AD7851ARS datasheet(27/36 Pages) AD

27 / 36 page

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REV. A

AD7851

extra bit on the DOUT pin.

If the user has control of the CONVST pin but does not want to

exercise it for every conversion, the control register may be used

to start a conversion. Setting the CONVST bit in the control

trol of the CONVST pin, a conversion should not initiated by

writing to the control register. The reason for this is that the

user may get “locked out” and not be able to perform any fur-

ther write/read operations. When a conversion is started by writ-

ing to the control register, the SYNC goes low and read/write

operations take place while the conversion is in progress. How-

ever, once the conversion is complete, there is no way of writing

to the part unless the CONVST pin is exercised. The CONVST

signal triggers the SYNC signal low which allows read/write op-

erations to take place. SYNC must be low to perform read/write

operations. The SYNC is triggered low by the CONVST signal

rising edge or setting the CONVST bit in the control register to

1. Therefore if there is not full control of the CONVST pin the

user may end up getting “locked out.”

SERIAL

INTERFACE

MODE

POWER-ON, APPLY CLKIN SIGNAL,

WAIT 150ms FOR AUTOMATIC CALIBRATION

DIN CONNECTED TO DGND

YES

START

WAIT FOR BUSY SIGNAL

TO GO LOW

PULSE

CONVST PIN

READ

DATA

DURING

CONVERSION

WAIT APPROXIMATELY 200ns

AFTER

CONVST RISING EDGE

APPLY

SYNC (IF REQUIRED), SCLK AND READ

CONVERSION RESULT ON DOUT PIN

PULSE

CONVST PIN

SYNC AUTOMATICALLY GOES LOW

AFTER

CONVST RISING EDGE

SCLK AUTOMATICALLY ACTIVE, READ

CONVERSION RESULT ON DOUT PIN

4, 5

2, 3

Figure 40. Flowchart for Setting Up and Reading from the AD7851

CONFIGURING THE AD7851

AD7851 as a Read-Only ADC

The AD7851 contains fourteen on-chip registers which can be

accessed via the serial interface. In the majority of applications it

will not be necessary to access all of these registers. Figure 40

outlines a flowchart of the sequence which is used to configure

the AD7851 as a Read-Only ADC. In this case there is no writ-

ing to the on-chip registers and only the conversion result data is

read from the part. Interface Mode 1 cannot be used in this case

as it is necessary to write to the control register to set Interface

Mode 1. Here the CLKIN signal is applied directly after power-

on, the CLKIN signal must be present to allow the part to per-

form a calibration. This automatic calibration will be completed

approximately 150 ms after power-on.

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