AD7701

REV. D

–14–

SYNCHRONOUS EXTERNAL CLOCK MODE (SEC)

The SEC mode (MODE pin grounded) is designed for direct

interface to the synchronous serial ports of industry-standard

microprocessors such as the COPS series, 68HC11 and

68HC05. The SEC mode also allows customized interfaces,

using I/O port pins, to microprocessors that do not have a direct

fit with the AD7701’s other modes.

As shown in Figure 20, a falling edge on CS enables the serial

data output with the MSB initially valid. Subsequent data bits

change on the falling edge of an externally supplied SCLK.

After the LSB has been transmitted, DRDY goes high and

SDATA goes three-state. If CS is low and the AD7701 is still

transmitting data when a new data word becomes available, the

old data word continues to be transmitted and the new data is

lost.

If CS is taken high at any time during data transmission,

SDATA and SCLK will go three-state immediately. If CS re-

turns low, the AD7701 will continue transmission with the same

data bit. If transmission has not been initiated and completed by

the time the next data word becomes available, and if CS is

high, DRDY will return high for four clock cycles, then fall as

the new word is loaded into the output register.

CLKIN (I)

DRDY (O)

SDATA (O)

DB15 (MSB)

DB14

DB2

DB1

DB0 (LSB)

HI-Z

HI-Z

SCLK (O)

HI-Z

HI-Z

CS (I)

72 CLKIN

CYCLES

Figure 19. SSC Mode Showing Data Timing Relative to SCLK

SCLK (I)

SDATA (O)

DB15

(MSB)

DB14

DB13

DB1

DB0

(LSB)

HI-Z

HI-Z

DRDY (O)

CS (I)

Figure 20. Timing Diagram for the SEC Mode