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MAX9921

Dual, 2-Wire Hall-Effect Sensor Interface with Diagnostics

Maxim Integrated 

Dual, 2-Wire Hall-Effect Sensor Interface

with Diagnostics

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 4 shows the behavior of the MAX9921 when a

Hall input is open. Figure 5 shows the behavior of the

MAX9921 when the open input is reconnected to a Hall

sensor. Figures 4 and 5 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.”

14V

VIN_

0V

HALL INPUT

SHORT TO

GROUND FAULT

INDICATED

5mV/ms

HALL INPUT

DISCONNECTED

FROM SENSOR

HALL INPUT

OPEN-CIRCUIT

FAULT INDICATED

VBAT - 25mV

TIME

IIN_

50µA

0A

TIME

Figure 4. Hall Input Ramps to Open-Circuit Fault When Short to Ground is Relieved

12 ______________________________________________________________________________________

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