Rainbow Electronics MAX9921 - Manual (Page 12)

MAX9921
Hall Input Short-to-Ground
The Hall input shorted-to-ground fault is effectively a
latched condition if the input remains loaded by the Hall
switch when the shorting condition is removed. The cur-
rent required to power the Hall switch is shut off and only
a 50µA pullup current remains. The Hall input can be
manually re-energized or it can be re-energized by the
ECU. A falling edge at DIAG initiates a restart with a
blanking cycle of any Hall input that has been shut down
due to the shorted-to-ground condition. During startup or
restart, it is possible for a Hall input to charge up an exter-
nal capacitance of 0.02µF without tripping into a shorted-
to-ground latched state. All other fault conditions are not
latched and when these other faults are removed, ERR
goes high and the Hall input is again functional.
Manual Method for Re-Energizing Hall Sensor
and Means for Diagnosing an Intermittent
Hall Sensor Connection
Figure 5 shows the behavior of the MAX9921 when a
Hall input is open. Figure 6 shows the behavior of the
MAX9921 when the open input is reconnected to a Hall
sensor. Figures 5 and 6 demonstrate how a shorted-to-
ground Hall input can be reset.
Resetting a shorted-to-ground Hall input involves
three steps:
1) Relieve the short to ground at the Hall sensor.
2) Disconnect the Hall input from the Hall sensor (open
input fault condition).
3) Reconnect the Hall input to the Hall sensor.
The MAX9921 restarts the Hall input with a blanking
cycle. If the Hall input is disconnected from the Hall
sensor for 10ms, it allows the Hall input to be pulled up
by the 50µA pullup current to register the open-input
fault condition. Then, reconnecting the Hall input to the
Hall sensor restarts the Hall input with a blanking cycle.
This provides a manual means of re-energizing a Hall
input without having to resort to the ECU to reset it. This
also demonstrates that an intermittent connection to a
Hall sensor can recover without intervention of the ECU.
This gives the ECU a means of diagnosing an intermit-
tent connection to a Hall sensor by using the diagnostic
mode to detect a diagnostic sequence of “open-circuit”
to “blanking cycle” to “no fault.”
Dual, 2-Wire Hall-Effect Sensor Interface
with Diagnostics
12 ______________________________________________________________________________________
14V
V IN_
I
IN_
0V
HALL INPUT
SHORT TO
GROUND FAULT
INDICATED
HALL INPUT
OPEN-CIRCUIT
FAULT INDICATED
TIME
V BAT - 25mV
5mV/ms
TIME
50 μA
0A
HALL INPUT
DISCONNECTED
FROM SENSOR
Figure 5. Hall Input Ramps to Open-Circuit Fault When Short to Ground is Relieved