AD7912AUJ-R2 datasheet(22/32 Pages) AD

22 / 32 page

AD7912/AD7922

Rev. 0 | Page 22 of 32

DAISY-CHAIN EXAMPLE

In applications where fast throughput is not critical, connecting

several ADCs in a daisy chain lets the user perform simultane-

ous sampling on all the ADCs contained in the chain using the

minimum number of I/O lines from the µC/DSP ports.

The user needs to alternate modes of operation in the ADCs.

While the parts are in normal mode, the conversion is per-

formed and the result from each ADC is stored in its internal

daisy-chain mode and the user can proceed to read the result

from each ADC by shifting the data from one ADC to the next.

For clarity in the following example, only two devices are

connected in a daisy chain. Both AD7912/AD7922 are

addressed by the same CS and SCLK signal. The devices are

configured as shown in Figure 33 for simultaneous conversion

and shifting read operation later. The output of the device on

the left, ADC1, feeds the input of the device on the right, ADC2.

During a normal conversion, the conversion result is stored

internally and output to the DOUT pin. In daisy-chain mode,

the value internally stored is output through the DOUT pin and

the information provided at the DIN pin is shifted into the

internal register.

When several AD7912/AD7922 are connected in a daisy chain,

the sequence is as follows:

Normal conversion.

Every AD7912/AD7922 performs a conversion on its

selected channel and the result is stored in the internal shift

Entering daisy-chain mode.

In this cycle, CS is brought high between the 10th and

12th SCLK falling edges and all the devices enter daisy-

chain mode.

Daisy-chain cycles.

While the AD7912/AD7922 are in daisy-chain mode, the

conversion results from all the devices in the chain are

read, and the parts are configured for the next conversion.

The user needs to perform as many read cycles as there are

devices in the chain. To keep all the AD7912/AD7922 in

daisy-chain mode, the CHN and STY bits in the DIN input

must always be inverted. Data is shifted through the

devices in the chain. Data is clocked into the device in the

chain by the same clock used to clock data out. The first

word clocked into the DIN pin once the devices are in

daisy-chain mode is eventually lost. The second word

clocked into the DIN pin contains the channel configura-

tion data for the last device in the chain, the third word

clocked into the DIN pin contains the channel configura-

tion data for the second last device in the chain, and so on.

Then the selected channel for the first device in the chain is

clocked in the cycle executed after all the data has been

read, that is, in the short cycle used to change modes of

operation. See Figure 34.

Enter normal mode.

After reading the conversion results from the AD7912/

AD7922, the devices need to be placed into normal mode

to perform a new conversion. Therefore, in this cycle CS is

brought high between the 10th and the 13th SCLK falling

edge. The DOUT line contains invalid data and DIN

contains the selected channel for the first device in the

chain. The remaining devices in the chain have already set

the channel for the next conversion as a result of the data

shifted in during daisy-chain mode.

Normal conversion.

A new conversion can be performed on the newly selected

channels. The process can be repeated by following the

previous steps.

Figure 34 shows the timing diagram for two AD7912/

AD7922 connected in a daisy chain, as shown in Figure 33.

The DIN signal corresponds to the DIN pin on the first

AD7912/AD7922 in the chain, and the DOUT signal

corresponds to the DOUT pin on the last AD7912/AD7922

in the chain. The words clocked into the DIN pin, which

set up the channel for the next conversion for the two

AD7912/AD7922, are shown as COMMAND1 and

COMMAND2. The first word clocked in, COMMAND3,

does not remain in any of the ADCs in the chain and is

eventually lost. The channel configuration data for the first

device in the chain, COMMAND1, is clocked in while

changing from daisy-chain mode to normal mode.

Figure 35 is a more detailed diagram that shows the data

presented on the DIN pin and clocked out on the DOUT

pin for each of the AD7912/AD7922 in Figure 33. If the

DOUT1 (or DIN2) signal is ignored, Figure 35 brings

about Figure 34.

SCLK

DIN1

DOUT1

ADC1

SCLK

DIN2

DOUT2

ADC2

DIN

DOUT

FSX

SCLK

µC/DSP

Figure 33. AD7912/AD7922 Connected in Daisy Chain

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