An application of analogue BIST to an automotive circuit

One of the main factors which reduces the efficiency of a spark ignition engine is a phenomenon known as engine knock. This occurs when a rapid combustion started by self ignition in the chamber wall induces a high frequency pressure oscillation in the cylinder. The method currently used to overcome this problem employs a pressure transducer to convert the oscillations into an electronic signal. This signal is fed into an engine knock IC which extracts the `knock´ related frequencies from the normal vibrations of the engine. The vibration spectrum of the engine consists of two resonance frequencies along with an amplification of background noise. The two resonance frequencies are dependant on the speed and the chamber wall temperature of the engine. Hence engine knock detectors require a degree of programmability to take account of these shifts in the knock frequencies. This is achieved through the use of Switched Capacitor (SC) techniques. However SC circuits are more prone to failure than normal RC circuits since the additional FET switches have more failure nodes and there is more probability of shorts occurring across the extra capacitors. This paper describes a built in test technique for testing the numerous SC circuits used in a standard engine knock IC. The technique uses M-sequences and has the advantage that most of the test hardware can be produced by reconfiguring the existing circuit designs