Accurate timebase

Posted by Circuit Labs on Monday, November 24, 2008

This circuit demonstrates how easily a timebase can be designed using a minimum number of components. This circuit consists of no more than an IC and an oscillator to which a few connections and decoupling parts are added. Use is made of a single IC from the 4000 series, the type 4521 oscillator / counter with 24 steps. Here we only use the counter section of the IC.u060573[5]

The IC is supplied with a clock signal by an oscillator module. The clock signal is divided down by the 4521 to obtain certain values. Discrete frequencies obtained in this way are available on the counter output pins of the 4521. For example, pin 10 (Q18) supplies the clock frequency (applied to pin 6) by a factor of 218 or 262,144. Likewise pin 1 (Q24) divides the input signal by 224 or 16,777,216.

By using a clock frequency of Accurate timebase 2.097152 MHz the following timebase frequencies are obtained at the output of the 4521:

• Pin 10 (Q18): 8 Hz;
• Pin 11 (Q19): 4 Hz;
• Pin 12 (Q20): 2 Hz;
• Pin 13 (Q21): 1 Hz;
• Pin 14 (Q22): 0.5 Hz;
• Pin 15 (Q23): 0.25 Hz;
• Pin 1 (Q24): 0.125 Hz.

In case other frequencies are required, a different crystal oscillator module should be selected. For flexibility, an IC socket is recommended so modules can be exchanged quickly. Alternatively, modules may be connected to the 4521 by means of a selector switch.

To keep the circuit as versatile as possible, the outputs of the 4521 counter may be made available for connection to the outside world by way of a pinheader. For the sake of convenience, the pinheader also supplies +5 V and GND.

Author: Thomas Pototschnig

(Elektor Electronics Magazine – 2006)

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