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Design Article

# Move over RC: silicon timing has you beat

## 11/27/2010 3:52 PM EST

One might say that the 1600s were a barbaric time, quite literally, for medical technology.  That’s because the barber surgeon was the solution for really big problems, and that often meant surgery, with no disinfectant and no anesthetic.  The barber surgeon was the specialist for the removal of whatever was impeding one’s path to good health:  hair, teeth, appendages, vital fluids, etc.  Fortunately for us, the barber profession has scaled back to a more modest level and specialized medical professionals now handle the “really big problems.”

The long history of the barber surgeon is a great illustration of how technology advances.  Sometimes, as in electronics, lifecycles are so short that we have no time to become enamored with any particular technology.  But that’s not true for the old RC-circuit, which has continued to be an extremely popular timing element for decades.  One need only observe the popularity of the 555 timer, invented nearly 40 years ago, which relies on an RC-circuit.  Even within the last few years, new versions of the 555 timer have come to market.  It’s not just the 555 timer, though; numerous integrated devices rely on an RC-circuit for timing because it has always offered the most simple, flexible and programmable choice available.  But, no matter how it is implemented, using an RC-circuit comes with a number of significant limitations.  This, however, is about to change with the emergence of a new class of timing devices, based on silicon oscillator technology.

Basic RC Circuit
Perhaps the simplest, most common electronic circuit is a resistor, in series with a capacitor, connected to ground. As shown in Figure 1, when a voltage is placed on the resistor, the voltage on the capacitor will exhibit an exponential response:  VC = VR (1 – e-t/Τ), and when the resistor is grounded, the voltage on the capacitor will exhibit a similar exponential response in reverse:  VC  = VINITIAL (e-t/Τ).  This simple, predictable time response makes this circuit an ideal solution for filtering noise, slowing down fast signal edges, protecting device inputs, avoiding race conditions, and countless other timing issues.  Even without adding a single resistor or capacitor to a circuit, this circuit is often present due to the resistance in traces or wire.

Figure 1: Basic Operation of an RC-circuit

With the addition of a few components, the predictable charge and discharge characteristics of the RC-circuit can be used as an electronic timing element.  A nice feature of the RC-circuit is that it can be used to set the timing of both a monostable and an astable response, as illustrated in Figure 2.  In monostable operation, a trigger opens the switch, the capacitor charges up, and the comparator resets the output when the capacitor reaches 2V.  Monostable operation is necessary to enable asynchronous timing such as starting, stopping sequencing, or delaying an event.  In astable operation, the feedback continuously changes the direction of charge and discharge of the capacitor, keeping the capacitor voltage between a fixed range (1V and 2V, in this example).  The result is a continuous pulse train, or oscillator, as long the circuit remains powered.

Figure 2: A simplified version of the 555 circuit illustrates how to harness an RC-circuit.