AD7674

RDC/SDIN = 1

INVSCLK = INVSYNC = 0

D17

D16

D2

D1

D0

X

12

3

16

17

18

BUSY

SYNC

SCLK

SDOUT

CS, RD

CNVST

EXT/INT = 0

03083-0-046

Figure 41. Master Serial Data Timing for Reading (Read Previous Conversion during Convert)

SLAVE SERIAL INTERFACE

External Clock

The AD7674 is configured to accept an externally supplied

serial data clock on the SCLK pin when the EXT/INT pin is

held high. In this mode, several methods can be used to read the

data. The external serial clock is gated by CS. When CS and RD

are both low, the data can be read after each conversion or

during the following conversion. The external clock can be

either a continuous or a discontinuous clock. A discontinuous

clock can be either normally high or normally low when

inactive. Figure 42 and Figure 43 show the detailed timing

diagrams of these methods.

While the AD7674 is performing a bit decision, it is important

that voltage transients not occur on digital input/output pins or

degradation of the conversion result could occur. This is

particularly important during the second half of the conversion

phase because the AD7674 provides error correction circuitry

that can correct for an improper bit decision made during the

first half of the conversion phase. For this reason, it is

recommended that when an external clock is being provided, it

is a discontinuous clock that only toggles when BUSY is low or,

more importantly, that it does not transition during the latter

half of BUSY high.

External Discontinuous Clock Data Read after

Conversion

Though maximum throughput cannot be achieved using this

mode, it is the most recommended of the serial slave modes.

Figure 42 shows the detailed timing diagrams of this method.

After a conversion is complete, indicated by BUSY returning

low, the result of this conversion can be read while both CS and

RD are low. Data is shifted out MSB first with 18 clock pulses,

and is valid on the rising and falling edge of the clock.

Among the advantages of this method, the conversion

performance is not degraded because there are no voltage

transients on the digital interface during the conversion process.

Also, data can be read at speeds up to 40 MHz, accommodating

both slow digital host interface and the fastest serial reading.

Finally, in this mode only, the AD7674 provides a daisy-chain

feature using the RDC/SDIN input pin to cascade multiple

converters together. This feature is useful for reducing

component count and wiring connections when desired (for

instance, in isolated multiconverter applications).

An example of the concatenation of two devices is shown in

Figure 44. Simultaneous sampling is possible by using a

common CNVST signal. It should be noted that the RDC/SDIN

input is latched on the edge of SCLK opposite the one used to

shift out data on SDOUT. Thus, the MSB of the upstream

converter follows the LSB of the downstream converter on the

next SCLK cycle.

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