Aniruddha Deb

Electronic Clock II - Generating the Clock Signal pt.1

In the previous installment of this series, I spoke about the requirements and the architecture of the clock. In this installment, I’m going to engineer the clock signal generator, the most crucial part of our clock

###Importance of the Signal in a clock Every clock is only as accurate as it’s signal. An atomic clock uses radiations emitted by cesium-133 as it’s signal, a quartz clock uses a 32768 Hz quartz crystal as it’s signal whereas a mechanical clock uses an Escapement powered by a hairspring (in wristwatches) or by a pendulum (in clocks). The signal has a cascading effect on the accuracy of a clock - even a 0.1% divergence in the period of the second hand causes a clock to lose or gain 86.4 seconds a day, or in larger terms, lose or gain almost half a day in a year!

Make an accurate electronic signal

Our clock (like most other clocks) is going to use a quartz crystal as it’s main signal. An RC or LC oscillator cannot be an accurate signal as the frequency will change with changes in temperature. Also, tuning an RC/LC oscillator to an exact frequency requires a ton of precision - the frequency has to be within a standard deviation of ±0.01%

The signal generated by this crystal has a frequency of 32768 Hz, which isn’t very useful to us. In order to use it to drive the seconds counter, we must scale it down to 1 Hz (1 pulse per second). In order to do this, I will use a HEF4060, which is a 14 bit binary ripple counter.

The HEF4060 has the capacity to give us a minimum frequency of 32768/214 = 2 Hz. This will later be scaled down using a flip-flop to give us a frequency of 1 Hz. The circuit for this looks like the following:

Clock circuit

After testing the above circuit on a breadboard, pin 7 (Q4) gave the following results on an oscilloscope:


These results were expected as 32768/16 gives us 2048, which is approximately 2.049 kHz. the extra 1 Hz is probably a result of breadboard capacitance, which should go away once the circuit is soldered on a PCB.

In the next installment, I will use a flip flop to halve the 2Hz signal to give us a resultant signal of 1Hz, which can be used to drive the seconds counter.

- Deb