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Initial mode:

The magnetic information is derived from a chopped Hall amplifier. The threshold

information comes from a PROM-register that may be programmed at any time, but only

once (no EEPROM). The magnetic information is compared against the threshold and

the output state is set correspondingly. Some hysteresis is introduced in order to avoid

false switching due to noise.

In case that there is no PROM-value available (PROM has not been programmed

before) the chip starts an auto-search for the actual magnetic value (SAR-mode). The

initial threshold value is set to this magnetic value. This feature can be used to find a

TPO-value for providing correct programming information to the chip simply by setting

the chip in front of a well-defined static target. In this case a moving target wheel is not

necessary.

In case there is a PROM value available, the open drain output will be turned on or off

by comparing the magnetic field against the pre-programmed value.

During rotation of the target wheel a self-calibration procedure is started in the

background. The IC memorises magnetic field values for adjusting the threshold to an

optimum value. The exact way of threshold adjustment is described in more detail in the

precalibrated mode.

Precalibrated mode:

In the precalibrated mode the IC permanently monitors the magnetic signal. To say it in

more detail, it searches for minimum (caused by a notch) and maximum (caused by a

tooth) values in the signal. Once the IC has found a pair of min / max values it calculates

the optimum threshold level and adjusts the system offset in such a way, that the

switching occurs on this level. The internal offset update algorithm checks also the

magnetical edge in that point in time when an offset update is to be released. Positive

updates of the offset are released only at magnetic rising edges, negative offset updates

only on magnetic falling edges. Otherwise an update on the wrong magnetic edge may

cause additional switching. The threshold adjustment is limited to increments of approx.

15mT per calibration in order to avoid totally wrong information caused by large signal

disturbances (EMC-events or similar). The optimum threshold level may differ depending

on the target wheel. For example, for regular gearwheels the magnetic signal is close to

a sinusoid and the optimum threshold value can be considered as 50% value, which is

the mean value between minimum and maximum signal. For camshaft wheels an

optimum threshold may be at a different percent-value in order to have minimum phase

error over airgap variations. See fig.4 for definition of this dynamic switching level.

In case that the initial PROM-value does not lead to a switching of the IC because it is

slightly out of the signal range the IC nevertheless does its switch value correction in the

background. After having corrected for a sufficient amount the IC will start its output

switching. The output switching includes some hysteresis in order to avoid false

switching. During 16 switching events updates (number of updates depend on the

magnetic signal) with 15mT are allowed.