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AD7298

Rev. B | Page 18 of 24

SCLK

DOUT

DIN

112

CONVERSION RESULT

FOR CHANNEL 0

CONVERSION RESULT

FOR CHANNEL 1

INVALID DATA

NO WRITE TO THE

CONTROL REGISTER

NO WRITE TO THE

CONTROL REGISTER

DATA WRITTEN TO CONTROL

SELECTED: REPEAT = 1

SCLK

DOUT

DIN

116

CONVERSION RESULT

FOR CHANNEL 0

CONVERSION RESULT

FOR CHANNEL 2

NO WRITE TO THE

CONTROL REGISTER

NO WRITE TO THE

CONTROL REGISTER

NO WRITE TO THE

CONTROL REGISTER

Figure 27. Configuring a Conversion and Read in Repeat Mode

REPEAT OPERATION

The REPEAT bit in the control register allows the user to select

a sequence of channels on which the AD7298 continuously

converts. When the REPEAT bit is set in the control register,

the AD7298 continuously cycles through the selected channels

in ascending order, beginning with the lowest channel and

converting all channels selected in the control register. On

completion of the sequence, the AD7298 returns to the first

selected channel in the control register and recommences the

sequence.

The conversion sequence of the selected channels in the repeat

mode of operation continues until such time as the control

selected in the control register, then the temperature conversion

will be available for conversion after the last analog input

channel in the sequence has been converted. It is not necessary

to write to the control register once a repeat operation is

initiated unless a change in the AD7298 configuration is

required. The WRITE bit must be set to zero or the DIN line

tied low to ensure that the control register is not accidentally

overwritten, or the automatic conversion sequence interrupted.

A write to the control register during the repeat mode of

operation resets the cycle even if the selected channels are

unchanged. Thus, the next conversion by the AD7298 after

a write operation will be the first selected channel in the

sequence.

To select a sequence of channels, the associated channel bit

must be set to a logic high state (1) for each analog input whose

conversion is required. For example, if the REPEAT bit = 1,

then CH0, CH1, and CH2 = 1. The VIN0 analog input is

converted on the first CS falling edge following the write to

the control register, the VIN1 channel is converted on the

subsequent CS falling edge, and the VIN0 conversion result is

available for reading. The third CS falling edge following the

write operation initiates a conversion on VIN2 and has the VIN1

result available for reading. The AD7298 operates with one

cycle latency, thus the conversion result corresponding to each

conversion is available one serial read cycle after the cycle in

which the conversion is initiated.

This mode of operation simplifies the operation of the device by

allowing consecutive channels to be converted without having

to reprogram the control register or write to the part on each

serial transfer. Figure 27 illustrates how to set up the AD7298

to continuously convert on a particular sequence of channels.

To exit the repeat mode of operation and revert back to the

traditional mode of operation of a multichannel ADC, ensure

that the REPEAT bit = 0 on the next serial write.

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