High Performance Automotive Sonar Intrusion – Data Sheet

AS8412

March 2001

Page 10 of 14

• SAS disabled (pin SAS = “0”)

The IC will keep the sensitivity fixed at the upper limit of the programmed range, regardless

of the environmental conditions. This mode can be useful in special applications that de-

mand a fixed or externally controlled sensitivity. The VCAP capacitor may have to be up to 4

times

bigger than it would be with the SAS enabled, to compensate the fixed high sensitivity and

avoid false alarms. Another use of this mode is to allow an easier characterization of the

upper sensitivity limit during the system development.

The self-test indicates an error with SAS=”0”. To generate a valid self-test, SAS must be ‘I’

during power-up. It may be switched afterwards.

Together with the AGC, the SAS loop provides improved controllability over the intrusion detec-

tion process, allowing the system to be little affected by changes in the external conditions,

such as temperature, supply voltage and sensitivity of the ultrasonic sensors.

In any case, the sensitivity can be very significant, so the AS8412 is not adequate to be used in

convertibles or with open windows.

DSP and Fuzzy-Logic Discriminator

Many external phenomena may affect the ultrasonic waves inside the vehicle. Sunlight, blows

at the glasses or roof, wind through the ventilation flaps are some examples.

Experiments have shown that a real intrusion can not be validated by a single specific charac-

teristic of the echo waveform. Several parameters must be observed at the same time and

also how they correlate with each other. Experimental data gathered from extensive field test-

ing were used to support the detection criteria embedded in the AS8412.

To implement those criteria, first the digitized echoes are processed by a DSP circuit to en-

hance the parameters to be monitored. Then, a fuzzy-logic discriminator continuously exam-

ines how those parameters change and correlate, to verify any possible intrusion.

Built-In-Self-Test

When power is applied and SAS = ”1”, the AS8412 goes automatically into a self-test routine

that checks the IC operation. It can also detect initialization errors due to a slow supply rise

time or a clock problem at OSCIN.

During the self-test period, the IC outputs are exercised and should be ignored. If the test is

successful, normal operation starts, indicated by the output LED pulsing periodically.

In the case of an IC malfunction, immediately after the self-test the LED and WARN outputs are

turned on (low) for about 4.4 seconds. If a light-emitting diode is connected to the LED output,

the self-test message may be seen directly by the user.

After an error message, the LED starts to blink again, as in a normal operation. Fig. 6 shows

the possible self-test waveforms after power-on.