Analog Devices

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AD5243
(Rev.:RevC)

Dual, 256-Position, I2C-Compatible Digital Potentiometers

Analog Devices 

AD5243/AD5248

I2C INTERFACE

I2C COMPATIBLE, 2-WIRE SERIAL BUS

The 2-wire, I2C-compatible serial bus protocol operates as follows:

1. The master initiates data transfer by establishing a start

condition, which is when a high-to-low transition on the

SDA line occurs while SCL is high (see Figure 45). The

following byte is the slave address byte, which consists of

the slave address followed by an R/W bit (this bit deter-

mines whether data is read from or written to the slave

device). The AD5243 has a fixed slave address byte,

whereas the AD5248 has two configurable address bits,

AD0 and AD1 (see Figure 10).

The slave whose address corresponds to the transmitted

address responds by pulling the SDA line low during the

ninth clock pulse (this is called the acknowledge bit). At

this stage, all other devices on the bus remain idle while the

selected device waits for data to be written to or read from

its serial register. If the R/W bit is high, the master reads

from the slave device. On the other hand, if the R/W bit is

low, the master writes to the slave device.

2. In the write mode, the second byte is the instruction byte.

The first bit (MSB) of the instruction byte is the RDAC

subaddress select bit. A logic low selects Channel 1 and a

logic high selects Channel 2.

The second MSB, SD, is a shutdown bit. A logic high causes

an open circuit at Terminal A while shorting the wiper to

Terminal B. This operation yields almost 0 Ω in rheostat

mode or 0 V in potentiometer mode. It is important to

note that the shutdown operation does not disturb the

contents of the register. When the AD5243 or AD5248 is

brought out of shutdown, the previous setting is applied to

the RDAC. In addition, during shutdown, new settings can

be programmed. When the part is returned from shutdown,

the corresponding VR setting is applied to the RDAC.

The remainder of the bits in the instruction byte are don’t

care bits (see Figure 10).

After acknowledging the instruction byte, the last byte in

write mode is the data byte. Data is transmitted over the

serial bus in sequences of nine clock pulses (eight data bits

followed by an acknowledge bit). The transitions on the

Data Sheet

SDA line must occur during the low period of SCL and

remain stable during the high period of SCL (see Figure 45

and Figure 46).

3. In the read mode, the data byte follows immediately after the

acknowledgment of the slave address byte. Data is transmitted

over the serial bus in sequences of nine clock pulses (a slight

difference with the write mode, where there are eight data bits

followed by an acknowledge bit). 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 47

and Figure 48).

Note that the channel of interest is the one that is previously

selected in write mode. If users need to read the RDAC

values of both channels, they need to program the first

channel in write mode and then change to read mode to

read the first channel value. After that, the user must return

the device to write mode with the second channel selected

and read the second channel value in read mode. It is not

necessary for users to issue the Frame 3 data byte in write

mode for subsequent readback operation. Users should refer

to Figure 47 and Figure 48 for the programming format.

4. After 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 10th clock pulse to establish a stop condition (see Figure

45 and Figure 46). In read mode, the master issues a no

acknowledge for the ninth clock pulse (that is, the SDA line

remains high). The master then brings the SDA line low

before the 10th clock pulse, which goes high to establish a

stop condition (see Figure 47 and Figure 48).

A repeated write function provides the user with the flexibility

of updating the RDAC output multiple times after addressing

and instructing the part only once. For example, after the

RDAC has acknowledged its slave address and instruction

bytes in write mode, the RDAC output updates on each

successive byte. If different instructions are needed, however,

the write/read mode must restart with a new slave address,

instruction, and data byte. Similarly, a repeated read function

of the RDAC is also allowed.

Rev. C | Page 16 of 20

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