When frequency stability is not of prime importance, a simple, yet reliable, digital clock oscillator can be made with the aid of relatively few components. High-speed CMOS (HCU/HCf) inverters or gates with an inverter function are eminently suitable to make such oscillators, thanks to their low power consumption, good output signal definition, and extensive frequency range. The circuit as shown uses two inverters in a 74HCf04 or 74HCU04.
Hcu-hct-based-oscillator - schematic

The basic design equations are: ForHCU: J~l!T; T~2.2RC; Vee 6V; I,~13mA For HCT: f~liT; T~2.4 RC; Vee 5.5 V; !,~2.2 mA With R, and R calculated for a given frequency and value of C, both resistors can be realized as presets to enable precise setting of the output frequency and the duty factor. Do not forget, however, to fit small series resistors in series with the presets, in obse=ce of the minimum values for R and R, as given in the design equations. The values quoted for I, are only valid if the inputs of the remaining gates are grounded.

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