High Performance Automotive Sonar Intrusion – Data Sheet

AS8413

March 2001

Page 11 of 15

• 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 demand 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 up-

per sensitivity limit during the system development.

The self-test indicates an error with SAS = ”0”. To generate a valid self-test, SAS must be “1”

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 AS8413 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 testing were

used to support the detection criteria embedded in the AS8413.

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 examines

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

Built-In-Self-Test

When power is applied and SAS=’1’, the master AS8413 goes automatically into a self-test rou-

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

time or a clock problem at OSCIN. The self-test does not operate in a slave IC.

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

cessful, 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.