AD5381BSTZ-5 datasheet(27/40 Pages) AD

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Data Sheet

AD5381

Rev. D | Page 27 of 40

Daisy-Chain Mode

For systemsthat contain several devices, theSDO 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 interfacelines.

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

chain mode is enabled. Thefirst falling edge of SYNC starts the

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

applied, the data ripplesout 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. Twenty-four clock

pulses are requiredfor each device in the system.Therefore, the

total number of clockcycles must equal 24N, where N is the

total number of AD538x devices in the chain.

When the serial transferto all devices is complete, SYNC is

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

daisy-chain and prevents further datafrombeing clocked

into the input shift register.

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

this is considered a bad frameand the data is discarded.

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

continuous SCLK sourcecan only be used if it can be arranged

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

gated clock mode, a burst clockcontaining the exact number of

clock cycles must be used and SYNC must betaken high after

the final clock to latch the data.

Readback Mode

Readbackmodeis invoked by settingthe R/Wbit = 1in the

serial input register write. With R/W= 1, Bits A5to A0, in

association with Bits REG1 and REG0, select theregister tobe

read. The remainingdatabitsin the write sequencearedon’t

cares. Duringthe next SPIwrite,the data appearing on the

SDO output will contain thedata fromthe previously

addressedregister.

For a read of a single register, the NOP commandcan be used

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

Figure 30 shows the readbacksequence. For example, to read

backthe m register of Channel 0 on the AD5381, the following

sequence shouldbe implemented. First, write0x404XXX to the

AD5381 input register. This configures the AD5381 for read

mode with the m register of Channel 0 selected. Notethat Data

Bits DB11 to DB0 are don’t cares. Follow this with a second

write, a NOP condition, 0x000000.

During this write, the data from the m registeris clocked out on

the DOUT line, that is, data clocked out will contain the data

from the m register in Bit DB11 to Bit DB0, and the top 10 bits

contain the address information as previously written. In

readbackmode,the SYNC signalmust 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 operationsof a readbackoperation, 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 30. Serial Readback Operation

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