April 9, 1998

Signals and Noise suggestion

In reference to the Design Idea "Edge detector runs off of single supply" (EDN, Dec 4, 1997, pg 140), a much simpler design exists. My simpler circuit consists of one EXOR gate, one resistor, and one capacitor (Figure 1).

When both inputs of the gate are at the same levels, the output is low. When the input changes state, one input of the gate is delayed by the RC network. During this time, the output goes high, dropping back low after the delay. Output pulse width  is determined primarily by the RC network. Input levels must be compatible with the gate used. I have used this circuit for several years in test and monitoring equipment.

John Pfouts
Engineer
Custom Equipment Company
Mount Victory, OH

Reader nickels and dimes us

Programming devices in-system by PC-based boundary scan is inarguably cost-effective, but I doubt it can change the basic rules of simple arithmetic, as is suggested by the box "Production programming--a nickel an IC" (EDN, Nov 20, 1997, pg 134). Here, we are told that a PC-based programming station costs $15 per hour to operate, in which case an IC requiring 20 sec for programming has a programming cost of five cents.

If we work out this third-grade arithmetic problem, we find that $15 per hour equals 25 cents per minute, or $0.004167 per sec. Therefore, a 20-sec programming operation costs $0.004167 times 20, or 8.33 cents--more like a dime a chip, not a nickel a chip!

Eric Kinast
Fellow
Datascope Corp
via the Internet

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