Column: Circuit drill
CMOS or TTL clock generator – which one is best?
frequencies below 7.5MHz either circuit can be used.
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions Company, Jordan
Figure 1: CMOS clock circuit using one CD4049B M
ost digital circuits require clock signal generator circuits. Tese can be easily built with components selected for their small size and low cost. However, the simplest clock circuits use inverters such as the CMOS CD4049B and the TTL 74LS00.
Both are cheap, small and easy to integrate, but which one is best for your circuit, and is there a difference when it comes to applications? To find out we’ve set up the following experiment.
Methodology The circuits in Figures 1 and 2 show simple clock signal generators in their simplest forms using CMOS and TTL ICs. In both, C1, R1, C2 and R4 determine the clock signal frequency. We used identical resistors and capacitors in both circuits, and monitored the generated frequency. Several simulations were done to discover the difference between these two circuits.
Assumptions Throughout the experiment, we used different resistance values for R1 and R4, ranging from 1Ω to 680kΩ. Changing these values changes the frequency of the generated clock signal. We used 0.001uF for C1 and C2. There is no need to vary the
capacitances of C1 and C2, since changing R1 and R4 is enough to alter the signal frequency. Also, it is much easier to choose different values for R1 and R4 than for the capacitors.
Our findings The TTL circuit showed it can generate a clock signal with a frequency much higher than the CMOS circuit – around five times higher. See Figures 3 and 4 for these circuits’ performance for different values of R1. Te TTL clock signal generator circuit yields a 43MHz
signal, whereas the CMOS circuit only generates 7.5MHz. Tis knowledge will help circuit designers to choose the most suitable circuit for their application. In short, use the TTL circuit if a clock frequency higher than 7.5MHz is required, whereas for
Figure 4: TTL circuit performance Figure 2: TTL clock circuit using a 74LS00
Figure 3: CMOS circuit performance
www.electronicsworld.co.uk September 2021 09
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