AD7708/AD7718
1. Write to the ADCCON register to select the channel to be
calibrated, its input range, and operation in unipolar or
bipolar mode.
2. Write to the mode register selecting chop or nonchop mode
of operation, select the reference, buffered/unbuffered opera-
tion on the AINCOM, and select zero-scale offset calibration.
Zero-scale calibration can be either self-calibration, where
the ADC determines the zero point internal to the ADC, or a
system calibration where the user must supply the zero-scale
voltage to the input for the duration of the calibration.
3. The calibration is initiated following the write to the mode
register. The user then needs to determine when the calibra-
tion is complete. This can be performed in two ways, by
polling the RDY pin or flag or by monitoring the MD2,
MD1, MD0 bits in the mode register. These bits are reset to
0, 0, 1 when the calibration is complete. The flowchart uses
polling of the mode bits in the mode register to determine
when the calibration is complete.
4. The next step is to perform the full-scale calibration. Full-scale
calibration can be a self-calibration or system calibration.
Using system calibration the user must supply the full-scale
signal to the analog inputs for the duration of the calibration.
Again the MD2, MD1, MD0 bits in the mode register are
monitored to determine when the calibration is complete.
Figure 18 shows a flowchart detailing the necessary program-
ming steps required to cycle through and read data results from
all channels in a multiplexed application. This flowchart assumes
that all channels have been previously calibrated. The following
are the general programming steps required when reading all
channels in a multiplexed application.
1. The AD7708/AD7718 is put into continuous conversion
mode. In this mode the part continually converts on the
specified channel and the RDY line indicates when valid
data is available to be read from the data register.
2. The ADCCON register is written to select the channel
for conversion, its input range and operation is unipolar/
bipolar mode.
3. In this flowchart hardware polling of the RDY line is per-
formed to determine when it is valid to read data from the
converter. When RDY is low, valid data is available in the
data register. The RDY line is set high on a channel change
and will not go low until a new valid data word is available.
Alternatively, the RDY bit in the status register can be polled
in software to determine when to read data from the converter.
4. When the data is read, increment the channel address pointer
to select the next channel, poll the RDY pin, or RDY bit in
the status register, and again read the data. Continue until all
channels have been read.
CYCLE THROUGH AND
READ ALL CHANNELS
WRITE TO MODE REGISTER SELECTING
CONTINUOUS CONVERSION MODE
WRITE TO ADCCON SELECTING CHANNEL N
AND ITS OPERATING RANGE
POLL RDY PIN
RDY
LOW?
NO
YES
WRITE TO COMMUNICATIONS REGISTER SETTING UP
NEXT OPERATION TO BE A READ OF DATA REGISTER
AND THEN READ DATA RESULT
INCREMENT CHANNEL ADDRESS (N = N+1)
NO
ALL
CHANNELS
READ
YES
END
Figure 18. Multichannel Read Operation
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