Hands-on CPLDs -Part I: Experimental rev counter （4）

Published:2011/7/20 22:46:00 Author:Li xiao na From:SeekIC

By A. Rosenkranzer

The remainder

Power for the circuit is supplied by the 12 V vehicle battery via connector Kl. Diode Dl prevents damage to the circuit if the supply polarity gets mixed up. LI filters noise spikes on the supply to the 5 V regulator IC2 while CI and C2 are also used to provide noise decoupling. The complete circuit takes about 0.5 A worst case so power dissipation in the voltage regulator is given by:

Pv = 0.5 A • (14.4 V - 5 V) = 5 W

It is important to ensure that the heatsink for IC2 can dissipate this amount of energy.

The six way DIP switch SI is used to select the vehicles engine cylinder count and the display mode. When a switch position is open circuited a 10 k£2 resistor pulls the line up to 5 V. R8 and R10 reduce the voltage on the ’light on’ signal to TTL levels while C8 reduces any noise spikes on this input.

The signal from the ignition coil is conditioned by the circuitry around transistor Tl, it produces a filtered TTL compatible output signal for IC1 from the ’raw’ ignition signal. C9 AC couples the signal. The negative transition of the pulse is clamped to -0.7 V by diode D3 while R9 protects D3 from excess power dissipation. Rll and zener diode D2 limit the positive level of the pulse to 4 V C10 filters out high frequency noise while R12, Tl and R13 form a transistor switch which produces a clean TTL compatible output pulse to the IC from the noisy ignition signal.

Resistors R14 and R15 control the hysteresis of a Schmitt trigger input. This is quite important because any noise on the ignition input signal would cause the tachometer to give a false reading. R15 provides the feedback from the output side of the input buffer. When the buffer changes state a small proportion of its output is fed back out from pin 30 on IC1 to reinforce the input signal via R15. This has the effect of shifting the voltage threshold at which the buffer can switch back (hysteresis) and so masking the effects of noise on the input.

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