AD5384BBC-5 datasheet(27/36 Pages) AD

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AD5384

Rev. A | Page 27 of 36

Daisy-Chain Mode

For systems that contain several devices, the SDO pin can be

used to daisy-chain several devices together. This daisy-chain

mode can be useful in system diagnostics and in reducing the

number of serial interface lines.

By connecting DCEN (daisy-chain enable) pin high, the daisy-

chain mode is enabled. The first falling edge of SYNC starts the

write cycle. The SCLK is applied continuously to the input shift

applied, the data ripples out of the shift register and appears on

the SDO line. This data is clocked out on the rising edge of

SCLK and is valid on the falling edge. By connecting the SDO

of the first device to the DIN input on the next device in the

chain, a multidevice interface is constructed. 24 clock pulses are

required for each device in the system. Therefore, the total

number of clock cycles must equal 24N where N is the total

number of AD5384 devices in the chain.

When the serial transfer to all devices is complete, SYNC is

taken high. This latches the input data in each device in the

daisy-chain and prevents any further data being clocked into

the input shift register.

If the SYNC is taken high before 24 clocks are clocked into the

part, this is considered a bad frame and the data is discarded.

The serial clock may be either a continuous or a gated clock. A

continuous SCLK source can be used only if it can be arranged

that SYNC is held low for the correct number of clock cycles. In

gated clock mode a burst clock containing the exact number of

clock cycles must be used and SYNC taken high after the final

clock to latch the data.

Readback Mode

Readback mode is invoked by setting the R/W bit = 1 in the

serial input register write. With R/W = 1, Bits A5 to A0, in

association with Bits REG1 and REG0, select the register to be

read. The remaining data bits in the write sequence are don’t

cares. During the next SPI write, the data appearing on the SDO

output contains the data from the previously addressed register.

For a read of a single register, the NOP command can be used

in clocking out the data from the selected register on SDO.

Figure 28 shows the readback sequence.

For example, to read back the m register of Channel 0 on the

AD5384, the following sequence should be followed. First, write

0x404XXX to the AD5384 input register. This configures the

AD5384 for read mode with the m register of Channel 0

selected. Note that Data Bits DB13 to DB0 are don’t cares.

Follow this with a second write, a NOP condition, 0x000000.

During this write, the data from the m register is clocked out on

the SDO line, i.e., data clocked out contains the data from the m

address information as previously written. In readback mode,

the SYNC signal must frame the data. Data is clocked out on

the rising edge of SCLK and is valid on the falling edge of the

SCLK signal. If the SCLK idles high between the write and read

operations of a readback operation, the first bit of data is

clocked out on the falling edge of SYNC.

SCLK

SYNC

DIN

SDO

UNDEFINED

SELECTED REGISTER DATA CLOCKED OUT

NOP CONDITION

INPUT WORD SPECIFIES REGISTER TO BE READ

DB23

DB0

DB23

DB0

DB23

Figure 28. Serial Readback Operation

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