Datasheet

TIME
VOLTAGE
V
HIGH
Limit
V
HIGH
- V
HYST
ALERT pin
(Active Low)
Measured Voltage
Over Range Alert
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TIME
VOLTAGE
Measured Voltage
V
HIGH
Limit
V
HIGH
- V
HYST
ALERT pin
(Active Low)
ADC081C021, ADC081C027
www.ti.com SNAS447C –FEBRUARY 2008–REVISED MARCH 2013
ALERT FUNCTION
The ALERT function is an "out-of-range" indicator. At the end of every conversion, the measured voltage is
compared to the values in the V
HIGH
and V
LOW
registers. If the measured voltage exceeds the value stored in
V
HIGH
or falls below the value stored in V
LOW
, an alert condition occurs. The Alert condition is indicated in up to
three places. First, the alert condition always causes either or both of the alert flags in the Alert Status register to
go high. If the measured voltage exceeds the V
HIGH
limit, the Over Range Alert Flag is set. If the measured
voltage falls below the V
LOW
limit, the Under Range Alert Flag is set. Second, if the Alert Flag Enable bit is set in
the Configuration register, the alert condition also sets the MSB of the Conversion Result register. Third, if the
Alert Pin Enable bit is set in the Configuration register, the ALERT output becomes active (see Figure 27). The
ALERT output (ADC081S021 only) can be configured as an active high or active low output via the Polarity bit in
the Configuration register. If the Polarity bit is cleared, the ALERT output is configured as active low. If the
Polarity bit is set, the ALERT output is configured as active high.
The Over Range Alert condition is cleared when one of the following two conditions is met:
1. The controller writes a one to the Over Range Alert Flag bit.
2. The measured voltage goes below the programmed V
HIGH
limit minus the programmed V
HYST
value and the
Alert Hold bit is cleared in the Configuration register. (see Figure 27). If the Alert Hold bit is set, the alert
condition persists and only clears when a one is written to the Over Range Alert Flag bit.
The Under Range Alert condition is cleared when one of the following two conditions is met:
1. The controller writes a one to the Under Range Alert Flag bit.
2. The measured voltage goes above the programmed V
LOW
limit plus the programmed V
HYST
value and the
Alert Hold bit is cleared in the Configuration register. If the Alert Hold bit is set, the alert condition persists
and only clears when a one is written to the Under Range Alert Flag bit.
If the alert condition has been cleared by writing a one to the alert flag while the measured voltage still violates
the V
HIGH
or V
LOW
limits, an alert condition will occur again after the completion of the next conversion (see
Figure 28).
Alert conditions only occur if the input voltage exceeds the V
HIGH
limit or falls below the V
LOW
limit at the sample-
hold instant. The input voltage can exceed the V
HIGH
limit or fall below the V
LOW
limit briefly between conversions
without causing an alert condition.
Figure 27. Alert condition cleared when measured voltage crosses V
HIGH
- V
HYST
Figure 28. Alert condition cleared by writing a "1" to the Alert Flag.
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