3.0 Watchdog Mode
(Continued)
TL/H/11021–48
FIGURE 9. Status Bit Normalization
4.0 A/D Conversion Modes
The ADC0851/8 can be used in two A/D conversion
modes. In ‘‘One A/D conversion’’ mode, the device oper-
ates as a multiplexed A/D converter and a conversion may
be initiated on any channel or channel pair configured in the
differential mode. In the ‘‘Automatic A/D conversion’’ mode,
an A/D conversion is done on a channel or channel pair and
after the output data is transmitted, conversion begins on
the next subsequent channel or channel pair. This process
will continue unless the device’s mode of operation is
changed.
Note that the A/D conversion time is determined by the
oscillator clock period and has no relation with the digital
clock signal, CLK. The oscillator clock’s frequency is set by
connecting a resistor from the OSC pin (pin 2 for ADC0851
or ADC0858) to V
CC
and a capacitor from the OSC pin to
ground. The conversion time of the A/D converter is eigh-
teen OSC clock periods maximum. Assuming that the oscil-
lation clock frequency is set at 1 MHz (with R
ext
e
3.16 k
X
and C
ext
e
170 pF) then the conversion time would be
18
m
s maximum.
4.1 ONE A/D CONVERSION MODE
This mode is used to initiate one A/D conversion on a single
channel or channel pair configured in the differential mode.
The necessary mode address as per Table I is 1 0 1 0. The
format for the input word is as follows:
Data Input (DI) wordDADC0851 or ADC0858.
TL/H/11021–49
(Table V(a) for ADC0851, Table V(b) for ADC0858)
The 4-bit data following the mode address is the channel
information address. These four bits assign the MUX config-
uration for the single A/D conversion. The channel informa-
tion addresses and the corresonding MUX configurations
are shown in Table V(a) and (b) for ADC0851 and ADC0858
respectively. Note that the ADC0851 only decodes the two
LSBs of the channel information data while ignoring the two
MSBs (I3 and I2). When a channel pair is configured in the
differential mode, it is important to note that the arithmetic
difference of the channel voltages should not be negative.
Negative difference voltage would result in all zeroes at the
output.
TABLE V(a). Channel Information for
One A/D Conversion (ADC0851)
Channel Information
Channels Enabled
I3
I2
I1
I0
X
X
0
0
CH0
X
X
0
1
CH0–CH1
X
X
1
0
CH1
X
X
1
1
Invalid
25