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AD7940

Rev. A | Page 16 of 20

SERIAL INTERFACE

Figure 20 shows the detailed timing diagram for serial

interfacing to the AD7940. The serial clock provides the

conversion clock and also controls the transfer of information

from the AD7940 during conversion.

The CS signal initiates the data transfer and conversion process.

The falling edge of CS puts the track-and-hold into hold mode,

takes the bus out of three-state, and samples the analog input.

The conversion is also initiated at this point and will require at

least 16 SCLK cycles to complete. Once 15 SCLK falling edges

have elapsed, the track-and-hold will go back into track mode

on the next SCLK rising edge as shown in Figure 20 at Point B.

On the 16th SCLK falling edge, the SDATA line will go back

into three-state. If the rising edge of CS occurs before 16 SCLKs

have elapsed, the conversion will be terminated and the SDATA

line will go back into three-state; otherwise SDATA returns to

three-state on the 16th SCLK falling edge as shown in Figure 20.

Sixteen serial clock cycles are required to perform the

conversion process and to access data from the AD7940. CS

going low provides the first leading zero to be read in by the

microcontroller or DSP. The remaining data is then clocked out

by subsequent SCLK falling edges beginning with the second

leading zero, thus the first falling clock edge on the serial clock

has the first leading zero provided and also clocks out the

second leading zero. The data transfer will consist of two

leading zeros followed by the 14 bits of data. The final bit in the

data transfer is valid on the 16th falling edge, having been

clocked out on the previous (15th) falling edge.

It is also possible to take valid data on each SCLK rising edge

rather than falling edge, since the SCLK cycle time is long

enough to ensure the data is ready on the rising edge of SCLK.

However, the first leading zero will still be driven by the CS

falling edge, and so it can be taken only on the first SCLK

falling edge. It may be ignored, and the first rising edge of SCLK

after the CS falling edge would have the second leading zero

provided and the 15th rising SCLK edge would have DB0

provided. This method may not work with most

microcontrollers/DSPs, but could possibly be used with FPGAs

and ASICs.

tCONVERT

2 LEADING ZEROS

3-STATE

SCLK

SDATA

ZERO

DB13

DB12

DB11

DB10

DB2

DB1

DB0

tQUIET

Figure 20. AD7940 Serial Interface Timing Diagram

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